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Encyclopaedia Audiophilica I: Room Design & Treatment

Danaxus

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For the past few months I have been doing a ton of research in an effort to design an audio room for my father, as a retirement gift. This involves everything from the ground up - from designing a room in a house currently under construction, to choosing speakers, electronics, finding the ideal listening position, and managing room treatment. I'm posting the results of my findings below in the hopes that a) someone else finds them useful and b) that others here, far more knowledgeable than myself, will supplement and correct my work (seeing as I have absolutely 0 expertise, I assume A LOT of corrections will be required).

Part 1: Intro and Components​

A sound system is composed of many parts (though often many of these parts are combined into a single piece of equipment), and the best analogy I've found is to compare it to a guitar. You have the source of the sound (the musician's brain), the conversion of that source into mechanical energy using a DAC, amp, and other electronics in the chain (the musician's fingers), the speakers (guitar strings), and the room (the guitar shape and materials).

The size, shape, and material of a room, just as with a guitar, flute, or any other instrument, vitally impact the sound you end up hearing. Just as the same set of strings will sound different on different guitars, so too will speakers sound different in different rooms. And as with guitar strings, speakers are merely a component of the system as a whole, not the be-all, end-all. Getting the room right, as well as treating and caring for your existing room, is paramount when trying to optimise your listening experience. The best strings in the world won't make a ****** guitar sound good, nor can the best guitar in the world sound good using ****** strings. Just as with the analogy, if any single one of your components is bad, it'll make the entire system sound bad (a musical who doesn't write good music, or can't play for shit, using old frayed strings, or playing on a dollar-store guitar). Similarly, improving any one component will elevate the entire system (a pro musician can play great music on a bad guitar, but whilst impressed, you'll still be wishing he had at least something semi-decent).

Hierarchy - Long Live Digital​

This little subsection will make me many enemies. Thankfully I don't live in the US, so I can't be swatted, doxxed in any meaningful way, and my lack of notoriety makes me immune to cancel culture. Some things in audio is subjective - but nothing digital is. Digital components work in 1's and 0's, and all digital components can be lumped into one of two categories: good, or not good. I don't mean this literally - you can have a mediocre DAC, but because good digital components are so prevalent and cheap, you either have an excellent one, or you don't. In other words, why do you have a mediocre DAC?!? Similarly no one compresses digital music to absurd degrees anymore, so any track you listen to, even if it's an mp3, should sound pretty damn good. And contrary to the preachings of many audio religions, cables either work or they don't.

Finally, know that digital lives at the top of the food chain. Digital processing can do a lot to ameliorate a bad room and/or bad speakers. But speakers and rooms can't fix broken digital (or even each other).

Part 2: The Room​

Why Does it Matter?​

The entire process of building and treating your room is for one reason: taming sound waves. Waves are undeniably awesome physical systems, but they're also little bitches that are a pain in the ass to deal with. Sometimes they coexist and ignore each other, other times they combine to form a big, loud waves, and other times the fuckers mutually annihilate each other leaving dead silence behind. You can buy two of the largest speakers known to man, point them at each other, crank the volume up to max and stand right in between them, and if you set it up just right, you will hear absolutely nothing. Because music is made up of waves, each with a different frequency, your room is like an all-out chaotic bar fight: some waves are teaming up, other waves are fighting each other, and others are drinking their beers, minding their own business.

By dealing with the room, we're trying to bring order to this chaos - we want to hear every wave clearly, with no big waves, no cancelled waves - ideally want every one of them minding their own business. This however, is no easy problem to fix. Sadly, every action you take will have unintended side effects. Break up one fight, and you'll accidentally start another in a different part of the room. So know this: the perfect room doesn't exist, it can't ever exist, so we're in the business of mitigation and damage control, not achieving perfection.

Limitations​

You'll be disappointed to know that the room's role in audio fidelity is the most complex and least well-understood aspect of your journey. There's a lot of voodoo out there, even some involving complex math with questionable utility, disagreements between self-proclaimed experts, and no end of snake-oil salesmen. I hope someone far more qualified than I shows up, and is able to distill information in a way that can be comprehended without engineering degrees in construction and acoustics, but for now, the below is the best I've got for you.

When trying to build a room, there three considerations that will play a role in how it will sound: Dimensions, Materials, and Sound Proofing. I have zero interest in sound proofing and so it won't be covered here. That leaves two, and we will be manipulating them as much as possible to achieve our goal of optimising for neutral sound.

Dimensions​

Perhaps the most difficult and least understood part of the entire system. But first a primer on why dimensions are important. A room, just like a guitar, is an instrument. Sound waves bounce around inside it, slosh around together, ultimately producing a unique cocktail which is what you end up hearing. And like cocktails, you're aiming for a specific flavour (neutral), and you shape, treat and massage your room to get as close as possible to that result.

Room Modes vs Waves​

You know how electricity and magnetism appear like two different phenomena, even though it's the same thing? If not, don't worry! Whilst low frequency (roughly 200Hz and below), are kinda different than anything higher frequency, They are essentially the same thing and pretty simple to understand with an analogy. Think of low frequency (deep bass notes), as being physically big. If you could spray sound waves with paint and turned on your speakers, a low frequency wave would either move all the air in the room at once, or be a gigantic fat vertical line a third, or even half the size of your room, either not moving at all, or moving back and forth very predictably. High frequency waves on the other hand, are small and agile. They're like pellets being sprayed out of your speakers, and they ricochet around the room smashing into walls and each other.

This analogy by the way, is completely wrong and doesn't hold up to scrutiny, but thinking of them this way is perfect for managing our rooms. If a big wave is just standing still in your room, then you'll hear it if you sit in it, and won't hear it if you don't. This leads to the weird situations as I mentioned earlier - you can play a deep note so loud it makes the windows rattle, but stand in the right place and it'll be so quiet you could hear a pin drop.

Consequences​

Sound waves as we've established, bounce around the room, and as we've said, their collisions can be bad news for us. If they combine they can amplify and sound a lot louder than they're supposed to, or they can cancel each other out and you end up hearing silence instead of that particular note. If you're an amateur like me, it might be hard to tell when you're hearing this; after all, the music doesn't suddenly go silent, only bits of it. You can still hear the fat lady sing, but her voice is distorted. You still hear the guitar string, but it's not vibrating the way it's supposed to - certain notes are too shrill, or too muffled. Your brain will do a lot to compensate for this, but hearing it played properly is like having the wool pulled from your eyes - suddenly everything is the right colour and clear - as it should be - and you didn't know it til you heard it.

Room Shape​

How projectiles or sound waves bounce around a room depends on its shape. You can plug the room shape into computer software, and it will calculate how all the pellets and waves will move around, precisely where they'll collide, and so, where they'll amplify and where they'll cancel. Math likes nice simple geometric shapes however, and in particular, nice straight lines. So if you build a nice rectangular room with 4 corners, you can predict before you start (in principle), if and where there is a good place to sit where all the sound waves are behaving nicely. If you have curved walls, walls which aren't at right angles, lots of corners, recesses, etc, then your model gets way more complicated.

I have good news and bad news now:
Good news: we don't care too much about the high frequency pellets. Because they are weak we can easily stop them, divert them, or otherwise control them. To do so we need to treat our walls, but it's a relatively simple affair. Still, you want your room to be nice and symmetrical, because we want sound reaching both our ears from all speakers to be even. We can manipulate them in a non-symmetrical room and get them to be even, but you'll need a lot of room treatment to do so, which will be ugly, take up space, and be expensive. Symmetry is good.

Bad news: Those truck-live waves? Yeah, good luck stopping those, your only practical option is to avoid them entirely, and this is where things start to get complicated, or at least, where my research hit its limits. As far as I can tell, there are three schools of thoughts:

School 1: Rectangular Room and Bolt Diagram​

I don't understand the reasoning, I haven't read the papers, but apparently a lot of people subscribe to the notion that there is an idea ratio of room width/length/height, that spreads out room modes in a favourable way. You can use the following calculator to see how your room fares (the values entered are for the room I'm building):
https://amcoustics.com/tools/amroc?l=680&w=410&h=290&r60=0.6

From what I've read, you want to achieve two things:
1) In the Bonello box (modes per third), you want the number to increase steadily, and not to stay flat or descend. I don't know why.
2) In the Bolt-area box, you want to be...well, in the bolt area. Again, not sure why.

I believe it has something to do with not wanting to have modes that are close in frequency. Perhaps because they're more likely to interfere, but honestly I have no clue.
The other number you care about is the Schoeder Frequency. Below that all frequencies are modes. Above that, they're pellets (and I suppose that means they're more easily dispersed with treatment).

In any case the logic behind this school of thought is that you can design your room to have ideal ratios, know your modes in advance as well as where they occur, and know exactly how and where to treat them.

Big Fat Caveat​

I've also read that Bolt is inaccurate and not even useful as a general guideline. @Avare on Gearspace was kind enough to point me to the following paper:
https://www.researchgate.net/publication/28578741_Room_sizing_and_optimization_at_low_frequencies
The TL;DR of it is that Bolt's calculations aren't up to snuff, but they've developed and tested better calculations. The calculations afaik have not been made available to the public, but in the paper they provide a list of ideal room dimensions for 50, 100, and 200 m3. Unfortunately they all call for really low ceilings or stupidly large rooms. Basically unusable dimensions for any real room.

School 2: Narrow at Front Wall, Wide at the Back, no Parallel Walls​

Basically if the front wall is a square or rectangle, then all angles leading away from it are over 90 degrees. Think of a square cone shaped room. One acoustics treatment business told me the side walls (usually parallel), should be moving apart at 10 or more degrees, ideally the ceiling should sloped too. I believe the reasoning behind this school of thought is that a sound wave requires far more bounces before it can interfere with itself, thereby weakening any modes, but again, I don't know, just postulating here. I've seen absolutely no scientific evidence for this, just the word of several acoustic experts, the most prominent of which is Ethan Winer:
http://ethanwiner.com/acoustics.html

Big Fat Caveats​

If it turns out the Bolt calculations are accurate, then by opting for a non-rectangular room, you are losing track of any room modes that exist. Basically you won't know where they are until you've actually built the room and gone in with a measuring microphone, therefore far too late to do anything about them if your only good listening position is dangling from the ceiling behind the speakers.

School 3: Hire an Expert to Design your Room from Scratch​

There are several purported experts. They charge stupid-money, come in and do massive amounts of treatment, and many make you sign an NDA that prohibits you from posting measurements. I've read many accounts of customers vouching for such solutions, but the complete lack of transparency and absence of any scientific rigour, makes me think it's a bunch of horseshit sceptical.

Conclusion: Winner - School 1!​

I personally opted for School 1 for my own room (still under construction). The reasons are thus:
Rectangular rooms are by far the most common of all room shapes, and therefore the most studied. In my case, a rectangular room was the most convenient - all other options would require sacrificing room volume. There is unanimous consent as to how to treat rectangular rooms (explained below). Finally, I don't know if DSP can handle all room shapes equally as well, but I do know they can definitely handle a simple rectangle.

In essence, a rectangle may or may not be the ideal room shape, but once it's built and it's time to move on to the next steps (speaker placement and room treatment), you really want to have a rectangle. Once I decided that rectangle was the way to go, I changed the room layouts to ensure it landed in the bolt area, because why not?

Finally, one thing almost everyone agreed on was that you can never have too much height, so if you can raise the ceiling or lower the floor, it's something really worth looking at.

Room Treatment​

As you can intuit, big truck-like waves need big truck-like wall treatment to counter. Putting up a puny little egg carton will no more stop a room mode, than it will a car. On the other hand, zippy little high frequency pellets may look scary as they zip around every which way, but can be stopped by basically anything - a blanket, some furniture (but no, not an egg carton either). As with everything else, there are some aspects of room treatment that are universally agreed upon, and other things which seem to be contested. I will attempt to clearly delineate between the two here.

Room treatment confers many benefits to a room, most of which cannot be combatted digitally using EQ:
  1. Tame as much of those freaky room modes as possible. As far as I understand, elimination is impossible below 200ish Hz, instead, you try to control them (I guess this means you kill them after a few bounces by sucking energy out of them after each bounce, rather than let them bounce from wall to wall for ages).
  2. Tame the high frequencies. Left to their own devices, they will bounce around causing little echoes called flutters. You don't want some frequencies bouncing around, whilst others don't. Your mind can filter out a lot of echo, but when it's unevenly distributed along the frequency spectrum, you're asking your brain to do a lot of work.
  3. Prevent sound waves interfering with each other, causing comb filtering (another way of expressing destructive/constructive interference)
  4. Prevent sound reflected from the wall from reaching you at the wrong time. If you hear a sound from the speaker, and then the same sound reflected off a wall, the timing between those two is important to the listening experience. According to GIK (https://gikacoustics.com/early-reflection-points), if you hear a reflection less than 20ms after the direct sound from the speakers, your brain won't register it as a reflection, won't filter it out for you, and therefore you'll perceive a distorted sound.
  5. Manage the reverberation time of certain frequencies. RT60 is a metric best used in large spaces (concert halls?). REW says a different metric called TOPT is best for listening rooms in homes. What TOPT is, what it measures and why it matters, is something I don't understand. All I know is something from 0.2-0.6 is good, but I couldn't tell you why.
  6. Ensure the sound reaching your left ear matches the sound reaching your right. Audiophiles talk a lot about "soundstage". They use words like "depth", "width", and "separation". To the best of my understand, all of this is related to how sound from each speaker, reaches your left and right ears. I postulate that ensuring the room symmetrically reflects sound, especially the first reflection points between you and the speaker, play a crucial role in this. As far as I understand, room correction software can do a better job at fixing this than treatment can, but getting things as right as possible using treatment can only help.
Below I'll examine the types of treatment and how to apply them:

Absorption​

The goal of this treatment is to kill as much sound reflection as possible, in as wide a frequency range as possible. Thin absorption will kill high frequencies, thick absorption which looks way uglier, will kill deeper frequencies. Nothing will kill low bass though - if something is blasting 20-40Hz around the room, nothing short of knocking down the wall and letting it escape into the world will get rid of it. Even then, there's a good chance it'll circle around the globe and come back to wake you up a few hours later.

Absorption is best used to tame room modes, and to ensure you don't hear wall reflections too early. As such there are three locations it is best used for:
  • Behind the speakers. High frequencies get shot out forwards from the speakers, but low frequencies are omni-directional. Apparently they can "wrap-around" the speaker enclosure and go every which way. I don't understand the physics of it, I studied genetics not acoustics or fluid dynamics. In any case, since we want to tackle those pesky low frequency room modes, this seems like the best place to start.
  • In every corner of the room. For more physics reasons I don't understand, room modes LOVE corners! I keep reading all over, that if you want to experience room mode pressure, play something low and go sit in a corner. Honestly I find it a bit disconcerting that an inanimate property of air has a seemingly conscious preference for room corners, especially as I too, being an introvert, also have a preference for corners. In any case, be it for acoustical reasons, or because a sound wave with conscious intelligence poses a threat to humanity, these corner-loving waves need to be killed with prejudice.
  • At the first reflection points. All sound waves, from low modes to high pellets, love these places. Sound that reaches you from here will reach you fast, shorter than the aforementioned 20ms. Therefore, all sound from these locations must be killed. First reflection points are easy to spot as the sound waves bounce much the same as light does. I constantly read about people putting mirrors on the walls and wherever they see a speaker, they throw up absorption. You need two sets of absorption per wall, as both speakers reflect off both walls (left speaker left wall, right speaker left wall, left speaker right wall, right speaker right wall).
    • The ceiling is also a first reflection point! You want absorption up there too
    • Somewhat surprisingly, the floor can be ignored to the best of my understand. The two reasons I've read for this are: 1) The brain is used to hearing floor reflections as the ground exists everywhere so it would be weird to have it be an acoustic void, and 2) Even if it is a problem, what are you going to do about it? Putting thick treatment on the floor between you and the speaker will not only be inconvenient, but it will look silly too.

Diffusion​

Diffusion's purpose is to take a sound wave, and scatter it in all directions (either horizontally, or in both axes depending on the panel type). The way I imagine this is if a pellet hits the wall and explodes into a bunch of smaller pellets, each flying off randomly. As far as I can tell, this serves two purposes:
  • Prevents wave interference by spreading things homogeneously around the room
  • Creates an even reverberation which is acoustically pleasant. It also prevents flutters, and uneven reverb (some frequencies reverb, but others, typically high frequencies, are absorbed, leading to a phenomenon people call a "dead" room).
Diffusion panels are often cleverly placed on top of absorption panels, giving you a two-for-one combo of killing room modes whilst also treating the high frequencies as mentioned above.

This is where what objectivity I strove to present, ends, and things get messy. Where and why diffusion should be used, is not something I understand. I've been in touch with GIK about treating the room I'm building, and as far as I can tell, they just haphazardly threw it into the design wherever it fit. My best guess is that diffusion makes the room sound good, and you can't really have too much of it. It probably helps, and doesn't seem to hurt.

Finally, what is the difference between horizontal and omni-directional diffusion? As far as I can tell, omni-directional diffusion (the stuff that looks like pixelated mountains stuck to your wall), is useless and a waste of money. I'm guessing lot of the sound either ends up bouncing around 'til it fades away without ever reaching you, or gets absorbed by your ceiling absorption. From what I've seen, you mostly only care about high frequency sound waves at your ear level (hence why the tweeter is always at ear level), so horizontal diffusion is the only type that matters, if at all.

Helmholtz resonators​

I don't know much about these. From what I understand they are stupid expensive, stupid hard to build properly, and are the only thing known to man that can actually kill a room mode. Each one is custom built to kill a single frequency, and single frequency only. I think they also need a hole in the wall leading outside? Not sure. In any case, these seemed so extreme that I didn't look into them at all. Still, it seems they are mankind's only defence against conscious intelligent sound waves, so if you feel threatened, this may be the answer you're looking for.

Part 3: Measuring the Room​

Once you've built the room, placed your essential room treatment (absorption as mentioned above), it's finally time to measure your room. I've got good new and bad news for you folks:
Good news is that an excellent tutorial for this already exists here:
Room Measurement Tutorial for Dummies Part 1
Room Measurement Tutorial for Dummies Part 2

Bad news is that it's not finished, and we've been waiting for part 3 for over 6 years now. I think @Amir might be a Half Life fan.
 
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kemmler3D

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Hey there, I highly support the "learn by teaching" method, and so this seems like it's very worthwhile for you.

I think you're not getting replies because 1) this post is super-long, (kudos for writing it, but all attention spans have limits) and 2) the average poster here is fairly sophisticated and has probably come across this information before.

I skimmed it and I noticed a couple alarming things.

1) You have the chance to purpose-build a listening room, but you're not considering soundproofing. :eek:
2) You are purpose-building a listening room, but you're not seriously considering non-rectangular shapes. Perhaps the contractor ruled out non-rectangular shapes, but if not, please have another look at that.

Rectangular rooms are considered suboptimal because flat surfaces facing each other tend to create more severe standing waves. All shapes end up with standing waves of some kind or another, but rectangular spaces are (IMO) worst for that. Selecting dimensions with the right ratios can go a long way, but alternative shapes or even building the room with some inbuilt diffusors or architectural bass traps could help. Selecting materials and locations of windows can really help. And how you insulate the room (or don't) and where, etc. This is actually a really deep topic and I think you're doing yourself a disservice by doing a general audio overview rather than going deep on room acoustics.

If I had the chance to build a room from scratch, there wouldn't be any rectangular surfaces, and the walls might be something like you see here (tuned for a certain amount of high frequency absorption from the beginning - ignore the hipster-chic sound system)
OJAS_EDITORIAL.jpg


Actually, check out materials on designing mixing / recording studios too. Not the same application but the same principles apply.

I used to sell acoustic foam for a living, and based on what I sold to people, and what kinds of problems they had, I can tell you that you are MUCH better off getting this right the first time. Not only are post-construction solutions expensive, they are generally bulky and ugly, usually all three.

I think if you're investing $XX,000 in this or more, you owe it to yourself to get an introductory textbook on acoustics and go a bit deeper on that topic. You only get one chance to build the room right.

I would at least start here: https://ethanwiner.com/basstrap.html Ethan Winer has written a lot about home studio / listening acoustics that's understandable and pretty practical.

Edit: I am a victim of my own skimming, you already checked out Ethan Winer's stuff. I can at least say that IMO he knows what he's talking about and IIRC his views are well in line with my understanding of physics / acoustics.
 
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kemmler3D

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Oh, and unless your dad will be the sole occupant of the house at all times, please do think about soundproofing. It is almost completely impossible to accomplish after construction is complete, and low bass cannot be tamed without careful planning.
 
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Danaxus

Danaxus

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Oh, and unless your dad will be the sole occupant of the house at all times, please do think about soundproofing. It is almost completely impossible to accomplish after construction is complete, and low bass cannot be tamed without careful planning.
I hear you - thanks for the feedback. I started with zero background knowledge so it's been a journey just trying to get to where I have. The two major obstacles I faced were: 1) A lot of information I came across was written by professionals, for professionals and I had no hopes of understanding it, and 2) I found very little in terms of controlled experimentation. The clearest information I found was from Jesco at https://www.youtube.com/c/AcousticsInsider.

Soundproofing would be great if I could do it, but that seemed even more complex (something something floating walls). The impression I got is that a) it will take up precious space, and less room volume means more difficulty controlling the sound, and b) it will be expensive and need specialists to do. The outer wall of the listening room is already finished now (the concrete has been poured), so whilst I still have months of time left before doing the internals, my options are more limited now regarding shape and size.


Finally, I'm not too worried about the lack of feedback. I could really use any extra knowledge, but I'm happy to keep plugging away at it myself. This thread was made with the intention of giving a head start to anyone in a similar position to me, though with a slim hope that someone might come along and post something really useful that might also help me before I'm done building :)
 

dominikz

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@Danaxus On the topic of soundproofing, may I suggest to also look at the "Home Recording Studio: Build It Like the Pros" by Rod Gervais? Though I read it long ago (so details are foggy), I vaguely remember :D it was written in an accessible style. Though you're not building a recording studio, soundproofing is soundproofing so it may give you some ideas (or at least references).

Good luck with your project!
 

kemmler3D

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I hear you - thanks for the feedback. I started with zero background knowledge so it's been a journey just trying to get to where I have. The two major obstacles I faced were: 1) A lot of information I came across was written by professionals, for professionals and I had no hopes of understanding it, and 2) I found very little in terms of controlled experimentation. The clearest information I found was from Jesco at https://www.youtube.com/c/AcousticsInsider.

Soundproofing would be great if I could do it, but that seemed even more complex (something something floating walls). The impression I got is that a) it will take up precious space, and less room volume means more difficulty controlling the sound, and b) it will be expensive and need specialists to do. The outer wall of the listening room is already finished now (the concrete has been poured), so whilst I still have months of time left before doing the internals, my options are more limited now regarding shape and size.


Finally, I'm not too worried about the lack of feedback. I could really use any extra knowledge, but I'm happy to keep plugging away at it myself. This thread was made with the intention of giving a head start to anyone in a similar position to me, though with a slim hope that someone might come along and post something really useful that might also help me before I'm done building :)

I think you're right about all this. Air gaps and separate spaces are basically required for soundproofing, it's not trivial. It's just not everyday someone gets a chance to build a listening room from scratch, so it seemed appealing. But I'm pleased to see the phrase "concrete" and "walls" here, that's a good material for blocking sound. :)

You're right that most reliable info on these topics is written by and for pros. Heck, a lot of people on this forum are pros. I even worked in the industry for a while, you're not wrong about that.

As far as not finding anything on controlled experimentation - that's probably because the theories around acoustics at macro scales (rooms) have been settled for a very long time and don't need further proof or exploration. For example, you might have heard of Helmholtz resonators... still in use today as bass traps... but Helmholtz died in 1894. Acoustics is basically an application of simple wave physics, and when it comes to architectural acoustics, the math has been settled for a long time. There's no ongoing debate about reflection, refraction, absorption, etc. So most of the writing you see on room acoustics online starts from a base of knowledge on those theories and takes it for granted.

Heck, (controversial opinion here) even most psychoacoustic research relevant to music listening is settled. The still-somewhat controversial Harman curve work is a decade old now. Acoustics is a much less fraught topic than psychoacoustics, and most basic research in that area is decades if not 100+ years old now.

If you want to do an excellent job on the interior of the room I would recommend a textbook that covers that stuff systematically. With that under your belt, all of this will get a lot easier, and you'll be able to interpret stuff like Ethan Winer's guides quite easily, and judge whether other guides even make sense.

In college I was assigned this book, it definitely helped, I would recommend it or anything similar: https://www.amazon.com/Physics-Psychophysics-Music-Introduction/dp/0387094709

It will get you further, faster, than reading random forum threads or "build your own..." guides online.

Starting from zero and learning enough to write this post is a lot of progress already, best of luck!

(PS: DIYaudio is another forum with a similar mentality to this one, it's mostly about speaker building, but the folks there know a lot about acoustics also, if you can cut through the inevitable noise on threads.)
 

kemmler3D

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But to your point of someone posting a really helpful idea... maybe just look for "How I built my listening room" threads out there and see if anyone posts really good measurements of theirs. Then just copy what they did. After all, acoustics is nothing new and there's no award for reinventing the wheel... ;)
 

abdo123

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@kemmler3D the only thing you're doing by not building a rectangular room is that you're taking the best place in the room to place bass traps, the corners. That's all.
 

abdo123

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Also in-wall speakers are the best case scenario in terms of speaker placement. That or something more than a meter and a half away from every boundary.
 

kemmler3D

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@kemmler3D the only thing you're doing by not building a rectangular room is that you're taking the best place in the room to place bass traps, the corners. That's all.

Hmm, seems I have been taking this idea too seriously over the years. On one hand - random shape gives random reflections, maybe a little more diffuse. On the other hand... they're random and hard to control. https://www.acousticfields.com/acoustic-urban-myths/ - seems to confirm what you're saying.
 

kemmler3D

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you could just multiply the numbers by a factor. Should behave more or less the same.

There are absolute thresholds on room size due to RT60 and audio wavelengths, so this is probably only good advice within a certain small-ish scaling factor. I would not expect similar behavior from the same shape room at 200% or 50% size, for example.
 
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JPA

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Danaxus, you could have saved yourself a lot of time and effort by learning from credentialed authorities rather that surfing the web. These two books are from experts in the field and tell you everything you need to know about building a great home theater room.
Master Handbook of Acoustics
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms

Kemmler3d, you are wrong about nonrectangular rooms and nonparallel walls. They do not eliminate room modes. All they do is move the modes around and make them impossible to predict or treat.
 

Curvature

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If you're interested @Danaxus
 
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Danaxus

Danaxus

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Danaxus, you could have saved yourself a lot of time and effort by learning from credentialed authorities rather that surfing the web. These two books are from experts in the field and tell you everything you need to know about building a great home theater room.
Master Handbook of Acoustics
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms

Kemmler3d, you are wrong about nonrectangular rooms and nonparallel walls. They do not eliminate room modes. All they do is move the modes around and make them impossible to predict or treat.
Sounds like the best place to start. I've ordered those books - many thanks for referring them to me!

If you're interested @Danaxus
Sounds like something that could be super useful if there's a pesky room mode you just can't get rid of. A brief look gives me the impression that these Limp traps are a better alternative to Helmholtz resonators, am I wrong?

My hope is that these won't be necessary for a listening (or a mastering) room, as multiple subs (even 1 or 2), seem to be the simpler option if you're optimising for a single fixed listening position, when you have a fixed source of sound (as opposed to a recording studio where you have instruments and microphones all over the shop, and therefore need the entire room to be mode-free and neutral).
 

Curvature

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Sounds like the best place to start. I've ordered those books - many thanks for referring them to me!

Sounds like something that could be super useful if there's a pesky room mode you just can't get rid of. A brief look gives me the impression that these Limp traps are a better alternative to Helmholtz resonators, am I wrong?

My hope is that these won't be necessary for a listening (or a mastering) room, as multiple subs (even 1 or 2), seem to be the simpler option if you're optimising for a single fixed listening position, when you have a fixed source of sound (as opposed to a recording studio where you have instruments and microphones all over the shop, and therefore need the entire room to be mode-free and neutral).
I would refer you to the books above for details. I urge you not to make quick decisions.
  • Sub integration is complicated. Two or more subs will not automatically make the situation better.
  • Multiple subs can achieve several goals. The most practical are better output and smoother frequency response.
  • Listening positions are never fixed. You are always moving your head and adjusting your body for comfort. So you have to optimize for a wider area (horizontal and vertical). Say you have friends over, for example. It would not make sense to have one prime listening position. The difficulty will depend on what speakers you get.
  • Membrane (not necessarily limp membrane) traps are lower Q (wider bandwith, less absorption) than Helmholtz. The latter have to be tuned to specific problem frequencies. It is not a question of better or worse but rather purpose and resources. In the link I gave you I outlined a general approach to room treatment assuming many of the normal limitations that people have in their rooms broken down into room/frequency regions.
 

Bjorn

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A few comments.

Slanted walls do not minimize room modes. Some have built slanted walls for that purpose but it's based on a misunderstanding. However, there are other reasone to use splayed walls and also slanted ceiling if possible.

Reverberation time doesn't exist in small rooms. That requires an isotropic sound field. What we have instead is specular energy and room modes. That's also why absorbers function very differently in small rooms compared to large rooms. If a mixed reverberant sound field well beyond Dc existed in these types of rooms, one could get away with quite thin absorption panels with a great result. But anyone who has worked with small room acoustics knows that broadband absorption gives a far better result without changing the spectral content. That is also easily mesasurable.
 
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Danaxus

Danaxus

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There are two things I'm unclear about.
1) The benefit of leaving an air gap behind absorption panels. I have 50cm to play around with (at least on the front and back walls). I can fill that with 50cm of rockwool, or do some combination of rockwool and air gap (25-25, 30-20, etc). Assuming I don't learn something that makes me opt for some sort of membrane-absorption panel thing, is there a benefit to having an air gap as opposed to stuffing the entire allotment with rockwool?

2) Is there a maximum absorption depth beyond which you get minimal returns? Using the calculator at http://www.acousticmodelling.com/8layers/porous.php, it seems that 40cm seems to be the upper limit, and adding more, even a metre of absorption, gives no additional bass absorption. Is this because of some exponential increase in the amount of absorption necessary, or is the calculator broken?
 

Triliza

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@Danaxus Read those recommended books diligently and post a short summary for us TL;DR fellows out here. Although I have to say I did read all of the first post, and learned a thing or two from it.

Great project, great retirement gift, keep us posted about it, I'd like to see how it turns up.
 

abdo123

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There are two things I'm unclear about.
1) The benefit of leaving an air gap behind absorption panels. I have 50cm to play around with (at least on the front and back walls). I can fill that with 50cm of rockwool, or do some combination of rockwool and air gap (25-25, 30-20, etc). Assuming I don't learn something that makes me opt for some sort of membrane-absorption panel thing, is there a benefit to having an air gap as opposed to stuffing the entire allotment with rockwool?

2) Is there a maximum absorption depth beyond which you get minimal returns? Using the calculator at http://www.acousticmodelling.com/8layers/porous.php, it seems that 40cm seems to be the upper limit, and adding more, even a metre of absorption, gives no additional bass absorption. Is this because of some exponential increase in the amount of absorption necessary, or is the calculator broken?

Number 2 has also an additional factor.

lower frequencies require depth because of their longer wavelength. Depending on how much 'air stopping' the material is (it's called flow resistivity) sometimes the air just doesn't penetrate all the way for low bass to be influenced.

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