John Sayers' Recording Studio Design Forum

Hello!

I have a studio in Stockholm, Sweden, and on the purchase of new monitors (genelec 8260) I had an acoustician visit me.
He recommended 2 options, either I improve the absorption with standard rock woll, or I flush mount the monitors.
He suggested I'd flush mount them in a wall made from thick, compressed rock wool (there's a term for it but I dont know it in english, they usually come 10cm thick and are quite rigid, the GIK standard basstraps are made from such material)

I have made drawings of what it may look like, and I'd like some help in deciding which way to go.

As i've understand it the pro's of flush mounting are better sound, however I'm not sure just how much better it will be (well, thats case by case of course).
The con's, again as I've understood it, of flush mounting are:
1. From reading about it, it seems many people think you shouldnt get into it unless you really know what you're doing
2. More expensive (more time to build it), though I'm not overly concerned by the price difference.
3. Tricky if you want to change monitors or tweak position.

I've made drawings of the flush mounting, and also the alternative plan of the absorption, if I were to go that way (which I probably will unless someone convinces me otherwise).
Also, first you can see the room layout (with the rough absorption plan drawed) and a floor plan.

Note. On the back wall, there will be 50cm of absorption with diffusor panels forming a broadband basstraps.

I hope my drawings are ok, I did my best

I'm thankful for any advice!

Hi Rasmus. Please read the forum rules for posting (click here). You seem to be missing something!

Anyway, I would definitely recommend flush mounting (soffit mounting) as being the best, without any doubt, for most speakers. It solves so many of the problems associated with speakers, and is well worth the extra cost / effort.

However, as far as I know, the Genelec 8260s are rear-ported, which is very hard to flush mount correctly, so that really isn't an option for your speakers, unfortunately. I may be wrong, but the line-drawings for that speaker seem to show a rear port, and if so that would mean that they should not be flush mounted. Speakers that are front ported, or not ported, can be flush mounted easily, but rear ported speakers are a problem.

In addition, the way shown in your diagram is not the correct way to do it, for several reasons: The front panel of the soffit cannot be soft, light-weight or absorbent. It MUST be massive, had, heavy, rigid and reflective. so you cannot use compressed rick wool for that: it is not rigid enough, or dense enough, or hard enough, or reflective enough.

Technically, that front panel must act as an "infinite baffle", which basically means that it must extend the front baffle of the speaker cabinet out to "infinity": Of course, "infinity" is impossible, but as long as the panel is large with respect to the size of the lowest frequency wavelengths, then you are fine. But the front panel you show in your diagram is not big enough to do that: it should be much wider than the original speaker cabinet in order to be effective: One "rule of thumb" is that the width of the panel should be 4 times the diameter of the woofer, at least.

Another issue is that the cavity inside the soffit should not be empty. Rather, it should be filled with loose insulation, to damp resonance inside there.

Much, much better! It eliminates problems such as SBIR, power imbalance, first reflections of the front wall, comb filtering, phasing, and a lot of other problems. With flush mounting, all of those problems are gone: they do not exist. So yes, it would sound much better, if you could do it with your speakers.

For normal speakers (front ported or not ported), it isn't that hard. The principles are quite simple: a very large, rigid, heavy, solid panel with a hole cut in it, to poke the speaker through, and a solid framework to support it.

Yes, but the difference is well worth it.

True, but there are work-arounds for that, such as making a cut-out in the middle of the panel, larger than the biggest speaker you might ever use, and then make "bevels" to fit in the gap around each type of speaker.


But all of that is not really applicable, since your speakers are not meant to be flush mounted anyway, due to the rear port, so the only option you have is to mount them on very heavy stands, close to the front wall, and with plenty of absorption around.

There are a few other things you should consider if you go with that alternative: You will also need treatment on the side walls at the first reflection points, but your diagrams shows that the absorption on the side walls only goes part way up the wall.

Also, an easy way to do the absorption for the corners, is with a simple absorption panel placed vertically in the corner, floor to ceiling, and turned diagonally across the corner.

Also, you show diffusers on your rear wall, but the room doesn't seem to be big enough to be able to use most types of diffuser. You also mention something about "Helmholtz" on the rear wall, but you don't show any Helmholtz devices there. What did you have in mind for that? Helmholtz devices are not easy to tune, and you don't even know yet if you will need any. They are normally reserved as a "last resort" if you still have specific resonant issues in the room, after you have installed all of the basic bass trapping and broadband treatment.

Finally, you don't show any bass trapping at all (except for the front corners, behind your speakers)! That's a small room, so you will need bass trapping, and it seems that you do have room to put good size bass traps in the two rear corners. 50 cm of absorption on the rear wall is good damping for some types of modal issues, but that wont to much for axial modes and tangential modes that do not touch the rear wall. Also, if you have 50 cm of space to spare at the back, then maybe hangers would be a better option than just pure absorption.

But anyway, to answer your original question: Flush mounting would be far superior, but is not advisable with your specific speakers, since they are rear ported.


- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

OK, after having written all that, I found that Genelec actually DOES have a flush mount kit for the 8260A! So I guess it is not rear ported after all!.

That's good news: it means that you can actually flush mount your speakers, and gain all the benefits!



- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Hi Stuart!

Thanks a lot for your reply! I did read the forum rules, and did my best, sorry if I missed something!

Thank you so much for your insight! As for the new wall (where I could flush mount the speakers), the drawings are a bit misleading. There is actually not more space to go out the sides, inwards of the speakers, as there's a double door on that wall. The drawings of the absorption plan shows this door accurately. I was told at another forum that this small size of the flush mount wall is a problem, because the tight corner will create diffractions of precisely the kind I'm trying to avoid (same as what you're saying I guess).

The absorption on the side of the speakers (first reflection points) is not a problem, that can easily be extended (and would be). However, I cant do nothing to increase the size of the actual new flush mount wall.. So it seems that rules out the flush mount option in that case.

As a further note though: The plan for making a wall using rigid rock wool was suggested by the acoustician, who is one of the most experienced in Sweden, and has designed countless studios. Of course, that doesnt mean he must always be right, perhaps especially with regards to small control rooms of less than ideal dimensions. But his experience is unquestionable, so perhaps there is more to the flush mount theory than only the super rigid wall? I'm not one to challenge paradigm, and I need to work for a living in my room, so maybe I'll go with a safer option.
But it would be interesting perhaps to hear this being discussed more in this forum?
The acoustician recommended this to me, could it have something to do with the fact that I cant make a rigid wall big enough? Just speculating..

As for basstraps. Sorry if that was not clear. The whole back wall will be a bass trap, with 50cm of absorption material from floor to ceiling. On top of the absorption will be placed wooden diffusor panels from floor to ceiling. They are 20cm wide, and will be placed 4 cm apart (there's room for a total of 14 diffusors on the wall). Thereby creating what I thought was termed a "broadband helmholtz trap", though I could be mistaken.
The bass trap is there in either case.

The biggest issue in the room is an interference dip at 70hz. The acoustician said this would be greatly helped if not completely reduced by the 50cm absorption in the back, combined with the absorption in the front.
Hopefully 30cm will be enough in the front/sides, as thats all that will fit.
I'll put a bit more in the ceiling above the speakers though, plus a little more in the corners just behind the speakers..

There's still one thing missing: It's the part in large blue letters, about 1/3 of the way down the post...

There still might be enough space there. If you post an accurate SketchUp model, showing the room dimensions and the correct location of the door, I'll try to figure it out for you.

If the soffit is built correctly, then the corner is no longer part of the room, acoustically. However, the walls around the soffit are still part of it, of course, and if the soffit panel isn't very wide (as compared to the wavelengths of interest), then there can still be some artifacts going on there. It won't be diffraction, though (if the soffit is built correctly). Diffraction is what soffits eliminate. Diffraction occurs at the edges of the speaker, for wavelengths that are similar to, or longer than, the dimensions of the speaker cabinet, and that is one of the major problems with speakers that are not soffit mounted. That's why Genelec rounds the corners of their speakers: to reduce edge diffraction issues. But the way to eliminate them completely, is to flush-mount (soffit mount) the speakers. If that is done correctly, then the edges of the speaker cabinet do not exist any more, acoustically, so there is no diffraction. That's one of the many good things that soffits do. Of course, if the soffit is not built correctly, and does not reach the side walls or front wall properly, then there can still be some amount of diffraction, but it will be at a lower level, and also at a lower frequency, which is good.

Great! The important part is the area where the sound waves from the speaker on the OPPOSITE side hit that wall, and bounce back towards your ears. There is an easy way to find out where that is. it is called the "mirror trick". Get someone to hold a mirror against the wall, while you are seated normally at the mix position. Ask them to move the mirror around all over the side walls, and any place where you can see one of the speakers reflected in the mirror, needs treatment. The same is true for the ceiling.

I'm not questioning his experience or methods: What he does might sound fine, but it is not soffit mounting, or flush mounting: it is simply surrounding the speaker with absorption. Perhaps he bases that on the old LEDE concept, that he modifies. But it is not the same as proper flush mounting, also commonly called "soffit mounting".

By definition, proper soffit mounting means that the speaker is surrounded by a very large, very massive, very rigid surface. Technically, that is called an "infinite baffle". It MUST be at least as rigid and massive as the front baffle of the speaker cabinet itself. (I have never yet seen a speaker cabinet made from fiberglass insulation batts...).

The principle is actually not hard to understand: If you have just the speaker itself (the "driver" alone), without any cabinet at all, it will sound terrible, since the acoustic impedance of the speaker element itself and the acoustic impedance of the air are very different. So you must have some method of matching the impedance. The speaker cone and the curved "horn" shape in the front panel of the cabinet (around the cone) do that. So now it sounds a bit better, but only for sound waves that are smaller than the size of that front panel (usually called a "baffle"): For waves that are larger than the baffle (front panel), as they radiate outwards along the surface of the baffle, they eventually reach the edge, and then "wrap around" behind it. And as they do that, the emit secondary waves right at the edge, which interfere with the primary wave. This is called "edge diffraction", and the frequency where that happens is set by the size of the baffle. With a small baffle, it happens at high frequencies. If you have a large baffle, it happens at lower frequencies. If the baffle is large enough, it does not happen at all, since the speaker can't produce frequencies that are low enough to cause a problem.

Of course, the baffle must be made so that the sound waves cannot make the baffle itself move! If the baffle moves along with the waves, then the baffle will emit sound! So the baffle must be prevented from moving, vibrating, and resonating. The way to stop the baffle from moving, is to make it very heavy, very rigid, and very hard. If it is not heavy (massive) then sound waves can make it move. If it is not rigid, then sound waves can make it move. And if it is not hard, it will allow sound waves to go through it, around to the back of the speaker, instead of forcing the sound waves to only go forwards, away from the speaker and into the room. This is why the front panels of loudspeakers are made from very thick high density plastic or wood: They have to be, in order to work as a baffle. If the baffle is not hard, rigid and massive, then it cannot make the waves behave correctly, and we go back to the original problem: a driver by itself sounds terrible.

So, the front panel (baffle) of the speaker must be large enough that the lowest frequency waves cannot wrap around the edges, and cannot cause diffraction. But if you made a speaker cabinet big enough to do that, it would be way too large to even fit in your car, to take home after you buy it! In fact, it would not even fit inside the shop, so there would be no place to buy it! So manufacturers have to compromise. In order to make speakers small enough to fit in cabinets that you can carry in your arms, put in your car, and put on the shelf in the shop, they have to make speakers that DO cause diffraction. It is simply a result of the size of the baffle (front panel of the cabinet), and the frequencies that the speaker can produce. By carefully shaping the box, it is possible to reduce the problem of edge diffraction, but it cannot be eliminated completely. That's why your 8260s have that rounded shape: it reduces edge diffraction. But not completely. Genelec knows this, which is why they also sell that flush mount kit for the 8260, so that things can be improved even more.

There is another problem with small speaker cabinets, that is even easier to explain: The hard, rigid front baffle obviously forces high frequencies to radiate forwards in a tight cone, like a beam of light. And we already saw that low frequencies do not do that: rather, they wrap around behind the speaker, and go the other way. so if you think about it, half of the power of the low frequencies is wasted! For example, if you send one watt of sound out the tweeter, it all goes directly forwards, so there is one watt of high-frequency sound heading directly towards your ears. But if you send one watt of sound to the woofer, half of it goes the wrong way! It goes backwards, behind the speaker, away from you. So only half a watt actually goes towards your ears. Therefore, you have a power imbalance: there is a 6 dB difference between high frequency power and low frequency power, because the high frequencies are radiating into "half space" (only forwards), while the low frequencies are radiating into "full space" (spread all around).

Manufacturers know about this, so they have to increase the low frequency power by 6 dB, in order to compensate for the lost power. This is called the "baffle step compensation", because it is caused by the size of the baffle. If you look at a frequency response graph of the speaker without any compensation, you see a "step" in the graph at the point where the wavelengths start to be longer than the size of the cabinet. So that's the point where the internal circuitry starts increasing the amount of power going to the woofer, in order to compensate for the power imbalance.

But if you put the speaker in a proper soffit, then you no longer have a baffle step! With soffit mounting, all of the the sound from the speaker goes forwards, towards you, and none of it goes out backwards. It radiates into half space across the entire spectrum. So you do not need baffle step compensation any more, because there is no power imbalance: the power is perfectly balanced, naturally (not artificially), so you can turn off that circuit. On the back of your speaker there are controls for doing that. They are called "bass tilt". If you set those to -6dB, there is no longer any baffle step compensation, and the speaker can radiate at its best, the way it should be, with no power imbalance.

There are other issues with speakers that are not correctly mounted in soffits: the sound that wraps around behind the speaker, can then bounce off the front wall and side walls of the room, and come back again, out of phase and delayed in time. When the reflected sound meets the direct sound, it causes cancellation where the waves are exactly out of phase, and interference patterns for other phase differences, as well as comb filtering, early reflections, and such like. This is commonly referred to as "SBIR", meaning "Speaker-Boundary Interface Response". Some acousticians only use SBIR to refer to the phase cancellation problem, others say that SBIR also includes the other terms, but which ever way you use that term, SBIR is a bad thing, and all of these problems are also eliminated by proper soffit mounting.

So, getting back to the original point: I'm not questing that the acoustician you hired has built good studios, and has used a method of mounting speakers by surrounding them with insulation. What I am saying, is that "surrounding a speaker with insulation" is not the same as flush mounting it (soffit-mounting), since the very concept of flush mounting requires that the wall around the speaker is massive, rigid, and hard. In fact, in their material on flush mounting, Genelec speaks of mounting the speakers in concrete... !

Perhaps. If you post the SKP here, I'll take a look and see if it is possible to do something useful with proper soffit mounting.

Yes, and like I said: that will work fine for the 1,0,0 axial mode, and for tangential modes that happen to involve the rear wall, but it will do nothing at all for 0,0,1 modes or 0,1,0 modes, which do not ever even see the back wall, nor will it do anything for tangential modes that don't happen to involve the rear wall.

That type of bass trapping that you show is essential for soffit mounting, so you already have that in place if you do decide to go that way, but it won't help with other room modes. It only works for the modes that the speakers can excite, which obviously are only those associated with the length dimension. And it won't work very well if the real wall is also partly reflective.

That would only work as Helmholtz slot wall if it is correctly sealed air-tight all around the edges of the cavity, and the top and bottom slats are but to half width. However, your diagram shows that there is a gap around the sides of the slots, to that will not work as a Helmholtz resonator. The cavity must be sealed in order for that to work.

Also, that will not be a bass trap, given the dimensions you mentions, as the tuned frequency is too high. You didn't say how thick the slats are, but assuming one cm thickness, that would be tuned to about 650 Hz., which is not bass. In order to get that down to the type of bass frequencies that you need, the slats would have to be at least 5 cm thick. That would give you tuning at around 290 Hz, which is still rather high.

That wall also will not be broadband, since all of the slat widths, depths and gaps are the same size! So the entire rear wall is tuned to one single frequency (647 Hz for 1cm slats, 457 if you use 2 cm slats), which is certainly not what you want. You could vary the depth of the cavity, the width of the slots, and the depth of the slats in order to make it more broadband, and while that would broaden the Q of the entire wall, it would also flatten the cruve downwards, thus making it less efficient.

So they way you shoe it right now, it is definitely not a "broadband bass trap". It would be a broadband bass trap if you take off all that wood, but with the wood in place, and working as a Helmholtz resonator, it would be neither broadband nor bass.

Like I said before, tuned traps are normally the last resort in treating a room. They are very difficult to tune accurately (despite the equations), and they must be placed in the exact location where the conditions are correct, and at the correct angle of incidence, in order to work at all. Helmholtz resonators are pressure devices, so placing them at a pressure null (velocity peak) would be pointless, for example. And placing them on the wall that does not involve the problem, or at a very low angle of incidence, would also be pointless.

Besides, Helmholtz resonators are not very effective at all at bass frequencies: very hard to do that.

It is far better to treat the room with the normal procedure, then measure the acoustic response, and design the remaining treatment accordingly.

Unfortunately, it isn't. If it is a Helmholtz resonator tuned to several hundred Hertz, then it just is not a bass trap. Sorry.

What do you mean by "interference dip"? Do you mean that you have a standing wave issue at 70 Hz? How do you know it is at 70 Hz? I guess you measured that, but it would be good to know how you measured it. Based on you room dimensions, you do not have any modes at 70Hz, so it would be interesting to know where that is coming from. Your closes modes are as follows:

66.2 Hz. (0,0,1 Axial)
72.7 Hz. (1,0,1 Tangential)
75.7 Hz. (2,1,0 Tangential)

As you can see, there is nothing at 70 Hz. You do have the 0,0,1 axial mode at 66.2, but that does not involve the back wall, so putting 50 cm of absorption on the rear wall would not help for that.

And if your problem is 70 Hz, then why is the entire slot wall tuned to way higher?

Maybe you could download REW, and run a complete acoustic analysis of your room, then post the results here so we can see what the problem might be, and possibly suggest some ways of fixing it.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Hi Stuart. First of all, thank you so much for sharing all this information. I'm very grateful! Its really cool of you to share all this info to someone you've never met on a forum. There seems to be hope for humanity after all!


Thank you so much for that offer! I'll definitely do my best with the sketch! I just downloaded Sketchup before I made those first drawings, so I have still to learn how to make them look a bit better/informative.
Just to let you know, I'm a songwriter/producer/part-mig engineer, making my own music for a living. That is keeping me busy enough, so I am by no means an acoustic expert. Some of your lingo flew right over my head, so I might come in with more questions on that.

My aim is to make a reasonable improvement to my room to improve my mixing enviroment, and also getting a bit of motivation push.
I brought this acoustician in to suggest changes. These were the two options he suggested, and the fact that he suggested those particular variations might also have to do with what he saw as reasonable changes in my room.
For example, building a concrete or other fixed rigid structure in my room might not be suitable, simply due to the laws and regulations of rented spaces in Sweden.
I'm debating the changes to my studio also from the perspective of how long I will actually stay in that room etc. So in short, with regards to concrete or brick structure, I'll be happy to come back to you when I manage to get my own space where I can build whatever I want, and where I know I'll be staying for a long time.


This may very well be correct. He never called that flush mounting either, that was simply my interpretation. Since you've described how it works I realize its not the same thing.
Granted this guy was not young, so if flush mounting have gained in fashion pretty recently it might have passed him by, or he might simply stick to his trusted methods. But it might also be that he judged it to be impractical in my room for some other reason that he didnt mention clearly to me, such as mentioned above.


Done. The side wall absorption (30cm) will cover both the reflection points from both speakers at both sides.


I'll be posting the SKP (I'm assuming this means Sketchup?) anyway, so you can have a look at the back wall as well, but if concrete/bricks are th only options for a true soffit mounting, then I think I will need to pass on that til the next studio. I'm very excited by that prospect though!


Like I mentioned briefly, that was my own interpretation, from my limited knowledge. He might not have meant it as a Helmholtz trap at all. In fact, I had a Helmholtz trap in there before. (reaching the full width of the wall, about 1.5 meters high and 30cm deep). But he did not think that was effective enough. He also said that I could keep the absorption without the diffusors on top, and that would work, but that the sound in the room would be better with the diffusors, so it seems that he did not construct it with those diffusors as an absolute necessity.
He just told me what to do basically.


Well, umm, yeah, I'm a bit embarrassed now, but I actually dont know what this means. Firstly, you may be right, it might actually be 72.7, or even 75.7. Its just that I dont remember the specific frequenzy.

Again, as for the acousticians advice, I'd be wrong to assume that he doesnt know what he's talking about. However, I'd NOT be wrong to assume I might have missed some explanation of his as to WHY he recommended what he did, rendering my explanation of it to you faulty in some way(s).

Thanks again! Will keep on chipping away at Sketchup now

:) That's what this forum is all about: people helping out other people. When I first started on my quest to learn about acoustics, a lot of really smart people helped me to understand the principles, so I'm just giving back some of what I learned: "passing it forward" I guess.

SketchUp is a great tool, but it's frustrating the first time you try it, as some of the concepts are a bit "strange"! So it takes a while to figure out, but once you do, it is a very powerful, useful tool. So your model doesn't have to look pretty: as long as it is accurate, the dimensions are correct, and shows the position of the important parts of the room, that's all I really need.

No problem! Like they say: "The only dumb question is the one you didn't ask!". Yeah., the lingo of acoustics is a bit weird sometimes! Some words that we all use every day have slightly different meanings in acoustics, so feel free to ask whatever you don't understand.

Then you are definitely in the right place here on the forum!

I hope I didn't give you the wrong impression! Concrete is not necessary for a soffit: it's just the material that Genelec mentions in their documents about soffit mounting. Most people here use ordinary wood, such plywood, MDF, OSB, or even drywall (plasterboard) for their soffits. As long as it is thick, rigid, hard, and heavy, that's what matters. There are many ways of doing that, and concrete is only one. Of course, if you can do concrete soffits, that would be really good! But very, very few people do that. A sheet of thick plywood with a hole in the middle for the speaker, is all that you really need.

You could make your soffit modular, held together with bolts and screws, instead of nails, so you can take it with you if you move, and re-use it in the new place.

To me, it looks like he is trying to combine a good mounting position for your speakers with bass traps in the corners. Bass traps work best in corners, because that is where all the room modes "terminate". So a bass trap in a corner should work on modes that involve any of the walls around it. For example, if you put a bass trap in the corner between one wall and the ceiling, then it should have an effect on the modes that touch that wall, and also the modes that touch the ceiling. So corners are usually reserved for bass traps, but if you don't have any other place to put your speakers, then they would have to go in the corners too. However, it is usually best to not put speakers in corners (for complex reasons), so I guess there really is no other option, and that's why your acoustician suggested the combination.

Yep! When you save a SketchUp model, it ends up with a ".skp" on the end of the name.

It might not have been in the right place! Let me explain: As a sound wave moves through the air, it has two components: pressure and velocity. At any point on the wavefront, the sum of pressure+velocity is the total amount of energy on the wave, but at some points pressure is higher and velocity lower, while at other points velocity is higher and pressure lower. For example, at the exact point where a waves is bouncing off the wall, the velocity is obviously zero (since the wave must slow down, come to a stop, then head back the other way, speeding up again: where it just touches the wall, it is not moving at all: velocity=0), so at that point the pressure is at its maximum. Out in the middle of the room, a half wavelength away from the wall, pressure is zero, and velocity is maximum. So there is a balance between the two that changes all over the room, and it is in different places for different waves and different frequencies. It is complex!

So, there are basically two types of acoustic treatment: the ones that work on the pressure component of a wave, and the ones that work on the velocity component. If you put a pressure device at the point where pressure is zero, obviously it will do nothing at all! The best place to put it, is where the pressure component for that wave is high. And the same for velocity treatment: if you put a velocity device at the point where the velocity of the wave you want to treat is very low, then it will not be very effective. So it is important to put the right treatment in the right place in the room.

For example, Helmholtz traps are pressure-based: If you have a Helmholtz trap tuned to (for example) 500 hz, and you put it at a place where the pressure component of a 500 Hz standing wave is always zero, then it will not work. It must be at the location where the pressure component for that wave is as high as possible.

Absorption is a velocity-based device. So absorption right up against a wall doesn't do much for waves that hit the wall at a 90° angle, but works well for waves that are coming in at a 30° (for example). That's why you often see recommendations here to leave a gap behind absorption panels: space them away form the wall, so they are in the velocity zone for a larger number of waves, coming from more angles.

So maybe the problem with your Helmholtz resonator not being effective, is that it was not in the optimum location.

Very true: diffusion is good, but not essential. And some types of diffuser cannot be used in small rooms, for other complex reasons. One thing you could do, is to space those slats so far apart that the will not work as Helmholtz resonators, and will only act as diffusion. If they are too far apart, then they just won't work as resonators: instead, they will reflect some high-frequency energy back into the room (which is good), and also will diffuse sound waves that are of a size similar to the dimensions of the slats. So you could also vary the width of the slats: make some a bit wider, others a bit narrower. In that case, the exact dimensions and spacing don't matter much, acoustically, so you could just arrange them to look good. As long as the gap between slats is at least a couple of cm, that would work fine. You can put black cloth behind them, to hide the insulation at the back.

That is all about room modes. I'm sure you already know that each tone, or frequency, has a specific wavelength, because sound travels at a constant speed. So, for example, since sound travels at 343 meters per second, a note with a frequency of 343 cycles per second (343 Hz.) will have a wavelength of exactly one meter, and a note with a frequency of double that (686 Hz) will have a wavelength of exactly half a meter. And a really low tone, such as a low "C" on a bass guitar, at 34 Hz, would have a wavelength of ten meters. Etc.

OK, so every frequency has a wavelength. And if a wave happens to fit in perfectly between two walls in the room, then you have a problem! As it bounces back and forth between the walls, the pressure and velocity components will always be in the same phase at the same point in the room! So the wave reinforces itself, building up louder and louder, and causing the entire room to "resonate" at that frequency: So if your room is exactly ten meters long, every time you play a low "C" on any instrument, the entire room will vibrate too, since that is the natural frequency for a room with those dimensions. So the low "C" will always sound much louder than it should be in a room ten meters long, but a low "E" would sound normal. Of course, that isn't good: you don't want the room to be "coloring" the sound like this! All notes should sound the same, not some louder than others.

So each frequency where the room has a natural resonance, due to the dimensions, is called a "mode".

And there is another problem here: Since the room is "tuned" to that specific note, and "resonates" at that frequency, it is doing the same thing as the string on your guitar: after you pluck it, it carries onnnnnnnnnnnnnnnn for a while. So if the mode is triggered, then the entire room carries on vibrating at that frequency even after you stop playing the note! The "mode" sort of stores some energy at that frequency, then carries on releasing it back into the room after the source has already stopped. So the room can ring for a while. In bad cases, it could be a couple of seconds!

So, every room has a set of modes that it will vibrate at, and those modes depend entirely on the the dimensions of the room: length, width, height. Of course, if two of those are the same, the things get much worse: so if you have a room that is ten meters long and also ten meters wide, then it vibrates in BOTH directions at the same time, and makes the mode twice as strong. So the worst shape for a room is a cube: A room that is 2.5 m wide, 2.5 m long, and 2.5 m high would be impossible to use as a studio. And this also applies to multiples: If one wave fits into 2.5 me, then two of the same wave will fit into 5 m, and three of that wave will fit into 7.5m, so a room that measures 2.5 m high, 5 m wide and 7.5m long will be just as bad as a cube.

So it is important to take care in choosing the dimensions of your room, and avoid bad relationships between them. This concept is called "room ratios", and there are many different good ratios, as well as many, many bad ones. There are also programs that calculate all of this for you,m which is where I got the numbers above, for your room. Those are the modes that your room will have, due to the dimensions you gave.

OK, so what are the "axials" and "tangentials"? It turns out that there are several ways a wave can bounce around a room. The obvious one that I was talking about is where you have two parallel walls, and the wave just bounces back and forth between those two walls: Those are "axial" modes, since they always happen along an "axis" of the room. But it is also possible to have a wave that bounces off four walls and then gets back to where it started, and those are called "tangential" modes, since they bounce of the walls at a "tangent". And there are also some ways that a wave can bounce of all six walls in the room, then get back to where they started. Those are called "oblique" modes. Axials are the most prominent, tangentials a bit less, and obliques less still, but they all matter.

It gets complex!

That's why I mentioned the problem of placing treatment in the right place. For example, if you have a problem in your room with the mode that happens between the floor and the ceiling, then putting treatment on the walls will not doing anything at all for that mode! It seems obvious when you think about it, but unless you understand modes, how they work, and where they go, then it isn't very intuitive. So if your problem is at 66.2 Hz, then it is the first vertical mode in the room (that's what the "0,0,1" means), and can ONLY be treated with absorption on the ceiling or in the floor. But if it is the mode at 75.7 Hz. (2,1,0 Tangential), then putting treatment on the ceiling will do nothing at all, since it never touches the ceiling! It only touches the front, back, and side walls.

Etc.

So acoustics and placement of treatment is a bit more complicated than most people imagine.

Anyway, I hope that helps to give you a better understanding of some of the issues. That's just the basis: there's plenty more to worry about...



- Stuart -

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I want this studio to amaze people. "That'll do" doesn't amaze people.

Thank you for the lesson in acoustics! Particularly, the terms axial/tangential/oblique I had never heard before!
Dont know what you do when you're not a samaritan for people with acoustical needs but if you ever need any advice on production, songwriting or recording, don't hesitate to contact me!

Right, so I've made a Sketchup doc of my studio, to the best of my abilities. Here's the link to that file:
http://www.yousendit.com/download/TEhXa ... V01LSk5Vag

I'm very much looking forward to hear your thoughts. The sketch portrays how it would look if I go the absorption way.

Some notes:
1. Because of visibility, I put the back wall back quite a bit in the sketch but I marked on the floor where it is.
2. The back wall shows the placement of the diffusors. Here is the diffusor model I will probably be using: http://www.diffusor.com/Diffusorpaneler.htm (model AD40 from that page. Comes in panel lengths of 2.40 meters max)
3. There's a sofa standing against the back wall (which makes me wonder if those diffusors are doing any good near the floor behind the sofa if they're not also helping to act in any bass trapping way.
4. I put those TriTraps in just for fun. They might not be necessary.
5. One thing I'm a little bit concerned about: The acoustician recommended 30cm of absorption in the ceiling, but that isnt possible in the whole room because of the window. Only 20cm can be easily put in. Do you think that will make a big difference? I could extend the absorption another 10 cm at the ceiling reflection points I suppose, though thats a bit of a hassle building wise.
6. I have 6 of these: http://hofa-akustik.de/pages/startseite ... sor_en.php The acoustician wasnt too impressed by them, but said they might do some good if I put some of them (3-4 I guess) in the ceiling right above the mix position (thats where the absorption ends)

Also, I have a couple of GIK TriTraps, Basstraps, and standard absorbers, for use if necessary. The acoustician wasnt too impressed by them, as he felt they were too rigid (the Basstraps and Standards) to act as first reflection absorbers, and he felt the TriTraps was too light to do any serious bass trapping. Do you think he's right?
If there's no use for them, I might just put them in a 3rd room, which I use very rarely as a spare recording room.