Absorption coefficient /Density? -- RFZ panels

[musictracer]

Hello,
I have been around this great forum for a while and got great knowledge on stuff I didn't know, so let me start by saying a big thanks to all the admins/moderetors/users and of course John himself for sharing their help with all of us! This is my first post and I will try to follow the posting instructions, but please correct me if I miss something.

Long-term goal:
Treat my bedroom studio.
So, as a starting point I decided to build a few acoustic panels (to create rfz) in my little bedroom studio and currently I am in the research process for the absorption material I should go for, so that I will have the optimum results. My room is small, and while this doesn't provide any useful information, I tried to sketchup as detailed as I could to give an Idea of the geometry of my place. The file is bigger than the size allowed here, thus the external link (i hope it is considered a short one:-) ): https://files.fm/u/g5au5d8x . ( Also please forgive me for leaving the drawer open, but it was downloaded as it looks from the free 3d sketchup warehouse, ) I have already made several measurements that give some idea on how the room behaves acoustically, but I don't own a decent dbspl meter to get the levels right, so I won't be posting them for now, but if requested I can provide them in the future. Maybe I will open a new thread in the future regarding my room treatment needs with decent measurements.

Goal of this post:
So the main goal of the post regards the relation between the Absorption coefficient and the density, and mainly what material should be used in first reflection panels, so that they would be optimally efficient in the low mid frequency area. I always believed that the more dense a porous material is, the higher - low frequency - absorption it provides. But by browsing several threads in the forum I realized that I was wrong. Similarly I browsed external scientific links that also claim the exact opposite of my assumption eg: [http://file.scirp.org/Html/1-1610168_73876.htm=LINK]Density / Absorption Coefficient[/http://file.scirp.org/Html/1-1610168_73876.htm].

Now particularly in my situation, I was planning to use 0.8m (3") "Rockwool COMFORTBOARD™ 80 ". The acoustic specs can be found here: [https://www.rockwool.com/products/comfo ... Sizes=LINK]COMFORTBOARD™ 80[/https://www.rockwool.com/products/comfo ... nsAndSizes]. This material is quite dense (128kg/m3) and seems quite effective even down to 125hz. The problem is that I haven't been able to find a dealer in my city, so I am looking for alternatives too.

So, to summarize, which product would you suggest for a good low-mid absorption on rfz panels in EU? I live in Greece, so no OC can't be found here as far as I know. Also, let me point that currently, due to the lack of space in my room, it is rather difficult to build superchunks (I won't to avoid room modifications). I am planning to turn my basement into a control room in the future, but currently I am forced to work in my bedroom. So, only rfz panels for now and maybe some covered rockwool placed in the corners. I don't want to modify the room itself , as it will be used temporarily.

Thanks in advance,
Good day!

[Soundman2020]

Hi there "musictracer", and Welcome!

Congrats on your first studio treatment!

I tried to sketchup as detailed as I could to give an Idea of the geometry of my place.

Nice SketchUp work! Good job. Everything we need to see is in there. I'm not sure why your file got so big: even after purging it, it's still big. Probably way too much detail in one of the models you downloaded.

I have already made several measurements that give some idea on how the room behaves acoustically, but I don't own a decent dbspl meter to get the levels right, so I won't be posting them for now,

As a very rough initial approximation, you can use a cell phone app as a sound level meter. It won't be accurate, and you'll still need a proper meter later, but just to get in the ball-park, it's fine.

Here's the instructions on how to do your calibration and first test: http://www.johnlsayers.com/phpBB2/viewt ... =3&t=21122 .

So the main goal of the post regards the relation between the Absorption coefficient and the density, and mainly what material should be used in first reflection panels, so that they would be optimally efficient in the low mid frequency area.

Great question! And as you have already discovered, it's not as simple as it sounds.

But first, it's important to understand that you cannot create a true RFZ with absorption alone. Absorption can most certainly help to deal with first reflections, but it does not create an RFZ in the sense that the RFZ room design concept lays out. Part of the RFZ concept is that those first-order reflections from the side walls are NOT absorbed there, but rather are sent to the rear of the room to be diffused (if the room is big enough) or absorbed more effectively (for smaller rooms) than they could be on the side walls. I realize that you are not trying to create a true RFZ room in there, but it's important to understand the concepts.

So, yes, you do need absorption on your first reflection points, and it does need to go as low down as you can get it.

Now, for the actual question about density: Simple answer... it isn't really the density that matters! Rather, it is something called "Gas Flow Resistivity", or GFR for short. That's a parameter of all types of insulation, and even of air itself (curiously), that measures how it reacts to air trying to flow through it. Technically, it is just the ratio of air flowing through a given thickness of the material, to the pressure that was applied, but what it actually measures is acoustic impedance, and it is measured in the rather obscure units of MKS rayls, which can also be written as Pa.s/m2, or pascal-seconds per square meter. Yep, very obscure! Most manufacturers of thermal insulation such as mineral wool, fiberglass, etc. don't even bother measuring this since it isn't really relevant to primary purpose of the product, which is THERMAL insulation, but some manufacturers do, and there are charts and tables for many other products that have been measured independently over the years, or extrapolated, or sometimes even guessed ( ) So if you wanted to do actual acoustic prediction of how a particular type of insulation will perform, then you would need to know the GFR number for it. Typical values for studio builders, are in the range of maybe 4,000 rayls to 50,000 rayls.

Fortunately, even if you can't find the actual GFR number for your insulation, you can still estimate it from rules of thumb: There is a rough relationship between the density of the insulation, and the GFR, but it is not really linear, and is different for every type of insulation. Here's a VERY rough approximation:

GFR-Gas-Flow Resistivity chart.jpg

Here's a chart published by Rockwool themselves, that shows that in reality there is a broad range of possible GFR values even for the same product, with different production settings:

Gas Flow Resisitivty Chart from Rockwool Marine PDF Small.JPG

And here's an even more useful one, published by Isover, showing that the relationship isn't really linear, and varies widely:

gas-flow-resistivity-vs-density-graph-CLPD.png

(in that chart, "Glasswolle" = Fiberglass, and "Steinwolle" = mineral wool) On that graph, there's a line going across at 5,000 rayls, and you can clearly see that to get that very SAME 5.000 rayls GFR for mineral wool and fiberglass, you would need to use a mineral wool product that has a density of around 27 to 40 kg/m3 (so maybe 34 kg/m3 would be good!), but if you use fiberglass then you would have to get something in the range 9 to 18 kg/m3 (so maybe 14 kg/m3). And if you moved that line upwards to the 14,000 rayls mark, then you can understand why we recommend mineral wool at around 50 kg/m3 or fiberglass at around 30 kg/m3, as being ideal for the insulation that goes inside isolation walls, when you are building a full isolated room.

So the answer to your question isn't that simple! But what is clear from all of those graphs, is that lower density products have lower GFR, which makes sense when you think about it.

In fact, to confuse you even more, it's not just the density of the product that matters, but also parametes such as the diameter of the fibers, the porosity of the product, and the "tortuosity", which is a real technical term that describes how twisted and "tortuous" the path of a sound wave through the fiber would be.... Yeah, it's complex.

Ok, so that the background in theory: but what about real life?

I always believed that the more dense a porous material is, the higher - low frequency - absorption it provides. But by browsing several threads in the forum I realized that I was wrong.

Correct! Intuitively, it just seems "right" that big, beefy, heavy, deep frequencies MUST need big, beefy, heavy insulation to treat them, but as you found out, a lot of things in acoustics are NOT intuitive: this is one of them!

As with most things in life, there is an optimal GFR for each application so there's an optimal density too. It's not just that you need to go lower and lower and get the lightest possible product, with the lowest possible GFR, because of you did that, you would end up just using air, which has a GFR of about 400 rayls (it seems strange to talk about how air resists air flowing through it, but that's exactly what sound waves are: air pressure waves flowing through stationary air). So clearly, something as low as 400 is no use, because you already HAVE that in your room, and it sure ain't working as a bass trap!

So, you do actually need something "fibrous" and "absorptive", because the way a bass trap actually works is that it absorbs some of the energy of the sound waves that are moving through it, and turns that energy into low-grade heat. In fact, a large part of how these things work is because the presence of the fibers actually changes the way the air deals with heat (from adiabatic to isothermal if you are interested, but you don't need to know that). So you need to have just the right amount of fiber in there to do the job. Any more, and the bass waves cannot efficiently transfer their energy into the fibers and the air: any less and there isn't enough fiber in there to do the job. If it is too dense, then the low frequency sound waves can't even penetrate into the structure very deeply, because the fibers are too close together, so the wave can't get absorbed. If it is not dense enough, then the waves just go right through, without touching many fibers, and thus they are not affected very much.

So how much is "just right"? Once again, there's no simple answer here. rayls is a measurement of acoustic impedance, and thus the actual effect it has on sound waves varies with frequency. Very dense insulation is great for high frequencies, but does nothing for lows. Very light weight insulation is no good for either.

There's also the question of thickness: Gas Flow Resistivity is a measure done in a laboratory on a specific thickness of the product, and obviously if you used a thicker piece then the resistance would be higher, or for a thinner piece it would be lower. But notice what I did there! I threw ina a new term, "resistance", which is NOT the same as resistivity!

Think of it this way: You probably now that all types of wire have resistance to the flow of electricity, and a very long wire has more resistance than a very short wire (which is why you should have your amp close to yoru speaker, not far away... but I digress). The RESISTIVITY of the long or short wire is the same, but the RESISTANCE is different. You can go to the manufacturer, and will tell you that the resistivity of his wire is 0.37 ohms per meter (or some such). So know you know that ever single meter of that wire has the same RESISTIVITY of 0.37 ohms, so if for some reason you wanted a RESISTANCE of 37 ohms you would need to get 100 meters of that wire. Get the difference? Resistivity is measured per unit length of the wire, and is the same at any place on the wire: If you measure a section 1 meter long at any point, it will always show 0.37 ohms. But the RESISTANCE is related to the entire length of the piece you are using.

Same with insulation: the Gas Flow RESISTIVITY is one thing, fixed, and the Gas Flow RESISTANCE depends on how thick you make it. The thicker it is, the more total resistance it has. Here's a quick prediction I did for the exact same material, with a resistivity of 15,000 rayls, where the blue line is for insulation that is 10cm thick (about 4") and the green line is for the SAME insulation, but 100cm thick (about three feet):

GFR-for-10cm-and-100cm-insulation.jpg

You can see that the thicker stuff is very much more effective for low frequencies.

Now for the interesting thing: what happens if, instead of using 100 cm thick insulation, I used only 50cm thick, and left 50cm empty air behind it? So it is still 100cm total depth, but half of that is empty air... This is what happens: the red line shows that exact case:

GFR-for-10cm-and-100cm--and-50+50airgap-insulation.jpg

Wow! It's practically identical! The red line and green line are almost right on top of each other!

This is good news, because it means you can get an excellent effect from only half the thickness of insulation, if you leave the same thickness of air gap behind it. I won't go into the details of why that works, but it does.

However, you don't want to take it to extremes. Here's what would happen if you kept the same total depth of 100 cm, but you only used 10cm thick insulation at the front: The orange line shows that case:

GFR-for-10cm-and-100cm--and-10+90airgap-insulation.jpg

Now you have a resonant effect, where the air trapped in the space behind the insulation is acting something like a resonant cavity, plus you have comb-filtering, and that messes up the absorption. That would work if you needed to treat a problem at 80 Hz or 220 Hz, but would do nothing at all of you needed to treat a problem at 190 Hz.

I'm sure I am confusing you greatly with all this, and I still have not answered your question! But in my experience, studio builders do a MUCH better job on their rooms if they understand WHY they are doing certain things, instead of just doing them blindly. "Do this because...." is a lot better than just "Do this...".

Now particularly in my situation, I was planning to use 0.8m (3")

Ummm..... 0.8m is about 31", not 3"!!!! So I guess you meant 0.08m? 8 cm? 80 mm?

This material is quite dense (128kg/m3) and seems quite effective even down to 125hz.

Right, but how well does it do in the LOW end? The 125 Hz mark on that graph doesn't mean that this is how well it performs at exactly that frequency: rather, it means that this is the average absorption coefficient for the one-octave wide band centered at 125 Hz. So it might be doing fantastic at 200Hz, but pretty bad at 100 Hz, and still get a rating of "0.75 - 125 Hz". Don't get me wrong! 0.75 at 125 Hz is pretty good, but it's important that you don't misinterpret that. And 125 Hz isn't very low anyway: Kick drums live at around 80 Hz, 5 string bass goes down to about 35 Hz, and a grand piano goes down to 27 Hz. A church pipe organ goes down even lower, to 20 Hz, and if you are doing sound effect for movies, that can go even lower, into the infra-sonic region. So your FIRST priority here, is to figure out how low you need to go! That is set by your speakers: Look at the specs for whatever speakers you are using, and that will tell you where they start to roll off (the "-3dB" point). Go down one octave below that, and that's about the limit for where your speaker will be putting out useful energy. You don't need to worry about absorbing anything below that, because there wont BE anything below that in your room!

So, to summarize, which product would you suggest for a good low-mid absorption on rfz panels in EU?

Terminology! As I mentioned, you can't make an RFZ with insulation alone. But there are several different things that you CAN do with insulation in a room, to greatly improve the acoustics. One of those is bass trapping, and for that the insulation needs to be thick, and low density: maybe 7,000 rayls, or a bit lower. So around 35-40 kg/m3 for mineral wool, or 15-20 kg/m3 for fiberglass. Bass traps are most effective in the room corners and across the back wall. It needs to be DEEP! THICK! At least 15cm thick across ALL of the wall, and 20cm would be better, plus even deeper in the corners.

Another thing you can do is to put absorption on your first reflection points, on the side walls, and ceiling, and that needs to be a little higher density. perhaps 10,000 rayls or so, which means roughly 45kg/m3 for mineral wool, and maybe 22 kg/m3 for fiberglass. That also needs to be fairly thick: around 15 cm, but no less than 10cm, and with an air gap behind it, if you have the space.

And one more thing you can do, is SBIR traps between the speakers and the front wall. Those need to be 10cm thick, and use the same stuff as for the bass trapping on the rear wall.

From what I explained above, you should be able to understand WHY I recommend those densities for those locations...

So that should help you find the stuff you need, even if you don't know what the acoustic characteristics are. As long as you know what TYPE of insulation it is you can figure out roughly what the optimal DENSITY needs to be.

But to be very honest, there's not a huge difference for what you are trying to do in your room: Anything in the ranges I mentioned above would do a pretty good job.

One other point: Putting all that large area of insinuation in the room is going to "suck out" the high end, and make it sound very dull and dry. You will probably need to cover some of the insulation with plastic sheeting, to keep the highs in the room, but then you need to be careful that you don't put the plastic at locations where it could send reflections back to your ears....


- Stuart -

[musictracer]

My god, Stuart, I can't thank you enough for for putting so much effort to explain the physics behind the facts, sir ! You really provide knowledge that equals the one given in the University, in a quite understandable way. You see, I happen to have studied Music Technology and Acoustics a few years ago, though I never really had the chance to practiced the Acoustics part, and so my memory on these things got weakened in time. Some stuff you said, I knew already, many others I couldn't recall, and you truly enlightened me, and some others confused me a bit. But in any case, you really showed a strong will to help and I deeply appreciate that!

As a very rough initial approximation, you can use a cell phone app as a sound level meter.

Well, I wanted to avoid that, since I realized that it wouldn't be an acceptable method by you or the rest moderators. In fact, that's the way my previous measures were made. Despite that, I am planning to make new ones using a bunch of different speakers, because, as you can see in the sketch, my main monitors are the hs5's, and though it's a wonderful speaker for its price, and very useful when mixing, it definitely isn't the flattest responding monitor in the planet.

Technically, it is just the ratio of air flowing through a given thickness of the material, to the pressure that was applied, but what it actually measures is acoustic impedance,

This line confused me a bit, because if GFR is proportional to the air flow and inversely proportional to the pressure applied, then it means that when its rate is high, the gas (air) penetrates more easily. To my small mind this comes in contrary with the term acoustic impedance which is the opposite (to my judgement). Obviously I am mistaking something here, could you please help me with this? To put it in a simple question:

1. Isn't high GFR rate meant to describe the "bigger difficulty" for the gas to penetrate? And if yes, then :
2. Does "bigger difficulty"mean that the absorption is more, or less? (Probably not that simple to answer this one, because I assume there are other factors that play part in the "equation)

The graphs that you provided are pretty enlightening!! Supposing that the answer to my first question above is yes, then it looks quite sensible to me. You see it never mattered to me whether the relation between density and GFR would be proportional, logarithmic or exponential (as it seems to be). It just wouldn't make sense to me that the one rate lowers when the other rises. This was the problem for me when I browsed the thesis (link) Ι posted in my first post, which claims that when density raises, then absorption lowers. I didn't spent much time reading the whole of it, but that is what it concluded. But then again, I feel that I am mistaking the relation between GFR and absorption. I feel that when the one rate rises, then the other one rises too, but I may be wrong, or it might not be that simple.

It's not just that you need to go lower and lower and get the lightest possible product, with the lowest possible GFR,

Ok , now that sentence makes me thing the answer to my second question is negative. Or am I confused again?

So you need to have just the right amount of fiber in there to do the job. Any more, and the bass waves cannot efficiently transfer their energy into the fibers and the air: any less and there isn't enough fiber in there to do the job. If it is too dense, then the low frequency sound waves can't even penetrate into the structure very deeply, because the fibers are too close together, so the wave can't get absorbed. If it is not dense enough, then the waves just go right through, without touching many fibers, and thus they are not affected very much.

Ok, now that starts to clear things in my mind quite a bit. I made a graph to show you the way I get it. If it is correct then things clear a lot for me:

Absoprtion vs density.jpg

Very dense insulation is great for high frequencies, but does nothing for lows. Very light weight insulation is no good for either.

Very useful information!!!!

You've also been very analytical in explaining the difference between resistance and resistivity. The way you explain it I understand that an academic definition could be that "Resistivity is an index of Resistance" , right? (I hope I am using the right terms here) Resistivity is a rate that regards the material itself, regardless of its dimensions. . In our case, when thickness comes into play, then we move from Resistivity to actual Resistance of a construction, so to speak, right?

Attachment:
GFR-for-10cm-and-100cm-insulation.jpg
GFR-for-10cm-and-100cm-insulation.jpg [ 73.45 KiB | Viewed 5 times ]


You can see that the thicker stuff is very much more effective for low frequencies.

Now for the interesting thing: what happens if, instead of using 100 cm thick insulation, I used only 50cm thick, and left 50cm empty air behind it? So it is still 100cm total depth, but half of that is empty air... This is what happens: the red line shows that exact case:

Attachment:
GFR-for-10cm-and-100cm--and-50+50airgap-insulation.jpg
GFR-for-10cm-and-100cm--and-50+50airgap-insulation.jpg [ 92.22 KiB | Viewed 5 times ]


Wow! It's practically identical! The red line and green line are almost right on top of each other!

This is good news, because it means you can get an excellent effect from only half the thickness of insulation, if you leave the same thickness of air gap behind it. I won't go into the details of why that works, but it does.

However, you don't want to take it to extremes. Here's what would happen if you kept the same total depth of 100 cm, but you only used 10cm thick insulation at the front: The orange line shows that case:

Attachment:
GFR-for-10cm-and-100cm--and-10+90airgap-insulation.jpg
GFR-for-10cm-and-100cm--and-10+90airgap-insulation.jpg [ 82.3 KiB | Viewed 5 times ]

I was aware of the two first graphs, but not of the 3d one. Very useful information here!!! Once again, many thanks!!!

I'm sure I am confusing you greatly with all this, and I still have not answered your question! But in my experience, studio builders do a MUCH better job on their rooms if they understand WHY they are doing certain things, instead of just doing them blindly. "Do this because...." is a lot better than just "Do this...".

Not at all, with this particular one. Generally speaking, some info are confusing for me and some are enlightening me, but either way, it all helps me to understand, and to do the job right! I really appreciate it!!

Ummm..... 0.8m is about 31", not 3"!!!! So I guess you meant 0.08m? 8 cm? 80 mm?

Correct, I meant 0.08m, or 8 cm, or 80mm. Please forgive me because I am rushing sometimes when typing... ^_^

So your FIRST priority here, is to figure out how low you need to go! That is set by your speakers: Look at the specs for whatever speakers you are using, and that will tell you where they start to roll off (the "-3dB" point).

Well, if you notice in the sketch of my room, there is an ADAM sub7 subwoofer placed next to my PC tower. Truth is I barely even use it. If I remember correctly it goes as low as 35Hz, so I am not expecting my room to be allowing these large wavelengths to sound right, (or to even sound at all). Currently, if I was able to get a bit smooth response down to 100 Hz I would be quite happy. I am not expecting any miracles to happen.

Regarding the bass trapping, as you can see there is no backwall at all in my bedroom. The only place there is room for it, coulbe the to corners (front and back) on the right (window side). But then again the front one is not a "corner" really , since there is this foundation column there. Also the back one is visible only from the dresser and upwards. Could I effectively build a trap there that would make a difference? Also it is very important for me not to modify the room (no screws or nailson walls or ceiling) which I understand makes things even harder. The panels, will be simply placed (not attached) on the bed, the floor against the window, the floor behind me, and on the desk against the front wall. That was my initial thought. Nothing on ceiling. I have a carpet on the floor that helps a bit with high frequency reflections. After the panels are made, and depending on my budget, I will consider making two bass traps as I mentioned before. I just hope there is a way of not "hurting any wall" just by placing them on the floor or the dresser.

Another thing you can do is to put absorption on your first reflection points, on the side walls, and ceiling, and that needs to be a little higher density. perhaps 10,000 rayls or so, which means roughly 45kg/m3 for mineral wool, and maybe 22 kg/m3 for fiberglass. That also needs to be fairly thick: around 15 cm, but no less than 10cm, and with an air gap behind it, if you have the space.

Very useful information!!! Just what I was looking for!!! Thanks again!!!

Putting all that large area of insinuation in the room is going to "suck out" the high end, and make it sound very dull and dry. You will probably need to cover some of the insulation with plastic sheeting, to keep the highs in the room, but then you need to be careful that you don't put the plastic at locations where it could send reflections back to your ears....

now that really makes me really concerned, because if I build the panels and the room sounds too dull, then I will have to take them apart to modify them, and that is an issue. I hope there is a way to predict that before I build them. Unless there is a way to modify them without taking them apart, but in a way that it looks good, aesthetically.

Anyway, I will be coming back with measurements, and with your advice if you will to help, I will design something and move on from there. I already have a good clue of the material I should choose and I really thank you for you directing me on this, but I will ask for your opinion before purchasing anything, if you don't mind.

Just one more question, have you ever encountered any problem with REW in terms of frequency shifting of graphs? I set a range of, say 65 to 20000 hz and the plot shows from around 70 to 20000 hz. I use a decent audient interface with a dbx mic and set both interface and app sampling rate in the same value. It just seems to me that it shifts the frequency plot about 1/10 octave lower than it should. But then again I might be wrong because when using just one tone, say 1khz and measure it, it shows up at exactly 1khz...

Anyway, will see about it
thanks again,
Good day!!!




Edit: So I am done with the measurements. Measured with yamaha hs5's and wharfedale pro 8.2 passive. I measured at 75 dbspl. It's in the middle between the instructions and the REW recommendations. Please forgive me for doing this, but I didn't want to stretch my speakers on this. My wharfedales are rated 30 watt rms!! The same reason applies for choosing a range starting from 55hz and upward. I just don't want to end up with blown woofers in my hands, I hope you understand. Besides, as I mentioned previously, I am not expecting to be able to get lower frequencies playing smoothly in such a small room. The measurement file can be found here : [ https://files.fm/u/kevc783e]measurement[/ https://files.fm/u/kevc783e] I also post the Left / Right results here, for anyone who might be interested, but is not familiar with mdat files.

hs 5 , wharfedale pro 8.2 L+R.jpg

Please accept my apologies, as I just realized I made a mistake on the graph title. Obviously the Blue regards the wharfedale's measurement, while the red regards the hs5's !! My bad!!

[musictracer]

Ok, so I did some more research on the "what material to use" issue, having in mind the density rates you proposed. Most of the mineral wool Ι can find in my city comes in 40kg/m3 or 50kg/m3. But for the first reflection panels (by the way, "frst reflection" is what I meant in the first place with the term "rfz panels", thanks for correcting me) you proposed roughly 45kg/m3. In the case I won't be able to find this particular density product, should I go for 40 or 50 for these 1st reflection panels? (Probably it wouldn't make that much of a difference, but I thought I might ask before choosing.)

Regarding the build of those panels, I am thinking about 10cm thick insulation with a gap of equally 10cm. If going for 5cm gap would not make a big difference, then I would choose that. What would you recommend?

As for the bass traps, I wasn't thinking about building any, but now I am reconsidering it, trying to find a way to accomplish that without attaching it on the corner walls. Do you think this could be possible?. Also how could I deal with that foundation column in the front right corner? Should I build the trap around it? Would such a trap still be effective?
Also, I can't think of any way to build an effective trap over the dresser on the rear right corner. It's a narrow niche of 49cm x 58cm. Is there a point on building a trap in there?

Also, while researching I came across this page: http://www.bobgolds.com/AbsorptionCoefficients.htm (I decided to post the link directly here, because from what I've seen the method described in the site instructions doesn't work)

fiberglass.jpg

mineral wool.jpg

Here, there are products with roughly double density rates than the ones you proposed (mineralwool or fiberglass respectively) that have a significantly better absorption coefficient. I hope you are not getting me wrong, I don't doubt the validity of your proposals by no means ! I am aware - from the 3 years I've been around this great forum - that you have huge experience on these stuff , and most likely (if not definitely) there is something I am missing here, but I am only trying to understand why those denser products seem to be more effective in low mid range.

Lastly, I am currently in the research on those plastic sheeting issue you mentioned. There is some info on the site, but if you think there is a particular topic that I shouldn't overlook in my research I would love you to cite it here.

Once again,
Many thanks,
Good day!

[DanDan]

If I may help out. Some other perspectives.
The Master Handbooks of Acoustics suggests that absorption varies little in a thin panel over a density range of 4:1
Absorption measured in Reverb Rooms uses approx a 10 Square Metre/Yard sample. Results are quite varied between Labs at LF.
Absorption measured in a Kundt (Impedance) Tube can present a full surface area, or boundary, to the incoming waves.
This is somewhat different, relatively, to a relatively small 10 M Sq sample in a large room, and very different from a single 1200x600mm panel on a typical wall.
So, from both, we can see that an acoustic tile ceiling, full surface area, a full boundary, benefits greatly from an airgap of up to 16:1
One might assume that a large area of panels, contiguous or perhaps closely spaced, would benefit from an airgap also.
We generally do, and recommend 1:1. But I haven't seen that actually tested, so a pinch of salt perhaps, and let's not presume big gaps work proportionately better.

Higher Density relatively thin panels do work well, but it's really not that simple. Carusobond Isobond is relatively low density and has amazing absorption.
There are factors other than GFR, or Density, e.g. Fibre dimensions, Tortuosity......

You would need to go deep to get the full facts. Old tests on 703, 705, studiotips.com ethanwiner.com.
Try to find the late great Eric Desart article called something like 'Trapping Traps' .

So, apart from all this doubt-casting and musing, what to do?
Caruso Isobond is fabulous, as is Autex afaik. In Europe don-audio.com
Locally, under 200mm I would go for 100KG. Isover might be available.
I would stick with 1:1 max airgap.
Straddling Corners causes a nice 80Hz boost, see studiotips.
Enjoy, DD

[Soundman2020]

My god, Stuart, I can't thank you enough for for putting so much effort to explain the physics behind the facts,

You are very welcome! That's what we are here for...

Well, I wanted to avoid that, since I realized that it wouldn't be an acceptable method by you or the rest moderators.

Cell phone "sound level meters" are not accurate, and I sure wouldn't want to use one for actually room tuning, but for a rough initial calibration, they are OK. As long as you update the calibration later, with a decent "REAL" sound level meter, that's fine. The issues in a typical empty room will be way, way larger than the small difference between a cell-phone app and a real meter.

my main monitors are the hs5's, and though it's a wonderful speaker for its price, and very useful when mixing, it definitely isn't the flattest responding monitor in the planet.

Right! At 5", it also doesn't go down very low. About 70Hz, I reckon. So it wouldn't be a lot of use for tracking or mixing instruments with a lot of bass content (drums, bass, electric guitar, keyboards, etc.)

This line confused me a bit, because if GFR is proportional to the air flow and inversely proportional to the pressure applied, then it means that when its rate is high, the gas (air) penetrates more easily. To my small mind this comes in contrary with the term acoustic impedance which is the opposite (to my judgement). Obviously I am mistaking something here, could you please help me with this? To put it in a simple question:

OK, I didn't state that very clearly, and perhaps gave you the wrong impression: Gas flow resistivity is defined as the change in pressure (or pressure drop, if you prefer) across the sample, divided by the flow rate. It's the same as electrical resistance basically: The pressure drop is equivalent to the voltage drop, and the flow rate is equivalent to the current:

R = Δp / qv

where:
Δp = air pressure difference (pascals)
qv = volumetric airflow rate (m3/s)

For an electrical circuit that is the same as classic Ohm0s law: R = V/I There's a lot of parallels between electronics and acoustics.

This was the problem for me when I browsed the thesis (link) Ι posted in my first post, which claims that when density raises, then absorption lowers.

Which is true, for a certain range of values which is different for each material. Don't get confused with resistance and impedance, though; if you know electronics, you know that they are not the same thing...

Ok, now that starts to clear things in my mind quite a bit. I made a graph to show you the way I get it.

That's basically it, sort of, maybe... I wish it were as clear-cut as a standard bell-curve, but it's a little more complex.... I'll get to that later.

Correct, I meant 0.08m, or 8 cm, or 80mm. Please forgive me because I am rushing sometimes when typing... ^_^

You and me both...

Well, if you notice in the sketch of my room, there is an ADAM sub7 subwoofer placed next to my PC tower. Truth is I barely even use it. If I remember correctly it goes as low as 35Hz, so I am not expecting my room to be allowing these large wavelengths to sound right, (or to even sound at all). Currently, if I was able to get a bit smooth response down to 100 Hz I would be quite happy. I am not expecting any miracles to happen.

If you treat the room correctly, it can sound very "right". However, do understand that the goal for a control room is not to make it sound "good"! It is to make it sound FLAT. And flat actually doesn't sound so good to many people... It sounds... welll... err... Flat! No hissing highs, no roaring lows, no exciting mids... just all things at the same level. In fact, if you put average Joe Public in a room with perfectly flat response, give him a graphic equalizer and tell him to make it sound "nice", you'd probably end up with the classic "smiley face" curve set on the EQ controls: Boosting the highs and lows, cutting the mids. Flat doesn't blow your socks of: But flat is what you need for accurate mixing. Flat allows you to hear what each instrument REALLY sounds like, and what your mix REALLY sounds like. It tells you the truth, whole truth, and nothing but the truth. I've had some people be a bit disappointment at first with their new rooms tuned flat, because it doesn't impress the hell out of anyone... until they start using the room, and suddenly appreciate that they can now hear things in their tracks and mixes that they never even knew were there. Then they "get it", and start loving "flat".

Actually, I don't normally tune rooms perfectly flat, because it's just a bit too lifeless (and part of the reason why true LEDE rooms are not liked, and no longer built by serious designers). Rather, I apply a "house curve" to the frequency response, which does put a slight lift in the bass (but still way less that "Joe Public" would do), then roll it off very gently from the mids all the way through the high end. There's a couple of curves that can be used here, but the point is the same: make it a little more interesting, a little more like a typical small domestic speaker would sound like.

So, back to your place: If you have a couple of decent mains and a good sub, you can get your room sounding quite decent, close enough to "flat with a house curve" that it can be very usable as a control room.

Don't short-change yourself on what can be done in your room! It won't be Studio Three or Blackbird, of course, but it doesn't have to be bad!

now that really makes me really concerned, because if I build the panels and the room sounds too dull, then I will have to take them apart to modify them,

Nope! Not if you build them right. Build them as a simple self-supporting frame filled with insulation, and test like that BEFORE you put the final finish fabric on. If you need to return some of the highs to the room, then add the plastic, perhaps in broad strips, or maybe even thin wood panels. Then when you have it right, THAT's when you put the final finish fabric on, to cover up all of the "ugly".

Just one more question, have you ever encountered any problem with REW in terms of frequency shifting of graphs? I set a range of, say 65 to 20000 hz and the plot shows from around 70 to 20000 hz.

It's probably not a frequency shift at all, but rather the way you are viewing the data. If you apply smoothing or filters, then you reduce the resolution, so REW cannot show you data lower than a certain point, because it AVERAGED that data into a smoother curve.

Besides: why are you only starting at 65 Hz? And why are you only ending at 20 kHz? Did you read the instructions on how to use REW to take measurements? I think you need to read this: http://www.johnlsayers.com/phpBB2/viewt ... =3&t=21122 .

I measured at 75 dbspl.

Yup... you DEFINITELY need to read those instructions!

but I didn't want to stretch my speakers on this.

Ummmm if your studio mains can't sustain 80 dBC each, then there's a SERIOUS problem! Yamaha doesn't publish the SPL specs for the HS5's, but I'll bet they can push 100 dBC without sweating too much.

The same reason applies for choosing a range starting from 55hz and upward. I just don't want to end up with blown woofers in my hands, I hope you understand.

I think you are the one who is not understanding! How would you blow the woofer on a speaker by sending it normal music frequencies? Have you ever listened to music on those that has a bass guitar? Or drums? Or keyboards? Or electric guitars? All of those put out substantial energy below 70 Hz.... If you didn't "blow" your speakers by listening to that music, why would you "blow" them now by letting REW do the exact same thing?

Made you are misunderstanding the specs of your speakers: When it says that the frequency response -3 dB at 74 Hz, or -10dB at 54 Hz, that does NOT mean that YOU have to turn the level down by 10 dB for tones under 54 Hz! It just means that when you play any sounds on those speakers, they will just produce all the 54 Hz tones at a level that is 10 dB lower than the rest of the tones they can produces, or for 74 Hz tones the level will be 3 dB lower. That's all it means. It does NOT mean that you will blow the speakers if you play music that has a bass guitar in it, that goes down to 35 Hz. Or keyboards with tones at 60 Hz. It just means that the sound level put out by the speaker for that part of the spectrum will be lower than for the mid-range of the spectrum.

Besides, as I mentioned previously, I am not expecting to be able to get lower frequencies playing smoothly in such a small room.

Why not? What does the size of the room have to do with the ability of your speakers to produce low frequency sound? There is no relationship. Can you hear low frequencies when you play music in your car? The interior of your car is FAR smaller than the interior of your room. Can you hear low frequencies when you listen to music in headphones? Sure you can! But the "room" between the headphone speakers and your ears is minute, compared to the size of your room. Can you hear bass guitar and drums when you listen on your iPhone ear-buds? Of course you can! Yet the room between the ear bud driver and your ear drum is miniscule: fractions of a cubic inch.

There is NO relationship at all between the size of a room, and the ability to produce and hear low frequency sounds. None at all.

You seem to be confusing the issue entirely here: Room acoustics is NOT about the ability to hear any frequency in your room! Rather, it is about using treatment in the room to deal with issues that would "color" the sound put out by your speakers, by adding extra stuff to it, or taking some things away from it. That's all. Your room, left to itself, will indeed color the sound: it will increase or decrease the levels of some frequencies, an it will increase or decrease the length of time that those frequencies tend to "bounce around the room" after the speaker stops playing them. Acoustic treatment is ONLY about that: getting those "coloration" things under control, so that you can hear exactly what comes out of the speaker, without it being changed by the room in any way.

The measurement file can be found here :

Unfortunately, those files are not much use, since you didn't follow the instructions. You ran the tests at a level that is ten times too low, and you cut off the frequency range that we need to see, so the data you provide does not show ANY of the modal activity of the room at all.

Think of it this way: You go to the doctor because you have a huge injury to your leg, and you tell him to fix it... but when he says that you need to roll up your trousers and take off the temporary bandage you wrapped around it, you refuse! You tell him "NO! I can't do that, because it might look bad, and it might hurt, and the bandage might get damaged...." Do you think the doctor is going to be able to do anything at all to fix your leg, if you won't even show him what is wrong with it? Do you think an acoustician will be able to fix your room, if you refuse to even show us what is wrong with it?

You have a decent sub: Hook it up properly, connect your HS5's, set the sub cross over to about 90 Hz, and do the REW tests like that, as the instructions direct. In this case, do a set of SEVEN tests: S--, L--, R--, LR-, LS-, RS-, LRS- Where "L" = left speaker on, "R" = right speaker on, "S" = sub on.

In the case I won't be able to find this particular density product, should I go for 40 or 50 for these 1st reflection panels? (Probably it wouldn't make that much of a difference, but I thought I might ask before choosing.)

Either of those would be fine, but do realize that you are showing two charts with two entirely different products: one is fiberglass, the other is mineral wool...

I am thinking about 10cm thick insulation with a gap of equally 10cm.

For first reflection points, that would be fine. 10cm of OC-703 would be great.

As for the bass traps, I wasn't thinking about building any, but now I am reconsidering it, trying to find a way to accomplish that without attaching it on the corner walls. Do you think this could be possible?.

You WILL need bass traps in there: that goes without saying. I don't even need to see valid REW data to tell you that: It's a small room, so it will need bass trapping. However, you are probably already getting some bass trapping from the things in the room... that helps, but it likely doesn't go far enough.

Since you don't want anything attached to your walls, build a self-supporting wood frame and put the insulation in there. Search for information on how to make a "Superchunk" style bass trap using OC-703. That's a very effective bass trap.

how could I deal with that foundation column in the front right corner? Should I build the trap around it? Would such a trap still be effective?

Yes. And yes. Yes build around it, and yes it will work.

Also, I can't think of any way to build an effective trap over the dresser on the rear right corner. It's a narrow niche of 49cm x 58cm. Is there a point on building a trap in there?

Any trap is better than no trap! Fill that entire gap as best you can with insulation.

Here, there are products with roughly double density rates than the ones you proposed (mineralwool or fiberglass respectively) that have a significantly better absorption coefficient. I hope you are not getting me wrong, I don't doubt the validity of your proposals by no means ! I am aware - from the 3 years I've been around this great forum - that you have huge experience on these stuff , and most likely (if not definitely) there is something I am missing here, but I am only trying to understand why those denser products seem to be more effective in low mid range.

As I mentioned in the first post, it is a lot more complex than just plain density. There's the issue of the size of the fibers that make up the insulation panel, how closely they are packed, how "straight" or "crooked" the air path is through those fibers (technically know as "tortuosity"), how flexible the fibers are, how much they are compressed in manufacture, the binder used to hold them together, the type off facing on the panel, whether or not the fibers run mostly in a certain direction, or are arranged randomly, the direction that sound moves through the panel, etc. It's nowhere near as easy as just looking at density.

There's also the issue of how those numbers where derived, and what they actually mean: when you see that 703 has a coefficient of 1.24 at 500 Hz, that does NOT mean that it absorbs 124% of all sound at 500 Hz.! Clearly, that is not possible. You have to understand that the test method used to derive those numbers only considers the frontal area of the panel, not the edges, and there are a number of strange and wonderful acoustic things that happen around edges and corners, so the TOTAL absorption of the panel can, indeed, by higher than just the absorption that happens through the front face. Some people don't get this, even some people who should know better, and they either say that the numbers are wrong, invalid, irrelevant, no use, etc., or they try to tell you that the panel really does absorb more energy than you throw at it... go figure!

As with most things in acoustics, if you want to get down to understanding exactl how things work, and exactly what the numbers mean, it's going to take a while and a bit of research on your part to get there. But if you just want to use the panels to fix the problems in your room, it really isn't necessary.

So, getting back to the point: Don't try to judge absorption characteristics of a product based only on density. As you can see form the graphs, the relationship is not linear, not constant, different for each type of insulation, and different for each manufacturer. If you use density alone, you'll be even more confused when you start looking at closed-cell foams, open-cell acoustic foam, melamine foam, and even metal foam....

Lastly, I am currently in the research on those plastic sheeting issue you mentioned. There is some info on the site, but if you think there is a particular topic that I shouldn't overlook in my research I would love you to cite it here.

There's a set of equations for foils, that gives you a very rough idea of what frequencies it will reflect, and which ones it will pass through:

F = 90 / m

F = The frequency at which the foil transmits 80% of the sound
m = The surface Mass of the foil in kg/m2

THEREFORE:

F*m=90
m=90/f

That defines the point where foil of any given surface density will be 20% reflective (80% transparent) to sound (allowing 80% of that frequency through to the other side, reflecting back 20%, or coefficient of reflection = 0.2). The curve rises to about 99% reflective above that (1% transparent), at about 6 dB/octave, and falls off to practically 0% reflective (99% transparent) below that, at the same rate.

However, once again, take that with a grain of salt. Different foils react differently, if you stretch it taught or leave it lose has a HUGE effect, leaving an air gap between that and the insulation (or not) has an effect, etc., etc. It's just a guideline to get you to the right general type of foil, then you can play around to fine-tune it to your needs, if you need to.

- Stuart -

[DanDan]

Rather, I apply a "house curve" to the frequency response, which does put a slight lift in the bass (but still way less that "Joe Public" would do), then roll it off very gently from the mids all the way through the high end. There's a couple of curves that can be used here, but the point i the same: make it a little more interesting, a little more like a typical small domestic speaker would sound like.

In a non feminist way...... Me Too! In a treated Control Room, even prosumer, the absorption (hopefully) removes colourations, so you hear the speaker direct field, more like headphones.
But if your treatment is good to LF, you also miss LF Boundary Gain. Overall Room gain in an untreated, normal room can be tilted from LF to HF by about 6-10dB.
So to emphasise Stuart's point, as a Recording Engineer, some LF boost and gentle HF roll off helps enormously with Mix or Master Translation.
DD

[Soundman2020]

So to emphasise Stuart's point, as a Recording Engineer, some LF boost and gentle HF roll off helps enormously with Mix or Master Translation.

Glad you agree, DanDan! I would add, though, that the house curve used to do that, should NEVER be the infamous X-curve... I have actually heard of people thinking they should apply the X-curve to their control rooms, so they could do sound for movies.... Sigh!


- Stuart -

[musictracer]

If I may help out. Some other perspectives...

Thanks a lot DanDan for your helpful contribution! All the info you provide are quite interesting and helpful!

We generally do, and recommend 1:1

Just to make sure, by the term airgap you mean the air space between the panel and the wall, right? Also the ratios you mention correspond to the : "thickness of panel" / "air gap" ratio, right? Which roughly means that for 10cm panel I could go for a 10cm gap for optimal results, right?

You would need to go deep to get the full facts. Old tests on 703, 705, studiotips.com ethanwiner.com.
Try to find the late great Eric Desart article called something like 'Trapping Traps' .

Great! Thanks!

Caruso Isobond is fabulous, as is Autex afaik. In Europe don-audio.com
Locally, under 200mm I would go for 100KG. Isover might be available.

I checked their site. Seems a good product, but a bit pricy for my budget. Also I don't think there is a dealer in my country for this, so it will get even more expensive to get it to my place.
Thanks a lot for your help anyways!

Right! At 5", it also doesn't go down very low. About 70Hz, I reckon. So it wouldn't be a lot of use for tracking or mixing instruments with a lot of bass content (drums, bass, electric guitar, keyboards, etc.)

Exactly! Also it is too mid-rangy. Helps a lot to deal with that specific part of the spectrum, though! Kind of like an ns10 would do...

OK, I didn't state that very clearly, and perhaps gave you the wrong impression: Gas flow resistivity is defined as the change in pressure (or pressure drop, if you prefer) across the sample, divided by the flow rate. It's the same as electrical resistance basically: The pressure drop is equivalent to the voltage drop, and the flow rate is equivalent to the current:

Ok, thanks for clarifying. Seems like my initial assumption on this was right

For an electrical circuit that is the same as classic Ohm0s law: R = V/I There's a lot of parallels between electronics and acoustics.

Yes, I recall from the University days, we often looked at Electrical as well as Mechanical equals to describe acoustics. Helps a lot, indeed!

Don't get confused with resistance and impedance, though; if you know electronics, you know that they are not the same thing...

Well, I know that the main difference is that impendance changes according to frequency, if that's what you mean. Despite that, they roughly describe the same thing, right?

That's basically it, sort of, maybe... I wish it were as clear-cut as a standard bell-curve, but it's a little more complex.... I'll get to that later.

I wouldn't believe it would be such a smooth curve, I just made the graph "on the go" to get rough idea of the relation.

Actually, I don't normally tune rooms perfectly flat, because it's just a bit too lifeless (and part of the reason why true LEDE rooms are not liked, and no longer built by serious designers). Rather, I apply a "house curve" to the frequency response, which does put a slight lift in the bass (but still way less that "Joe Public" would do), then roll it off very gently from the mids all the way through the high end. There's a couple of curves that can be used here, but the point is the same: make it a little more interesting, a little more like a typical small domestic speaker would sound like.

A few months ago I had the chance to tune a JBL system in an auditorium in my city with a colleague . We did this by ear, by adjusting every frequency of the spectrum that needed to be adjusted, while using a sweep tone. When done, we listened to the result and we were a bit dissapointed because it was lacking a lot of bass. Then we decided to boost the whole thing about 10db (!!!) from 200hz and bellow and I can't tell you how amazed we were by the result! The spectrum still sounded ballanced as we tuned it to be but also even and warm, without being muddy at all! We had the sensation it was tuned "flat". Later, by looking at the specs of the controller, it stated that, in case the auto tuning procedure was chosen (which wasn't, but anyway) , there was an extra automated function for boosting the lows in case it sounded too lifeless. The whole thing was a good experience for me, but I never thought this would also apply in critical rooms, like a control room for instance. Thanks a lot for sharing that (along with all the rest )

Nope! Not if you build them right. Build them as a simple self-supporting frame filled with insulation, and test like that BEFORE you put the final finish fabric on. If you need to return some of the highs to the room, then add the plastic, perhaps in broad strips, or maybe even thin wood panels. Then when you have it right, THAT's when you put the final finish fabric on, to cover up all of the "ugly".

Great!! Quite encouraging!! I would have thought it could be done like that but previously you mentioned that I should be careful not to redirect the plastic sheeting reflections back to my ears, and I couldn't figure out how that could be done without angling them in some kind of way. If it is indeed done with angling, then the final finish fabric will have bumps at the points where the strips/sheeting is placed underneath it, right?

It's probably not a frequency shift at all, but rather the way you are viewing the data. If you apply smoothing or filters, then you reduce the resolution, so REW cannot show you data lower than a certain point, because it AVERAGED that data into a smoother curve.

Great! You must be right! I'll check it out!!

Besides: why are you only starting at 65 Hz? And why are you only ending at 20 kHz? Did you read the instructions on how to use REW to take measurements? I think you need to read this

Yup... you DEFINITELY need to read those instructions!

Of course I read the instructions,the reason I hesitated to post my measurements in the first place is that they didn't meet the requirements of the instructions (ie. the lack of decent spl meter) and if you didn't ask for them I wouldn't have post them.

Ummmm if your studio mains can't sustain 80 dBC each, then there's a SERIOUS problem! Yamaha doesn't publish the SPL specs for the HS5's, but I'll bet they can push 100 dBC without sweating too much.

Well, you definitely have a point here. It truly stands to reason.

Made you are misunderstanding the specs of your speakers: When it says that the frequency response -3 dB at 74 Hz, or -10dB at 54 Hz, that does NOT mean that YOU have to turn the level down by 10 dB for tones under 54 Hz! It just means that when you play any sounds on those speakers, they will just produce all the 54 Hz tones at a level that is 10 dB lower than the rest of the tones they can produces, or for 74 Hz tones the level will be 3 dB lower. That's all it means. It does NOT mean that you will blow the speakers if you play music that has a bass guitar in it, that goes down to 35 Hz. Or keyboards with tones at 60 Hz. It just means that the sound level put out by the speaker for that part of the spectrum will be lower than for the mid-range of the spectrum.

No, not at all! Look, of course I am aware of what the "frequency response" is and the difference with the "frequency range" in the specs. But there are 2 main reasons why I want to be careful with the levels. First of all I am using a cellphone as a dbspl meter. The microphone of my cheap cellphone is focused on speech frequencies and thus it is incapable of capturing low frequencies. This could lead to the indication of an a lot smaller dbspl value of the actual one triggering the microphone membrane. The other reason is that I have matched my interface level with the speakers. And for measuring at 80 db each, I have to push a little beyond that level.
Also, you can check out this test (https://sonarworks.com/blog/yamaha-hs5- ... or-review/) by Sonarworks (I'm sure you are aware of their software products) that clearly states that:

"The unusually early bass roll-off on the HS5 might make these speakers harder to calibrate, especially in well treated rooms. The room gain mostly takes care of the bass, so the plug-in only needs to tame it down a little, but for HS5 in treated rooms bass boosting was needed. On higher SPL’s the 45W amp could start running out of steam. Maybe some of the larger HS series speakers would do better in this regard due to bigger woofers and more juice behind them."

Regardless, I will repeat the measurements as you propose, but having added the subwoofer in the procedure as well. It is a good idea. (I will explain later why I avoided that in the first place)

Why not? What does the size of the room have to do with the ability of your speakers to produce low frequency sound? There is no relationship. Can you hear low frequencies when you play music in your car? The interior of your car is FAR smaller than the interior of your room. Can you hear low frequencies when you listen to music in headphones? Sure you can! But the "room" between the headphone speakers and your ears is minute, compared to the size of your room. Can you hear bass guitar and drums when you listen on your iPhone ear-buds? Of course you can! Yet the room between the ear bud driver and your ear drum is miniscule: fractions of a cubic inch.

There is NO relationship at all between the size of a room, and the ability to produce and hear low frequency sounds. None at all.

You seem to be confusing the issue entirely here: Room acoustics is NOT about the ability to hear any frequency in your room! Rather, it is about using treatment in the room to deal with issues that would "color" the sound put out by your speakers, by adding extra stuff to it, or taking some things away from it. That's all. Your room, left to itself, will indeed color the sound: it will increase or decrease the levels of some frequencies, an it will increase or decrease the length of time that those frequencies tend to "bounce around the room" after the speaker stops playing them. Acoustic treatment is ONLY about that: getting those "coloration" things under control, so that you can hear exactly what comes out of the speaker, without it being changed by the room in any way.

Very enlightening and encouraging post! Thanks! The way you put it, it really stands to a good reason! You see it has been commonly thought by many that large wavelengths need larger rooms to be reproduced right. That's what I thought as well, but maybe it is myth. But please, I need your opinion in this also: Don't large speakers need large rooms as well? I mean, I am aware that every inch added in the woofer diameter adds roughly 2 db boost to its low end reproduction. But is this a good reason for not using a large speaker in a small room?

ou have a decent sub: Hook it up properly, connect your HS5's, set the sub cross over to about 90 Hz, and do the REW tests like that, as the instructions direct. In this case, do a set of SEVEN tests: S--, L--, R--, LR-, LS-, RS-, LRS- Where "L" = left speaker on, "R" = right speaker on, "S" = sub on.

Great, I'll certainly do! One thing that concerns me though is the level I should set my sub at. It has to remain unchanged during all measurements, if not forever

Either of those would be fine, but do realize that you are showing two charts with two entirely different products: one is fiberglass, the other is mineral wool...

Yes, I know, this is the reason I used the word "correspondingly" in my previous post.

For first reflection points, that would be fine. 10cm of OC-703 would be great.

Great! Well I wish I could have it in my city. Do you know any equal alternative to the OC-703 for EU?

You WILL need bass traps in there: that goes without saying. I don't even need to see valid REW data to tell you that: It's a small room, so it will need bass trapping. However, you are probably already getting some bass trapping from the things in the room... that helps, but it likely doesn't go far enough.

Of course I will need basstraping in here to make the room sound right. Ok, let me clarify: My initial goal was to only build a few first reflection panels to kind of get a smoother room response down to the low mid range (thus the narrow range of measurements...) The reason is that the place would be used temporarily as a mixing room. But the whole discussion here triggers my yearning to go a step further. Biggest part for this plays your encouragement and strong will to help me with this, which I deeply appreciate!

Since you don't want anything attached to your walls, build a self-supporting wood frame and put the insulation in there. Search for information on how to make a "Superchunk" style bass trap using OC-703. That's a very effective bass trap.

Great! I was thinking of superchunks as well, but I see there is enough room for a wider bass trap (square base, not triangular, thus deeper). Would that be better if I can build it like that?
Also, how important is it to get up to the ceiling with that? I am asking mainly because there is a curtain mechanism about 16cm hanging from the ceiling that prevents me from reaching the top (ceiling). Is that a big issue?
In addition, how deep should the trap be (the height of the isosceles triangle --- face to corner)?

Regarding the plastic sheeting all the info you provided are very helpful. Thanks! But if I ever get there, I will definitely need a little more help with that, because for instance, I haven't yet realized how it should be placed so that it won't redirect reflections to my ears. Probably with an angle or a curve, but then again , how would the fabric cover the construction without a bump? Anyway, it's not to be discussed for now. First things first!

Another issue that concerns me is the effect of the heater on the front right corner which will be almost touching the bass trap and also too close to the right panel. Is there any danger of any kind of reaction with the porous material health wise?


So that's for now.
I'll come back soon with fresh measurements!

Once again, many thanks!
Good day!

[musictracer]

Hi again,
Quick question here...
Been trying to spot Rockwool (company) products in my city but no luck so far. I found these two though:

http://s000.tinyupload.com/?file_id=238 ... 5965297686
http://s000.tinyupload.com/?file_id=253 ... 8769930813

The density is within the range you propose, Stuart, although the GFR is higher. I don't know, what do you think of these? I am afraid they won't do much in the lows and low mids, but I thought I might ask...

By the way, I've been looking at the "per octave" absorption of the 703-FRK . It looks amazing compared to everything else I've looked at. It's the most even one comparatively. I wish I could find something similar where I live. I'm afraid the ones I can get will suck out all the highs in my small room. Really, when people talk about 703 so much, do they mean the plain or the FRK version? Cause FRK looks a lot better to me!

[Soundman2020]

Been trying to spot Rockwool (company) products in my city but no luck so far. I found these two though:

That looks like it should work fine. It's good that they show decent acoustic data for their product! So many manufacturers don't do that.

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I am afraid they won't do much in the lows and low mids, but I thought I might ask...

The PDF's you linked to only show data for the 50mm panels, but you would need the 100mm (4") or maybe even the 160 mm (6"). I would go with the B-040 version, in 100mm or 160mm. Perhaps contact the manufacturer and see if they have the graphs for the thicker stuff. I think you'll find that it is considerably better than the 50mm. The 50mm stuff is getting 0.6 at 250 Hz, and .2 at 125 Hz, which is quite good. Not as good as OC-703, of course, but it's actually very close to OC-701. I would expect that the 100mm version would likely be over 0.9 at 250 Hz and maybe .65 or so at 125 Hz. That's not fantastic, but still very usable.

I'm afraid the ones I can get will suck out all the highs in my small room.

That can be fixed! There are techniques for ensuring that the bass trapping doesn't kill the highs...

Really, when people talk about 703 so much, do they mean the plain or the FRK version? Cause FRK looks a lot better to me!

Mostly people use the plain un-faced 703 for acoustic treatment, but the FRK is excellent for bass traps, as long as you have the foil side towards the room. I you have it the other way around, there's not a lot of improvement. On bass traps the foil helps in two ways: 1) it seems to act something like a membrane, to a certain extent, and improves low frequency absorption, and 2) it reflects back some of the upper mids and highs to the room, which is desirable. I have heard of people spray-gluing their own facing onto panels (usually very thick paper or thin cardboard), but I have no idea how that worked out for them, or if the results were anything like FRK... I have my doubts!

- Stuart -

[musictracer]

hat looks like it should work fine. It's good that they show decent acoustic data for their product! So many manufacturers don't do that.

Yes indeed! And to my surprise it is a Greek company!! Well done!!

The PDF's you linked to only show data for the 50mm panels, but you would need the 100mm (4") or maybe even the 160 mm (6"). I

This is the only spec sheet there is for the product. I will be using 10cm thickness with 10cm gap.

I would go with the B-040 version, in 100mm or 160mm. Perhaps contact the manufacturer and see if they have the graphs for the thicker stuff. I think you'll find that it is considerably better than the 50mm. The 50mm stuff is getting 0.6 at 250 Hz, and .2 at 125 Hz, which is quite good. Not as good as OC-703, of course, but it's actually very close to OC-701. I would expect that the 100mm version would likely be over 0.9 at 250 Hz and maybe .65 or so at 125 Hz. That's not fantastic, but still very usable.

From your words I can see you consider the 051 very usable and quite close to oc701, but despite that you say you would go for the 040. I am a little confused.

Regardless, I don't think it will do the job quite right. You see, all the measurements I've done clearly show a 125hz peak which must be a 2nd order axial mode on z axis (floor to ceiling). I will definitely need much absorption down there in the spectrum. Also there is very little space available for bass trapping, so... I think it won't be that efficient. The fact that I won't be using any absorption on the ceiling makes things even worse...

I'll search for something better...

Also I am going to borrow an spl meter from a friend because my cellphone must be distorting from 75dbspl and onward. This is why I am delaying the measurements.

Finally, some people have warned me about having health issues with fiber material like rockwool or glasswool. Their mouth dried out and they had some allergy kind of symptoms. One of them switched to sheep wool. Do you know anything about the issue? Ever used or heard about sheep wool?

[musictracer]

Quote:
I would go with the B-040 version, in 100mm or 160mm. Perhaps contact the manufacturer and see if they have the graphs for the thicker stuff. I think you'll find that it is considerably better than the 50mm. The 50mm stuff is getting 0.6 at 250 Hz, and .2 at 125 Hz, which is quite good. Not as good as OC-703, of course, but it's actually very close to OC-701. I would expect that the 100mm version would likely be over 0.9 at 250 Hz and maybe .65 or so at 125 Hz. That's not fantastic, but still very usable.

From your words I can see you consider the 051 very usable and quite close to oc701, but despite that you say you would go for the 040. I am a little confused.

Please accept my apologies, you were referring to the 50mm panels which I mistook for 50kg/m3. My bad! Still I am skeptical about the product though... For reasons I mentioned above...

[Soundman2020]

Finally, some people have warned me about having health issues with fiber material like rockwool or glasswool. Their mouth dried out and they had some allergy kind of symptoms. One of them switched to sheep wool. Do you know anything about the issue? Ever used or heard about sheep wool?

Fiberglass and mineral wool are probably the most common insulation materials around. In millions of homes, offices, shops, schools, churches, theaters, etc., all around the world. If there was a widespread problem such as your scaremongering friends seem to think, these products would have be banned long ago.

Fiberglass is glass, made from sand. Mineral wool is rock. So for people who are allergic to glass or rock, I guess they would also be allergic to this tpe of insulation.

Yes, there can be problems for some people with some types of the binder used in manufacturing the insulation, but not the insulation material itself, so make sure to use the type that does not use anything you might be allergic to personally, or that might cause other health issues. A few decades back there was a big scare about formaldehyde being a carcinogen, and it was used to make the resin that binds the insulation fibers together. The USA briefly banned formaldehyde in insulation, but then reversed the ban a year or so later, as the evidence was found to be weak. But new manufacturing processes were created anyway, to greatly reduce the release of formaldehyde from insulation.

If you are concerned about this, then don't buy insulation that uses urea-formaldehyde in the manufacturing process, as that can release formaldehyde gas. The ones that use phenol-formaldehyde are much better: the only release very low levels of the gas: lower than you'd find in normal air inside a typical building anyway. There are also some "formaldehyde-free" insulation products these days, more expensive, and not really "formaldehyde free" anyway! They can still release formaldehyde if they come into contact with a hot surface.

Here's some information I found on the issue:

https://www.certainteed.com/resources/IARCQandA.pdf

The issue of other potential health concerns was also discussed over at GS a few years ago. Here's the link: http://www.gearslutz.com/board/bass-tra ... eview.html

If you are concerned about formaldehyde in your room, then you will also have to avoid many types of carpet, bamboo flooring, hardwood flooring, MDF, plywood, and many types of cabinet and other furniture: they are very often made using products that contain various types of aldehydes. So check carefully for ALL of the building products that you use.

During the actual process of installing the insulation, it is ALWAYS advisable to wear safety glasses, gloves, a mask, and long sleeves, regardless of the type of insulation you are installing, as it can release dust and fibers while you cut it and handle it. It's not pleasant stuff to work with in general. Once it is in place, there's no further problem.

I have not used sheep wool insulation in studios. I do know it has good thermal properties, and some environmental advantages, but I'm not aware of any extensive independent research on the acoustic properties of sheep wool. There probably is some information out there, but I've never gone looking for it. So I can't help you much there!

- Stuart -

[DanDan]

A list of Alternative fibre absorption. http://johnlsayers.com/phpBB2/viewtopic.php?f=3&t=21797

Any hemp I have seen seemed too light and open. Sheep's wool can be very smelly. I haven't been able to find credible absorption or GFR data on either.

DD