John Sayers' Recording Studio Design Forum Archive

OVERVIEW::

So as stated in the title, I want to set up a studio in my home, intended for track creation, mixing and occasional mastering.
I'd like to get the best acoustically I can out of the space and am willing to construct whatever acoustic treatment is needed to
do so. I am not willing to make any serious renovations however (any mounting work needed is ok eg. ceiling hanging hooks, but nothing
beyond that. There are some filing cabinets and a separate area for DJ livestreams in the same room. Please see the attached plan

You can see the space here :: https://imgur.com/a/LVfq5VQ

I've attached the dimensions of the room below. The ceiling is approx 2.74 meters. To the left on the attached plan, there's a set of cavity sliding doors
and to the right a window. I have currently got my desk sitting against the window. The floor is carpeted and has a thick underlay.

My questions:

1. How much acoustic treatment would I realistically want to get and which varieties (thick bass traps, large broadband panels, diffusers etc)
2. Where would you personally place absorption and diffusion and what would your dimensions for each be?
3. Where would you place the desk in the room and why?
4. Where would your place the monitors and why?

Honestly, it's hurting my brain trying to figure out what's best to do from the advice out there, so I could really use your personalised insights as to how to go about setting everything up. If you need any further detail, i'm happy to provide!

Thanks!

Hello Tom. Welcome to the forum!

I have bad news for you.

- In order to have an amazing listening room, you need at least 20 square meters. Your room is shy of that at 18.36.

- You cannot have a great listening room that doubles as a great tracking room. You can have a mediocre version of each, but it would come with great planning and building and using variable acoustic treatment.

- You aren't willing to make major changes to the room. Without doing so, you will have what I would call a "ghetto" studio. You have carpet. If you want your room to sound remotely good, you need to yank that out and have a hard flooring material (concrete is the best, laminate, hardwood, tile, linoleum). Carpet does exactly the opposite for sound that we want to achieve.

- For a great studio environment, you'd want to have isolation (no sounds getting into your recordings or masking your listening. Also, no sound going out of your room to have the cops called on you). This cannot happen without major construction.

- For great acoustic treatment, you need to do serious construction. Now, you can get okay results with things like broadband panels and somewhat anchoring up super chunk bass traps, but it'll never be amazing.

To more directly answer you question:

- Your room is too small for any diffusion. Period.

- Super chunk bass traps is what you want.

- I like where you have your desk oriented. You need your speakers on sturdy stands pushed up agains 4" thick of insulation.

- You need broadband panels at your first reflection points.

- You need a cloud, hard backed, 4" insulation on the bottom, stuff a bunch of insulation above it. Angle it towards the back of the room.

- The back wall of your room needs a ton of insulation, but that probably can't happen with your room lay out.

Greg

Hi Greg, thank you for the super detailed response!

Honestly, tracking is not a concern for me. There'll probably be vocal tracking here and there down the line, but for the moment, it's not a priority to have a great sounding tracking room and i'd record elsewhere if needed. The focus is on mixing and occasional mastering. I'm more of a synths, drum machines and software guy.

Regarding the super chunk traps, go as thick as you can? From my understanding, the thicker, the lower the frequency range the absorption covers.
Also, with regards to the cloud, i'm a bit unclear on your description. You say 'stuff a bunch of insulation above it'... I'm unfamiliar with cloud absorbers. Do they differ much from broadband absorbers (you said they'd be hard backed but then also with insulation above)? I ask asI'm building my own panels.

Thanks again

Hi Tom, and Welcome to the forum!

You can see the space here :

Please check the rules for posting... There's a reason for the rule about taking the time to upload your images directly onto the forum: If you look around at design threads and build threads posted by members, you will often come across threads that sound fascinating, just the thing that you need to build your own place better, but are actually useless because the images under discussion are gone... the website where they were linked to changed name, or got reorganized, or went out of business, or the user's contract expired, or whatever. A thread where people are talking about things in the photos, but there's no photos, is a dead, useless, pointless thread, and very frustrating for others who come along years later and want to learn. It also makes things a lot easier if the images are right there, in line with your own comments, so we don't have to g flipping around between websites to see what you are talking about! So please take the time to upload the pics of your room.

I am not willing to make any serious renovations however (any mounting work needed is ok eg. ceiling hanging hooks, but nothing
beyond that.

OK; so isolating your room is out of the question, then. Zero chance that you can stop unwanted sounds getting in, or your own sounds getting out. The way it is right now is as good as it gets: you cannot get any more isolation ("soundproofing") than you already have. I assume you are OK with that?

I've attached the dimensions of the room below.

So 5.1m long, 3.6m wide, and 2.74m high? That's about 18 m2, and 50m3. As Greg mentioned, the minimum recommended floor area for a critical listening room is 20m2 (according to various specs, such ITU BS.1116-3, for example). At 18m2 you are a little short on that, but that doesn't mean your room is unusable! It just means it will be harder to to treat than a larger room, and cannot be quite as good, acoustically. On the other hand, you do have a nice high ceiling, and your room volume is over 50 m3, where the minimum recommended is about 47 m3, so that helps a bit. Your room can be quite decent. Certainly good for normal mixing, but probably not for precision mastering.

I have currently got my desk sitting against the window.

The orientation is correct, but the desk should not be tight against that wall. The desk should be located comfortably in front of the listening position (the place where your head will be while mixing), which must be about 185 cm from that window. The SPEAKERS will go up tight against the wall/window, yes, but not the desk. You should start by getting the room geometry correct, then get the treatment correct.

The floor is carpeted and has a thick underlay.

Rip it out. That's the very first thing that has to go, if you want that room to ever be usable as for mixing, and even more so for mastering. Carpet is a pretty good way of trashing your room acoustics, since it does the exact opposite of what small rooms need.

Carpet absorbs high frequencies very well, mids to a certain extent but randomly and rising with respect to frequency, then does absolutely nothing at all to low frequencies. That's the total opposite of what a small room needs. All small rooms need huge amounts of low frequency absorption, some in the mid-range but decreasing as frequency rises, and with little to none in the high end. Carpet makes your room sound dull, boomy, thuddy, muddy, lifeless, etc.

Secondly, it is on the floor (duh!), which means it destroys the reflections from the floor that your brain relies on to build an "acoustic picture" of the room. All your life, wherever you go, your ears are exactly the same height above the floor that you are standing on, and your brain is very, very accustomed to figuring out the acoustic signature of the room based on the reflections it hears from the floor. If you sit down, your brain recognizes that, and adjusts it's "image" of the room accordingly. It does not use the ceiling or the walls for that, because the distance from your ears to the walls and ceiling changes all the time, many times per second as you walk around and move from room to room, indoors, outdoors etc, so the "signature" is not constant or consistent. Ceilings are different heights, and when you walk outdoors, there is no ceiling at all! But there is still a floor, and it is still the same distance from your ears as every other floor.

If you have carpet on the floor, your brain no longer has any reflections to use for this, so your ability to judge frequencies, directions, and space is greatly deteriorated.

So forget the carpet. You'll find it really hard to have a good acoustic setup in a room with thick carpet on the floor. It messes up your psycho-acoustic perception of the speaker locations, as well as your ability to determine directionality, so you'll never get an accurate sense of the real sound-stage, and never have an accurate stereo image. Carpet is pretty good at messing with spatial perception.

Have you ever noticed that world-class control rooms practically never have carpet on the floors in the front half of the room? And ditto for professional studio live rooms / rehearsal rooms? Never any carpet on the floors. There's a reason for that. if you want your room to be the best it can be, do what the pros do, and get rid of it.

1. How much acoustic treatment would I realistically want to get and which varieties (thick bass traps, large broadband panels, diffusers etc)

In theory, you will need about 420 sabins of absorption in that room, but that doesn't tell you much, because it's not just about the TOTAL absorption: it's about WHERE it is located in the room. For example, if you put that all on the side walls and the middle of the ceiling, the room would still be terrible: the treatment must go in the right place.

In addition, as Greg pointed out, your room is too small to be able to use any type of numeric-sequence based diffuser (eg, Schroeder, QRD, PRD, BAD, etc.) You might be able to use some non-tuned diffusion, such as polys for example, but nothing tuned.

2. Where would you personally place absorption and diffusion and what would your dimensions for each be?

Impossible to say for certain without testing the room. Here's how: http://www.johnlsayers.com/phpBB2/viewt ... =3&t=21122

That said, what you WILL need without any question, is abundant bass trapping, which is most effective n the room corners. But how MUCH of that you need, and the DIMENSIONS you will need, and the LOCATIONS where it needs to be, are best determined by actually testing.

3. Where would you place the desk in the room and why?

This is starting to sound like an acoustics finals exam, for Acoustics 101!
There is a theoretical optimum location in the room for your ears, and that is 38% of the room depth (distance from front wall to back wall). That's the point where the modal response of the room is the least unfavorable. It is unfavorable EVERYWHERE in a small room, but 38% is where it is the LEAST unfavorable. However, way too may people take that number as "written in stone", and that simply isn't true. It is NOT a rule that must be obeyed religiously! It is a starting point. There are many reasons why there might be a better location a few percentage points further forward or backward, and there are things you can do with treatment to move that spot. As long as it is not at 50% and more, or 25% and less, and is always symmetrical left-to-right, then you should have reasonably good modal response characteristics. When I'0m designing rooms, I generally try to keep it in the range 33% to 42%, and certainly not outside 29% to 46%.

All of that is for a normal basic rectangular shaped room. Different "rules of thumb" apply to other room shapes.

4. Where would your place the monitors and why?

Am I getting a passing grade on my exam paper so far?!

It's a small room so the speakers absolutely must go up against the front wall. The reason is SBIR (look it up on the forum). SBIR is directly related to the distance between the wall and the speaker. The further you move the speaker away from the wall, the lower down the spectrum the SBIR dip moves. I the room is large enough, you can move the speakers so far away from the front wall that the first SBIR dip is off the bottom of the scale, below the cut-off frequency for those speakers,and therefore it is not an issue. But that means a distance of at least 3m from the speakers to the front wall, and considering the RFZ guidelines, your room would need to be about 15m long at least to be able to do that. In small rooms, it is impossible to get enough distance between the speaker and the front wall. And considering the wavelength of that lowest SBIR dip, it is practically impossible to treat it effectively with absorption or diffusion for the typical distances that are possible in a small room. Therefore, the ONLY other option is to put the speakers as close as you can get them to the front wall. That forces the lowest SBIR dip up into the mid range, where it CAN be treated with absorption.

Thus, my recommendation earlier that hte speakers need to go up tight against the front wall / window, with only a 10cm gap, where you will insert a 10cm thick panel of OC-703 or OC-701, which do have good absorptive characteristics at the frequency where that first SBIR dip will be.

How am I doing? Did I pass my exam?

Honestly, it's hurting my brain trying to figure out what's best to do from the advice out there,

Very true! There's such a huge amount of pure garbage about acoustics floating around the internet. Some of it is just ignorance and myth, but well-meaning (see all the "how I built my amazing studio" videos on YouTube, for example: probably 80% are rubbish, but not intentionally). But there's also a lot of snake-oil acoustic salesmen out there, with their weird theories, who want to sell you their unbelievable products at unbelievable prices, based around exotic materials and even more exotic claims... Simple truth is, there are no magical materials, or magical ways of putting materials together, or magical dimensions, or magical incantations. All products and all materials obey the laws of physics. There's no getting around that, no matter how much pixie dust, carbon powder, or unicorn hair they put into it, nor how cubicly cuboid your dimensions are. Everything obeys the laws of physics, and acoustics is a sub-set of physics. The ways that sound waves interact with objects and materials is rather well understood today, so as long as you build your devices based on the equations that have been derived to explain how sound works, and using materials whose properties are known and have been tested and quantified, then you will get good results. That's what we do on the forum: guide you through the process of applying empirical acoustic knowledge to YOUR studio. Here's an example of what happens when you do it right: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=20471 . And here's an example of how a room can be tuned, using this same procedure involving equations, correct use of materials, correct techniques, and proper acoustic measurement: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21368 That one is in progress right now: it's the final tuning of a control room, showing how all of this comes together in practice, when done right. Your room could end up similar to these, if done right.

As you can see from those two, and from all the many, many other threads on this forum, as well as all the many, many rooms that John has designed over the years, you really are in the right place here. And I promise we won't be telling you to use pixie dust, unicorn hair, speaker cables that cost US$ 5,000 each, space crystals, weird dimensions, or any other magical, mythical, mystical method or material. Here you will find plain old science, and plain old empirical designs that just work, and have been proven to work, countless times. You are in the right place here.

Honestly, tracking is not a concern for me.

Cool! So design it to meet ITU BS.1116-3. chapters 7 and 8, and you will be fine!

The focus is on mixing and occasional mastering.

Your room can probably work reasonably well for mixing, but maybe not so much for high-end mastering, first because of the size and secondly because you have placed limits on what you are prepared to do. That also places limits on how good it can be, acoustically (as Greg pointed out), and the limits you are imposing on the room prevent it from being great for mastering.

Regarding the super chunk traps, go as thick as you can? From my understanding, the thicker, the lower the frequency range the absorption covers.

Basically, yes, but you won't know how low you need to go until you measure! And the bigger you make the bass traps, the more they will suck out the high en, which you do NOT want to happen, so you will need to deal with that in your bass trap design: you want to ensure that the bass traps only trap bass, without messing with the mids and highs. So it's not just a matter of making ginormous superchunks in all room corners. If you did that, yes, you'd have great bass, but terrible mids and highs. The room would be dead, and useless.

Also, with regards to the cloud, i'm a bit unclear on your description. You say 'stuff a bunch of insulation above it'... I'm unfamiliar with cloud absorbers. Do they differ much from broadband absorbers (you said they'd be hard backed but then also with insulation above)? I ask asI'm building my own panels.

Take a careful look at the ceiling in that thread I linked you to above, for Studio Three. That large maroon colored "thing" is the cloud. In that room, it is made up from several segments, each built differently, and each hung at a specific angle. Some segments are hard-backed, some are not. Some have masses of insinuation above them, some don't. In any room, the cloud can be designed to perform just one specific purpose, or several purposes once. The cloud segments in that room do several things, such as creating the reflection free zone around the mix position in the vertical plane, blurring the modal issues in the vertical plane, providing deep bass trapping, providing the needed degree of absorption at the needed frequencies, while not damaging the high-end, and also looking nice! You can design yours to do as much or as little as you need. In the other thread I linked you too, the cloud is not in yet: In the the corner control room we will do that as pretty much the last "cherry on the cake" treatment, and I will re-tune the cloud design for him just prior to him building it. We will likely also adjust the angle of the cloud, based on the acoustic measurements after he initially hangs it. He will finish the speaker soffits first, then the side wall slot resonators, and the cloud will be the last bit of physical acoustic treatment, prior to the digital tuning (which can ONLY be successful if the physical treatment is already maxed out).

There's a process here, a sequence that you need to follow in the design, construction, and treatment of your room. It's a slow process, and it needs to be done one bit at a time, without jumping steps, or taking them out of order. That order can be different for different rooms (the cloud went in early at Studio Three, for example, but will be right at the end in the corner control room studio) for many rooms, and part of the design process is understanding why that might be, and when to change it. Studio design isn't just throwing a bunch of stuff on the walls and ceiling, and certainly not taking the "one size fits all" approach that some manufacturers want to fool you into following! Every room is different, each room needs to be analyzed properly to determine what the best sequence and treatment would be. If you only want a mediocre room that s sort-of OK for fooling around a bit, then you can get by with just putting up a few bits and pieces. But if you want a serious mixing room, it's going to need more effort, and if you want a serious mastering room, that's another thing entirely: it demands precision design, precision construction, and precision tuning. It all depends on you.


- Stuart -

Hi Stuart, thank you so much for taking the time to write such a comprehensive response! Wow, you guys are dedicated

To specifically address to your responses:

- Apologies for the image being hosted elsewhere. I had an issue getting it to upload. It's on my personal IMGUR account, so it shouldn't be going anywhere

- No chance of further modification to the room construction to increase isolation sorry. I know that the existing design features extra soundproofing in the walls and heavier cavity doors intended to keep the media room more isolated as is.

- 'Precision mastering' is not really in the scope of what i'll be trying to achieve in the space. I generally get my mastering done externally.

- This is one of the the issues I ran up against when positioning when reading advice online...

Placing the speakers against the wall on stands and having the desk so my listening position would be at 38% of the room length seemed like my monitors would be too far away from both my listening position and each other. Going by the standard '60 degrees' 'equilateral triangle' guidelines, you'd think that your stereo image would be too stretched out with your monitors that far apart. I didn't want to come too much closer to the wall with the desk however as that puts the listening position at around 25% of the room length away (also bad). You can see how I was getting frustrated

- While getting rid of the carpet isn't an option for me, would it serve the same purpose to get some more reflective floor material and cover that half of the room? That I could realistically do.

1 - This is the product I was planning on using for building the traps and broadband absorbers (either the HD or XHD 75 but it seems like the XHD has negligable benefits for a significant price bump) https://www.bradfordinsulation.com.au/- ... 60C5C0CABB

2 - Will look into testing, thanks for the link

3 - I'll do some more experimenting between 33% and 42%. As I said above, it did seem like the monitors would be too far apart, but i'm probably wrong.

4 - Should there be an air gap behind the absorber panel? So it'd go; back wall > 10cm air gap > broadband absorber > desk > listening position (33-42% of room length from back wall)?

It's a relief to have such a no-nonsense knowledge base at my disposal I must say! I'll be reading your resources on measurement and i'll let you know how I get on. Hopefully I can get the absolute best out of the space

Placing the speakers against the wall on stands and having the desk so my listening position would be at 38% of the room length seemed like my monitors would be too far away from both my listening position and each other. Going by the standard '60 degrees' 'equilateral triangle' guidelines, you'd think that your stereo image would be too stretched out with your monitors that far apart. I didn't want to come too much closer to the wall with the desk however as that puts the listening position at around 25% of the room length away (also bad). You can see how I was getting frustrate

Yup! It sure can be confusing, with all that hype, myth, ignorance and misinformation out there on the internet.

Just so you feel a bit more comfortable, let me go into a bit more detail.

First, the famous "38% rule" is not a rule at all. Wes is sorry he ever even put that out in public, as so many have misunderstood it completely, and don't even understand what he was saying anyway! Simply put: all rooms have "modes", which are the frequencies where standing waves form, due to the dimensions of the room. You can't get rid of them, and even if you could, doing so would be a BAD idea. The problem is that there are not enough of them in the low end. Plenty in the mid range, and abundantly plenty in the high end, but few and far between in the low end. Wes did some math, noting where room modes always form in rooms as percentage of the length, and came to the conclusion that the location in the room where modes are theoretically at the least noticeable, is 38%. That's all. The modes are still there! They don't go away at 38%. They are still very active all over the room, just a little less so at 38%. The problem is, some people don't get it at all, and go nuts trying to get things to work out so their ear holes will be exactly 38.00000% of the room depth, accurate to the nearest nanometer... without realizing that if they happen to lean forward just a bit in their chair, they will already be waaaayyyy of from 38%. Lean forward 6 inches in a room 10 feet long, and you went from 38% to 33%! Lean back 6 inches, and you went from 38% to 43%! Oh no! It's the end of the world! (not.) So unless those misguided folks have their heads clamped in a steel vice, they will NEVER be at 38% of the room depth. And would never even notice, in any case! When was the last time you were mixing, leaned forward a bit in your chair, and said "Damn! I can hear that darned 91.38 Hz modal peak here! I better lean back a quarter inch again"....

So no, you don't need to be at 38%. 33 to 42 is fine, and even outside of that a bit can still be acceptable. In fact, most engineers feel that the room sounds better when they are a bit in front of that supposedly perfect 38% location closer to 34% seems to be preferred... So 38% is a good starting point, but that's all. There are many reasons why it might be a good idea to be several % points away from there, as long as you avoid being close to 25% or 50%. Those really are bad spots.

Second, the famous "60° equilateral triangle" is also not written in stone. Yes, you see it in nearly every speaker manual ever published, but it's only there because it's a good starting point, and it works in the majority of rooms. That doesn't mean that it is the BEST layout: just that it works. They have to put something in the manuals, so it's a safe bet to put that, as it nearly always gives acceptable results. Not necessarily great, but at least reasonable. However, there is no psycho-acoustic reason why the speakers should be 30° off from your ears to the left and right! In fact, human hearing is most sensitive about 70° azimuth at high frequencies, and about 45° off for mids (lows are not so directional). Overall, best sensitivity is around 50°-55°. (which is why a crossed stereo pair mic setup sounds pretty darn convincing in the mix, as each mic is pointed 45° off axis, very similar to our ears... and if you set them at 110°, instead of 90°, then it gets even better... ).

Here's the actual pattern of hearing sensitivity for the average human ear (right ear in this case): That's called the "Head Related Transfer Function" or HRTF, if you are interested.

Of course, that's for each individual ear: when your brain combines the "images" from both ears, you get a roughly omni-directional result.

So, there's no biological reason to have the speakers 30° off azimuth. That's not the peak for our ears. However, you also don't want to go to 90°, because then your head itself is masking each ear from the speaker on the other side, which creates a low-pass filter starting at about 800 Hz or so for sound coming from the other side, and also messes up other methods that your brain uses for determining direction and frequency (such as time delays and phase shifts). You do want each ear to hear part of the signal coming from the opposite speaker. But it's also a bad thing if you have the speakers right in front of you, next to each other, where both ears hear both speakers equally, and there is no shadowing: that doesn't give your brain ENOUGH clues to work with. It turns out that any angle between about 20° azimuth and 50° azimuth is acceptable, but best is the range 25° to 45°. Interestingly, 30° is NOT in the middle of that range: 35° would be optimum, mathematically... So much for equilateral triangles...

Summary: it is fine to have your speakers anywhere in that range: 28° to 40° is really good, but you can go down to 25° or up to 45° if you need to. 29° to 38° works very well in most rooms. Don't get stuck on nailing that exact 30° angle!

Yes, there are consequences: changing the angle does, indeed, change the stereo imaging and sound stage, and having the speakers at a higher angle will, indeed, increase the width of the sound stage and exaggerate the stereo imaging. But that's actually a GOOD thing, as it improves your ability to resolve directions and positions on the sound stage! So if you are into panning things here and there in your mixes to make them more interesting, then you can do so much more precisely if your speaker angle is greater than 30° azimuth. It also extends the sweet spot in the front-to-back direction, which is good if you do a lot of leaning forward and backward as you mix, but it does "squish up" the sweet spot from side to side... however, that usually isn't a problem unless you have a very large console and need to lean (or roll your chair) a lot to the left or right as you mix. But in most small rooms with DAWs or compact consoles, that isn't an issue. You tend to sit on the center line, and lean forwards/backwards, but not side-to-side. So once again, having the speakers further apart and more angled, is a good thing.

In other words, it is perfectly fine to ignore that pretty triangle that looks so cool in your speaker's manual, and you are allowed to change the angles as necessary. I promise you, you will not be arrested by the Speaker Angle Police if you set yours at 37.5° Really!

However, you DO need to keep the speakers the same distance from your ears: so if the left speaker is 118 cm from your left ear, then the right speaker also needs to be 118cm from your right ear. That's important for symmetry of the stereo image and sound-stage. BUT!!!! They do NOT also need to be 118 cm apart! You can spread them further apart, or closer together, as needed. Once again, the world will not come to an end, and you won't be arrested for not having that speaker-to-speaker distance the same as the speaker-to-ear distance.

Now for the "speaker to front wall" thing. This one is real easy: It is called "SBIR", which stands for "Speaker Boundary Interference Response", and it is exactly what it sounds like: the sound wave that heads out from your speaker, hits a boundary, comes back, and interferes with itself, upsetting the frequency response. Here's how it works: at mid and high frequencies, your speakers are directional: they send out sound in a cone shape, aimed straight at you. But at low frequencies, they don't. As you move down the scale, that cone gets wider and wider, until at one point it is 180° wide. As you go even lower, the sound starts "wrapping around" the back of the speaker, and heading towards the wall behind the speaker, which is the front wall of the room. The frequency where the sound is spread out 180° is governed by one thing alone: the size of the speaker cabinet! That frequency is known as the "baffle step response critical frequency". It occurs at the point where the wavelength of the sound is the same as the width of the speaker. For very small speakers, that happens higher up the scale, and a lot of stuff "wraps around" to hit the front wall, while for really big speakers, the critical frequency is lower down the scale.

But regardless of what the frequency is, there is always a point where the speaker stops being directional and starts radiating in all directions, including towards the front wall of the room. So those waves hit the front wall, bounce back the way they came, and totally mess up the entire universe! Ok, slight exaggeration: that just messes up the frequency response in the room. Because that wave is now interfering with itself: parts of it are cancelling itself out, where it is 180° out of phase with itself, and parts of it are reinforcing itself, where it is exactly in phase with itself. So you if you play a sine sweep on your speaker, there will be some frequencies that are amplified by 6 dB (where the wave is in phase with itself) and some frequencies where there is a huge dip in intensity, theoretically infinite dB (total cancellation), but in real life more like about 30 dB. That is MASSIVE! The lowest dip in the frequency response will be at the frequency that exactly corresponds to the quarter-wave distance from the front wall, and it will be a HUGE dip. Why 1/4 wave? Because the sound went "there and back" and arrived 180° out of phase! So the total distance it traveled is one half wave, which is a quarter wavelength to hit the front wall, then a quarter wavelength to get back again.

But that's not all! That first SBIR dip is just one of MANY such dips: there will be a very large number of other dips in the frequency response, all related to that same distance, starting at twice the frequency of the first dip, three times, four times, five times, six times ... bah blah ... ten times, ... blah blah ... 50 times, etc.. all the way up to 20 kHz (and beyond). In other words, if you look at a graph of the frequency response, it will resemble a comb where each "tooth" on the comb is one of the SBIR dip frequencies, and for this reason, this issue is referred to as "comb filtering". So SBIR doesn't just affect that one single low frequency. It affects the entire spectrum, and comb filters the entire response of your speakers. Here's what a comb filter frequency response loos like:
If you don't do something about it, that's what your real frequency response will look like. Not the nice smooth, flat graph that you saw in the manual for your speaker, that convinced you to buy it!

Ok, that's the problem in theory: now for real-world implications: You can diminish the SBIR dip by using suitable absorption on the front wall, directly behind the speaker. The only problem is that the absorption has to be quite large with respect to the wavelength: at least 7% of the wavelength, and ideally more. So for low frequencies you need very, very thick insulation. The wavelength at 50Hz, for example, is about 22 feet, so the insulation would need to be nearly 2 feet thick to be effective . On the other hand, if you could arrange things so that the dip occurs at, say, 280 Hz, the wavelength is only 4 feet, and 7% of that is about three and a half inches, ... which just happens to fit a 4" thick panel of OC-703 insulation very well! And as luck would have it, the absorption coefficient of a 4" panel of OC-703 against the wall, at 280 Hz, is 1.24! Pretty amazing...

In other words, you need to get your speaker close enough to the front wall that a reasonable thickness of insulation is able to deal with the SBIR, and attenuate it. Or you need to get it far enough away from the wall, that the first SBIR dip is below the bottom of the audible spectrum. Wouldn't it be nice if somebody would do the math for us, and just publish a chart, to make it easy?

Fortunately, the great folks at Neumann (who make some amazing speakers!) did exactly that for us:
Bingo! There it is, all laid out neatly, and color-coded too! If the distance between your speaker and the wall falls in the red zone, that's really bad: major SBIR problems are all yours to keep forever, because you can't treat those with any realistic treatment, so your room is doomed. If you are in the blue zone, that's marginal. Maybe OK for a home theater or something, but not for a studio or audiophile listening room. If you are in the green zone, then you are fine. However, note that to be in the green zone, your speaker either has to be more than 3.2m away for the front wall (about 10' 5") or it has to be closer than 20 cm (about 8"). To get your speaker 3.2 m away from the front wall, and have your listening position decently far from those speakers (about 2m), and have your head about 38% from the front wall, and have an equilateral triangle, you need a very, very big room! About 14m long would be good...

The only other realistic option, is put them against the front wall.

So, that's the full story: if you want to minimize the huge, ugly room-killing SBIR monster, then put your speakers against the front wall with only a 10cm (4") gap, and put a 4" thick panel of OC-703 in that gap. (Even better is to flush-mount them, in "soffits", since that totally eliminates front wall SBIR, but that's a different story....)

Now, since your speakers MUST go against the front wall, and they must NOT be right in front of you, and must NOT be in the room corners, and they must NOT be at 25% of the room width... that doesn't leave a lot of options! They need to be some place around 29% of room width in most cases, give or take a few points. And since you also need large bass traps in the corners, your options are even more limited...

And since your head needs to be around 33% to 42% of room depth, and the speakers need to be angled about 25° to 45°, ... well, add up all of those limitations, and you can see why the only real options you have in a small room like yours, are the ones I gave you!

So there's the solid, sound, scientific acoustic reasoning why I said what I did. It is all based on well documented research papers published by recognized researchers and institutions, as well as information provided by speaker manufacturers, and well-known equations, and common sense.

( Aren't you glad you found this forum, and got a few hundred dollars worth of free consulting? )

Of course, not all of the above effects are equal! Some are more important than others. The most important of all is SBIR, right up there with bass trapping, and the least important is the 38% "rule". The others fall somewhere in between. So if you do have to compromise on these when laying out your room, then compromise on the least important ones, in favor of the more important.

There's one more thing I told you yesterday that I haven't explained: I said to aim the speakers at a spot BEHIND your head, not AT your head. The reason here is also simple: If you look at all of those famous equilateral triangle diagrams, they have a circle to represent your head, and the acoustic axes from the two speakers intersect in the middle of that circle. At first glance, that seems fine and logical. Now look again: where does the axis from each speaker arrive at your head? Yup. In the middle of your EYE, roughly... so if you get your ears surgically transplanted onto your eyeballs, then that would be fine: your ears would be on-axis to your speakers. But for the rest of us, whose ears are stuck out on the sides of our heads, our ears are OFF AXIS with that setup! You need to rotate the speakers a bit to get the axes pointing at your ears. And in reality, it turns out that your ears perform best psycho-acoustically if the axes just grazes by the edge of your pinna (that fleshy thing that sticks out on the side of your head). And to get that right, you need to aim both speakers so that the axes intersect at a spot that can be anywhere between about 12" or so and about 24" or so behind you head, depending on all the other factors mentioned above, but mostly room size and shape.

Sorry about the long rant, but I find it often helps when you understand WHY you are doing certain things, rather than just obeying blindly.

1 - This is the product I was planning on using for building the traps and broadband absorbers (either the HD or XHD 75 but it seems like the XHD has negligable benefits for a significant price bump) https://www.bradfordinsulation.com.au/- ... 60C5C0CABB

You would need either the HD100 or the XHD 100 to get close to the performance of OC-703. Both of those have GFR that is similar to 703, and either would work fine.

- While getting rid of the carpet isn't an option for me, would it serve the same purpose to get some more reflective floor material and cover that half of the room? That I could realistically do.

Certainly! That will help a lot. For example, put down some 3/4" plywood, OSB or MDF ove the front half of the room. That will do the trick rather nicely. And if you can extend it a bit further back then half way, that's even better. If you wanted to make it look neater, then lay laminate flooring on it. That's also great, acoustically.

3 - I'll do some more experimenting between 33% and 42%. As I said above, it did seem like the monitors would be too far apart, but i'm probably wrong.

Having the monitors well separated is a good thing, not a bad thing! They have to be REALLY far apart to be bad. ITU BS.1116-3 recommends the best distance for speaker separation as being 2 to 3 m, and up to 4m is acceptable under some conditions. It also recommends that the distance from each speaker to the listening positions can be as much as 1.7 times the distance between them, and even up to 2 times that distance in special cases. Closest distance is half the speaker separation. So for example if your speakers are exactly 2m apart, the distance from each speaker to the corresponding ear could be anywhere between 1m and 4m. That blows the equilateral triangle thing completely out of the water! That's all in chapter 8 of the document, if you are interested. It also recommends that the sweet spot does not need to be wider than a a radius of about 0.7m around the nominal mix position. There's no point in trying to listen outside that circle, no matter how the speakers are arranged, or how good the room is.

4 - Should there be an air gap behind the absorber panel?

Not really. There can be a very small air gap, but don't forget that as you move the panel further away from the wall, that also means your speaker has to move further away, which moves the SBIR dip further down the scale to a more problematic frequency that needs exponentially more absorption.... Catch 22! So a gap of 1" would be fine, but a gap of 5" would push you outside the green area...

back wall > 10cm air gap > broadband absorber > desk > listening position (33-42% of room length from back wall)?

Nope. Too much air gap. Keep it to about 2 or 3cm. So it wold be:

FRONT wall > 3cm air gap > 10cm broadband absorber > speaker > desk > listening position (33-42% of room length from FRONT wall)

Note that the listening position is measured form the FRONT wall, not the BACK wall. The front wall is the one in front of you... The one you are looking at as you mix. Where the speakers are. The back wall is behind your head, far away, and you can't see it unless you turn around.

It's a relief to have such a no-nonsense knowledge base at my disposal I must say!

We think so too!

I'll be reading your resources on measurement and i'll let you know how I get on. Hopefully I can get the absolute best out of the space

I'm sure you can. Think of it this way: if you can find a dozen things to do that each improve the performance by just 1 dB (not even audible), then you improved it in total by more than ten dB! (VERY audible)


- Stuart -

Absolutely amazing response Stuart, thank you again for your time and knowledge. I'll keep you posted on how things are going with the build.

Tom