John Sayers' Recording Studio Design Forum Archive

Hello!

I've been reading this forum for a while and have learned a lot of acoustics. This is my first post.

I have a small space next to my house (L3m x W2,75m x H2,60m) which I'm going to treat acoustically. I'm going to use the room for writing music and for mixing. I don't need to record any instruments in this space. Currently there's no acoustic treatment at all, except two movable 10cm thick rockwool panel absorbers that I built for testing purposes. I'm ready to do whatever needed to make this room sound as good as possible. More information about the room in the pictures. My monitors are Genelec 8330A and go down to 45Hz. The listening level will be quite low. There's no need for soundproofing since the exterior wall insulation is 150mm thick due to our cold winters. My monitors are against the front wall, 75cm away from side walls. The listening position is 120cm from the front wall. I followed the Soundman2020 procedure for REW and measured the room. Here's the REW file: https://bit.ly/2wBUjPP I think that low end null needs a lot of bass absorption. I'm going to treat the whole rear wall, there's enough space for 60cm deep absorption. I red the posts about acoustic hangers and I'm wondering if it would be a good idea to build hangers to the rear wall of my room. One thing with all the treatment I need to take care, is that I need to leave 5-10cm air gap between the exterior wall and bass traps. If I place thick insulation tightly against the exterior wall, that can cause the dewpoint to form in the wrong place and then there's a risk of mold problems. So I was thinking the structure could be: 5cm air gap at the back and both sides- 5cm rockwool across the rear wall and both sides- hangers 50cm wide angled 40 degrees - 5 or 10cm rockwool at the front. Maybe slats too. I don't know how to use sketchup so I finished my plan with photoshop. It's not in scale but you get the idea. Would this be a good design for the rear wall? I would also like to hear suggestions for the front wall and the ceiling.

Thanks,
Matt

Hi Matt,

Welcome to the forum. Have you thought about what design you are basing your design on? RFZ, MyRoom, LEDE?

With your room being very small you may find difficulty getting a great sounding room, although you may not have as many problems with low frequency modes as you might expect due to the fact that they will likely escape directly through your boundaries, and won't be contained.

Have a look here at a shipping container studio john designed:

http://johnlsayers.com/Studio/Mainpage/MP-Mark.htm

You'll notice that john even soffit mounted the speakers in here. From the pics it looks like the rear wall uses corner bass traps and absorption in the middle with an angled slot resonator in the center to avoid reflections heading directly to the mix position. It also features slot resonators on both side walls. These are tuned to the problem frequencies in the room.

Thanks,
Dan

Thanks for your response! And sorry for the delay with my reply.

I think I’ll forget that hanger idea... Actually I didn’t realize I have different design types to choose from. I was just thinking I need the basic bass traps on the corners, rear wall, first reflection points, a cloud and so on. But if my room allows me to do more specific and better design, then why not. I don’t know which design type would be the best. Do you have an opinion on that? What would be a cost effective and best way to treat this room?

Thanks,
Matt

Hi Matt, and Welcome t the forum!

Just a couple of quick comments:

There's no need for soundproofing since the exterior wall insulation is 150mm thick due to our cold winters.

Contrary to popular belief, insulation does not isolate, and there is no such thing as "soundproofing" ! Blunt statements, yes, but true. First, "soundproofing" is not a term that acousticians use much, since it has no technical definition, and means different things to different people. Literally, to make something "sound PROOF" means to end up with something that sound cannot penetrate or affect in any way. That's impossible. Any sufficiently loud sound will penetrate any conceivable barrier. The loudest sound ever heard on Planet Earth was heard around the world, was able to crack concrete 300 miles away, and flattened everything within a radius of dozens of miles. There is some astronomical evidence of waves similar to sound waves traveling though hot interstellar gas in some wild regions of the universe, and it is easily getting through hundreds of billions of miles of "empty" space. So no, you can't "soundproof" anything. You can isolate, sure, but you can't "proof" anything against sound. Even at Galaxy Studios, the best "soundproofed" studio on this planet, you could still hear a gunshot from one room to the next.

So, what I think you meant, is that you do not need additional isolation beyond what you have now. In other words, there are no external sounds around you that could get into your room and upset a writing/composing/mixing session, annoying you and breaking your train of thought. Sounds such as thunder, rain, hail, cars outside, aircraft overhead, helicopters, sirens, your neighbor mowing his lawn, dogs barking, the vacuum cleaner running, washing machine, microwave, sump pump, radio, TV, telephones, people taking, car radios, trains, etc. You do not have any of those sounds occurring around your room, so you do not need isolation. Is that what you are saying?

The other "common belief" is that, even if you do need isolation, you can accomplish it with insulation. Nope. Doesn't work. Insulation does not stop sound from getting through. If you don't believe me, go buy a sheet of 4" thick OC-703 (which you will need for your treatment in any case, so it won't be wasted money to do this experiment), play loud, bass-heavy music on one of your speakers, then put that sheet of 703 in front of the speaker. Can you still hear the music? Yup! You sure can! Muffled a bit, certainly, but you can still hear it. Add another sheet, so it is now 8" thick. Still hear it? Yup. End of experiment. Insulation does not isolate, And OC-703 is about the best there is. So having 150mm of insulation does not, by itself, mean that your walls isolate well. The acoustic purpose of insulation inside a wall is NOT to stop the sound from getting through: it is to damp the acoustic resonances going on inside the wall, and to change the way the air deals with heat from adiabatic to isothermal, as well as slowing down the speed of sound. All of that causes the resonant frequency off the wall system to be reduced, which is a good thing for isolation, but the insulation itself does not isolate. It helps the isolation system to work better, more effectively, but by itself, it does not isolate.

So assuming that you do not need isolation, then all you need is treatment.

I looked at your REW data, and yes, you do need treatment. A LOT of treatment. The modal issues are very clear in the low end: 57.9, 76.9, 116.8, 127.2, 169.7, 185.8, 215, 255, 289, 316, 365, 382, 405, 419, 460 Hz. Lows and mid lows (it's a small room). I could correlate those against the actual room dimensions, to see which peak corresponds to which room mode, so you'd know where to treat each one of those, but you ca do that yourself. You also have some pretty strong reflections, as well as SBIR and other issues. Yes, yo do need a lot of treatment.

My monitors are against the front wall, 75cm away from side walls. The listening position is 120cm from the front wall.

Great! Sounds about right. You could try moving a little forward from there: there's probably a better spot a few cm closer to the front wall.

I followed the Soundman2020 procedure for REW and measured the room.

Actually, you didn't! Your tests were done at around 75 dBC, not at 86 dBC as I wrote in that tutorial. Here's what your tests look like:
If you would have done your test at 86 dBC, then your SPL curve would be roughly averaging where the horizontal cursor line is. I set that at 86 dB, so you can see where it should have been. Your curve is actually around 75 dB, give or take a bit. It looks like you followed the REW instructions initially (which o tell you to use 75 dB, but that's for home theaters, not studios), but then didn't calibrate your system correctly to 86 dB after that. Or maybe your hand-held sound level meter is very faulty. Or you forgot to set it to "C" weighting and "Slow" response. But something is wrong there, and your tests were done at a volume ten times too low (ten dB too low). So it is possible that you didn't trigger all of the modes in your room...

I think that low end null needs a lot of bass absorption.

Probably, but I think you'll find it hard to do that in practice! That is most likely a "floor bounce", or a ceiling bounce. You can see that it is not related to the side walls, since it does not change at all with the treatment you did for the last reading.

I'm going to treat the whole rear wall, there's enough space for 60cm deep absorption.

Great! That's a good start, and will certainly help to get your low end under better control.

I red the posts about acoustic hangers and I'm wondering if it would be a good idea to build hangers to the rear wall of my room.

Yes it would: they are very effective. Take a look at this thread: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368 One of the first things we did in that room was with hangers in the rear corners (along with the tuned slot resonators). You can see how effective they are.

If I place thick insulation tightly against the exterior wall, that can cause the dewpoint to form in the wrong place and then there's a risk of mold problems.

I don't understand. You said you have 150mm insulation inside your walls, and that was for thermal purposes. So how come you have a condensation problem? You should get that fixed first, BEFORE you do any treatment. If your room is properly built, with air barriers, water barriers, and vapor barriers in the correct places inside the walls, and your HVAC system is properly designed, then that should not happen. Fix this first!

So I was thinking the structure could be: 5cm air gap at the back and both sides- 5cm rockwool across the rear wall and both sides- hangers 50cm wide angled 40 degrees - 5 or 10cm rockwool at the front. Maybe slats too.

Excellent! That will work quite well for bass trapping. Make the hangers in the corners bigger than the ones across the middle.

Would this be a good design for the rear wall?

Yes!

Have a look here at a shipping container studio john designed:

Your link doesn't seem to be working, Dan.... This is the one you want: http://johnlsayers.com/Studio/Pages/Container_1.htm

and

http://johnlsayers.com/Pages/Spark_1.htm

From the pics it looks like the rear wall uses corner bass traps and absorption in the middle with an angled slot resonator in the center

Right, but you really have to know what you are doing to use resonators on the rear wall of a control room. Not easy to accomplish that...

I think I’ll forget that hanger idea...

Why?

I don’t know which design type would be the best. Do you have an opinion on that? What would be a cost effective and best way to treat this room?

RFZ. Absolutely. Right now, there's no better design concept out there, especially if we are talking about small rooms. Pretty much all of the designs I do are based around the RFZ concept. The one I linked you to above, for example is RFZ based, even though it is rather unusual.

You do have options, but I'd suggest RFZ would be a really good choice for you. Not cheap, of course, but it will get you the best acoustics, if you design it well and build it well.

- Stuart -

Hi Stuart and thank you for your detailed response!

I just want to let you know that it's not easy for me to express all my thoughts in english, so I might use wrong terms sometimes or some of the things I write may be unclear... But I try my best and explain more if necessary. I meant Isolation when I wrote soundproofing.

So, what I think you meant, is that you do not need additional isolation beyond what you have now. In other words, there are no external sounds around you that could get into your room and upset a writing/composing/mixing session, annoying you and breaking your train of thought. Sounds such as thunder, rain, hail, cars outside, aircraft overhead, helicopters, sirens, your neighbor mowing his lawn, dogs barking, the vacuum cleaner running, washing machine, microwave, sump pump, radio, TV, telephones, people taking, car radios, trains, etc. You do not have any of those sounds occurring around your room, so you do not need isolation. Is that what you are saying?

Yeah, that's what I meant, I only need the treatment. I know that good isolation is one of the most important things and you always emphasize this. This is a very quiet place where I live, a forest around me and one neighbor nearby. No cars driving all the time, no trains, aircrafts... just wind or rain sometimes, those won't bother me. Usually I hear nothing from the outside. My closest neighbor won't hear anything, I tested this by playing tracks at high volume. This space is also separated from my house. Adding isolation would also make this room even smaller and cause even more problems.

Actually, you didn't! Your tests were done at around 75 dBC, not at 86 dBC as I wrote in that tutorial. Here's what your tests look like:

You wrote in your first post of the tutorial: "Yes, I know that the REW manual says to use 70 dB, but I prefer 80 dBC to make sure that there's a good signal to noise ratio, and that all of the room modes are triggered." So I measured at 80dBC. I didn't read the later posts where you said 86dBC... but I found out what I did wrong, so I'll do it again later at 86dBC and let you know when the REW file is updated.

I don't understand. You said you have 150mm insulation inside your walls, and that was for thermal purposes. So how come you have a condensation problem? You should get that fixed first, BEFORE you do any treatment. If your room is properly built, with air barriers, water barriers, and vapor barriers in the correct places inside the walls, and your HVAC system is properly designed, then that should not happen. Fix this first!

No, there's nothing wrong with my walls, they are all properly made. I didn't mean there's a condensation problem now. I talked about my room with two acousticians here in Finland, both are very experienced studio designers, the other one works for Genelec and another works for Akukon. https://www.akukon.fi/en They both warned me about this and said that in our climate conditions if you place a thick bass absorber tightly against the exterior wall, the vapour barrier stays in between two thick layers of insulation and it's not meant to be there. That can cause problems. It can be avoided by leaving an air gap between the wall and the bass absorber.

Why?

I was just thinking I forget the hanger idea for now, until I have decided which design consept I choose. But RFZ seems good. Does it always require soffit mounting on speakers? Can I soffit mount Genelec speakers with rear ports?

-Matt

it's not easy for me to express all my thoughts in english,

You are doing great! I would never have guessed that English is not your first language. Not a problem!

I meant Isolation when I wrote soundproofing.

Don't worry: many, many English speaking people make the same mistake! "soundproofing" is the common term that the general public uses: you see it all over the place. But technically it isn't correct, because there's no technical definition for what "soundproofing" actually is. So this is not something you got wrong due to not speaking English perfectly! It's something that EVERYBODY gets wrong. It means different things to different people: for some people, if you put up an absorber panel on the wall and the internal room acoustics sounds better, they call that "soundproofing"....

Anyway: OK, so you do not need more isolation in your room: That's good news!

You wrote in your first post of the tutorial: "Yes, I know that the REW manual says to use 70 dB, but I prefer 80 dBC to make sure that there's a good signal to noise ratio, and that all of the room modes are triggered." So I measured at 80dBC. I didn't read the later posts where you said 86dBC...

Right! It is 80 dBC for each individual speaker, when you calibrate them one at a time, but that automatically means the level will be 6 dB higher when they both play the SAME sound together. That's not true for playing music, of course, since music is not normally correlated between the speakers (unless you only record and mix mono!). For music, the increase will be closer to 3 dB, but for correlated sound, the increase will be exactly 6 dB. So with both speakers set for 80, together they produce 86. And with both set for 70, together they produce 76.

but I found out what I did wrong, so I'll do it again later at 86dBC and let you know when the REW file is updated.

It can be avoided by leaving an air gap between the wall and the bass absorber.

I guess you must have unusually high humidity where you live? But if that's the normal situation where you live, then there's no problem with that, acoustically.

I was just thinking I forget the hanger idea for now, until I have decided which design consept I choose.

There are alternatives, but hangers are about the most effective way of doing it.

But RFZ seems good.

Yes! Highly recommended.

Does it always require soffit mounting on speakers?

I have seem some people try to do it without soffit-mounting, but never seen it succeed properly. The thing is that all speakers will always produce artifacts when they are just inside the room: it doesn't matter which speaker it is, or who made it: it will produce artifacts. Some speakers more, others less, but ALL of them produce artifacts, just from the fact of being in the room. You can eliminate many of those artifacts by taking the speakers OUT of the room: putting them in soffits. Technically, a soffit is a "wall", so by soffit mounting the speaker, it is no longer in the room: it is in the wall. And therefore it CANNOT create those artifacts any more.

Some people try to reduce the artifacts with absorption or diffusion around the speakers, but that's complicated... it's a lot easier to just soffit-mount your speakers.

Can I soffit mount Genelec speakers with rear ports?

Yes you can. The vast majority of speakers can be soffit-mounted, even rear-ported ones. The only speakers that CANNOT be soffit mounted, are those that have actual physical drivers on the sides, top or bottom. Pretty much all others can be soffit mounted, if the soffit is designed correctly.

Take a look at this thread: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 Those speakers are Eve Audio SC-407's and they have very, very large bass reflex ports on the rear. Huge. Yet there they are, mounted in soffits, and performing spectacularly... If you look around the forum, you'll find many threads where people have soffit-mounted rear-ported speakers. As long as you design the soffit taking into account the rear port, then it will work. Genelec actually makes a flush-mount kit for the 8330. You do NOT need that! The kit they make is for basic flush mounting: proper soffit mounting is done differently, but the simple fact that they themselves offer a flush-mount kit, clearly demonstrates that it can be flush mounted / soffit mounted. No problem at all.

And soffit-mounting is, indeed, the best way to do it.

- Stuart -

I updated the REW file. https://bit.ly/2wBUjPP

I don't know why, but my hand held meter shows 80dBC when only one speaker is on, and 84dBC when both speakers on. I calibrated REW to 80dBC with one speaker on and my hand held meter was showing 80dBC.
-Matt

I tried to find the correct angles for splayed walls and did ray tracing. Did I understand this right? Splayed walls are reflective material and the back wall absorbs most of the reflected soundwaves? I used this tool: https://amcoustics.com/tools/amray

I tried to find the correct angles for splayed walls and did ray tracing. Did I understand this right?

Yes, but do it on the vertical plane as well.

Splayed walls are reflective material

The splayed walls must have a lot of mass in order to effectively reflect lower frequencies as well.

and the back wall absorbs most of the reflected soundwaves

"absorbs" and "most" are probably the wrong terms to use here. But yes, you want mostly absorbent material (insulation) on the back wall. Some of the things this will do and achieve are:
- slow the speed of sound down a bit (increasing your ITDG)
- convert the sound energy from adiabatic to isothermal
- refract the sound

These property changes will help you achieve a diffuse sound that is later and quieter than the initial sound you hear which is the end goal.

Greg

I tried to find the correct angles for splayed walls and did ray tracing. Did I understand this right? Splayed walls are reflective material and the back wall absorbs most of the reflected soundwaves? I used this tool:

You are on the right track, yes, but leave out the back wall in that tool. You don't want to consider first reflections from the rear wall for RFZ: it just muddies up the image. RFZ is mostly about specular reflections and therefore is mostly about mids and highs, not lows. There's several reasons for that, but I don't have time to go into that: suffice it it say that, in a well designed room, there won't be much in the way of specular reflections returning from the rear wall. The rear wall is a problem for other things, such as SBIR, and modal stuff, for sure, but in a well designed room it won't be sending any major specular reflections above the Schroeder frequency, back to the mix position. So leave out the back wall in your ray-tracing, and just look at reflections from the front and side walls, ahead of the mix position, and out to about 50 or 60° (but that depends on the dispersion characteristics of your speakers, too...)

- Stuart -

Ok, here's the new version. What do you think about the measurements? Is there anything I should change? The listening position is now 43% / 128cm from the actual front wall of the room. Is it ok, or is it too close to the speakers? I didn't aim to 30° angle for the soffits, but it gives a nice RFZ zone. I'll do the vertical version after this one is ok. -Matt

Your speakers look close to the 25% width of your room position. Can you share those measurements with us? 25% is not a good place for them. They can be between 28-34%, just not 25%!

Greg

My speakers are 75cm from the side walls. Room width is 275cm. So that's 27%. In a small room like this 1% is only 2,75cm, so it's not a big difference if I change it to 28%... Stuart wrote somewhere that speaker location should be 25%-40% of room width.

I'm wondering if should I move the tip of the triangle behind my head a little bit further towards the backwall?

-Matt

Here's another version. I brought the mixing position just a little bit closer to front wall, now it's 41% of room lenght. Also some other minor changes.

Soundman2020 wrote:

It can be avoided by leaving an air gap between the wall and the bass absorber.

I guess you must have unusually high humidity where you live? But if that's the normal situation where you live, then there's no problem with that, acoustically.

It's not the humidity as such, but the extreme temperature differences. The outside air can be -40'C/-40'F on winter, so a huge bass trap leaning against the inside of a outer wall can make the wall cold behind the trap, and the vapor barrier therefore ends up being on the wrong, "cold" side. This is a problem I too am scared of, as my own build project is just a thought at the moment. I don't have any idea what would be the best and safe way to deal with this. Place the vapor barrier only behind the second, inner wall, if you build the whole house? Air gap around every panel if the outer wall is already built with the normal vapor barrier? I'll rather have more nulls versus ruin the house and health with mold in the walls!