Sound dampening for practice space

[TWU_Chapel]

Hello,

I facilitate music rehearsals for our school's chapel programs, and am researching sound dampening options for our rehearsal space (a school portable). Our music teams consist of the usual pop/rock instruments (bass, drums, electric guitar, keyboard, acoustic guitar, and vocals). Because we use acoustic drums and electric guitar, everything needs to be amplified.

GOALS:

• To decrease the overall dB level within the rehearsal space as much as possible (sound bleed outside the rehearsal space is not a major concern). Unfortunately, making precise dB measurements is not possible right now as all of our students are gone, but I will be happy to make measurements when I can assemble a music team to play in the space.
• To improve the overall sound clarity and balance. Right now things are loud and muddy.

REHEARSAL SPACE CONSTRUCTION:

• Location: Langley, British Columbia, Canada
• Structure: Britco Portable
• Exterior Siding: Metal siding & metal fascia
• Interior Walls: Vinyl-clad paneling (Norwich Pearl)
• Exterior Door: Solid-core door with passage set and deadbolt
• Interior Door: Hollow-core door, prefinished wood with passage set
• Floor: Loop pile carpet (commercial grade)
• Ceiling: Vinyl-clad gyproc (white)

REHEARSAL SPACE DIMENSIONS:

• Windows: 4'-0" x 3'-0" XO horizontal slider with insect screen and security bars
• Width: 15 feet
• Length: 24 feet
• Height: 8 feet
• Floor Elevation: 2 feet, 7 inches (21" from the ground plus 10" for the flooring joints)
• Total portable size: 25 feet long by 40 feet wide. The rehearsal space is located on one side of the portable. Adjacent to the portable is a meeting room, unused during rehearsals, but measuring an additional 25 feet wide.

BUDGET:
An exact budget has yet to be determined. Basically, I am researching options and need to submit a proposal to my employer for approval. That said, with the research I've done to-date, I am hoping to get this room treated for $1500 CAD or so. If this is impossible, I'd like to know that too.

PROPOSED ACOUSTIC TREATMENT:
(Detailed diagram attached to this post)

• 3 "super chunk" corner bass traps (a 4th is impossible due to doorway obstructions) made of AK Manson rigid fiberglass (local equivalent to OC 703) stacked from floor to ceiling then covered in acoustically transparent fabric (likely Guilford of Maine or DMD).
• 7 2'x4' acoustic panels on the right wall
• 6 2'x4' acoustic panels on the left wall.
• 2 2'x4' acoustic panels on the back wall.
• I'm thinking I'll make the "super chunk" traps myself, and am unsure whether to make the other panels myself or just go with Acoustimac panels as they seem to be well-reviewed and constructed.
• Wall panels would be mounted 9" apart in a vertically "zig-zagged" pattern (the first panel 1 foot from the ceiling, the second panel 1 foot from the floor, alternating down the length of the wall).

practice-space.jpg

Or for those who prefer Sketchup: practice-space.skp (apologies for the external link, but it was too big to upload directly to the forum). Measurements are an inch or so off, but pretty close.

QUESTIONS:

• I know sound plays no favourites and the ceiling is just another surface, but affixing panels to the ceiling is problematic due to restrictions. Will wall panels be sufficient?
• Is there a better strategy for panel placement and/or number? In the diagram, panels are mounted in a "zig-zag" pattern vertically, alternating a foot from the ceiling then a foot from the floor. Horizontally, they are spaced approx. 9" apart.
• Is alternating panel depth recommended? I was going to do all the panels 4" deep for broadband bass absorption, but in reality, relatively even sound absorption across the frequency spectrum with just a bit more dampening for the bass is desirable. Would a pattern like 2'x4'x4", 2'x4'x6", 2'x4'x2", etc. better achieve this?
• Rather than plastering the wall with panels, would I be better off mixing diffusion and absorption techniques (bearing in mind that the primary goal is overall sound dampening)?
• Finally, given the dimensions of the room, what would you recommend for speaker placement (2 speakers)?

Any assistance you can provide will be greatly appreciated! If more details are needed, just let me know and I'll do my best to supply them.

With thanks,

Jared

P.S. My apologies to Soundman2020 for missing a few posting rules. Hopefully these recent post edits (and the inclusion of my location in my profile) rectify these issues. Looking forward to your reply!

[TWU_Chapel]

BUMP. Any feedback? I am hoping to begin panel construction shortly, and just hope I haven't missed some really important information. Thanks!

[Soundman2020]

Hi Jared. Welcome. But please read the forum rules for posting (click here). You seem to be missing a couple of things!

But that's actually not the reason I didn't reply yet: it's just that there are a whole bunch of other posts that I have lined up to reply to, and it's only fair that I treat them in "first come, first served" order. I'll try to get to yours as soon as I can! Promise! But in the meantime, please take a look at those rules...

- Stuart -

[TWU_Chapel]

My apologies, Stuart. I've edited my initial post to include more detail as well (and will wait patiently for a reply rather than bumping!)

Jared

[Soundman2020]

Much better!

researching sound dampening options for our rehearsal space

When you say "sound damping", do you mean in order to isolate the room ( so that sound cannot get out and disturb other classes), or do you mean treatment (so that the room can be made to sound good for rehearsals)? That's two very different things, but they are often confused.

Because we use acoustic drums and electric guitar, everything needs to be amplified.

Translation: "It is LOUD in there!". Been there, done that.... Understand you perfectly.

OK; but I do see some mics on stands in there: how can you record vocals with mics in the same room as the drums, bass and guitars?

To decrease the overall dB level within the rehearsal space as much as possible

There might be a misunderstanding here, about what acoustic treatment does. Even if you lined all the walls, floor and ceiling with a foot of absorption, the peak level inside the room would not change: If the drums produce 115 dBC, then they produce 115 dBC, and no amount of treatment will make them produce less. What it WILL do, is to damp reflections, resonance, and reverberation of the room, to a certain extent. So the room might sound quieter, simply because the sound only reaches you once at a loud level, instead of multiple times, but in reality the peak level in the room will not have changed. The only way to reduce the dB level within the room is to teach the musicians to play more quietly, and turn down their amps.

Unfortunately, making precise dB measurements is not possible right now as all of our students are gone, but I will be happy to make measurements when I can assemble a music team to play in the space.

That would be useful: Measure with "C" weighting, and "Slow" response. I'm betting you'll see levels of 115 dB and upwards with a typical rock band going strong in there.

To improve the overall sound clarity and balance. Right now things are loud and muddy.

That can certainly be fixed! No problem there.

Floor: Loop pile carpet (commercial grade)

I guess there's no chance that you can take the carpet out?

Width: 15 feet - Length: 24 feet - Height: 8 feet

There's a problem, right there. The length is exactly three times the height, and the width is very close to twice the height. So you will have some modal issues going on in there, especially around 71 Hz, 80 Hz, 103 Hz, and 113 Hz, plus their harmonics. I would guess that drums sound very ugly in there: boomy, muddy, unclear?

I am hoping to get this room treated for $1500 CAD or so. If this is impossible, I'd like to know that too.

You can probably do some pretty good stuff with that, to get a rather noticeable difference inside the room, in terms of resonance, flutter, and overall balance, but it's probably not enough to deal with the ore serious modal issues.

3 "super chunk" corner bass traps

Those are great, but from the diagram they don't appear to be big enough. The front face should be 36" across, for a room like that.

7 2'x4' acoustic panels on the right wall
6 2'x4' acoustic panels on the left wall.
2 2'x4' acoustic panels on the back wall.

Theoretically, you need about 600 sabins of absorption in there, to get that room under control. I would go for a bit less than that for a rehearsal room: say around 500. That implies about 500 square feet of perfect absorber. Your panels total around 110 square feet, so you are coming up a bit short there. And you are missing out on the single most important surface in the entire room: the ceiling. That needs treating.

Wall panels would be mounted 9" apart in a vertically "zig-zagged" pattern (the first panel 1 foot from the ceiling, the second panel 1 foot from the floor, alternating down the length of the wall).

Great, but also "stagger" them across the room, so that each panel on one side is facing a piece of empty wall on the other side, and vice-versa. "Checkerboard" is another good option for that type of room. I would also vary the panel thickness: make some 2", some 4", and some 6", and also vary the spacing away from the wall: some right against the wall, some with a couple of inches of empty air behind them. Another thing I would do, is to make sure that there are thick panels at the 25%, 50% and 75% locations along each wall.

Or for those who prefer Sketchup: practice-space.skp (apologies for the external link, but it was too big to upload directly to the forum). Measurements are an inch or so off, but pretty close.

I tired to download it, but there doesn't seem to be anything there?

Will wall panels be sufficient?

Not really, no. The ceiling is the single largest surface in the room, and the most important, acoustically. I would really look at options for hanging things from the ceiling. They don't have to be heavy: you can make light weight absorption panels that can be hung from hooks, wires, or something like that.

Is alternating panel depth recommended? I was going to do all the panels 4" deep for broadband bass absorption, but in reality, relatively even sound absorption across the frequency spectrum with just a bit more dampening for the bass is desirable. Would a pattern like 2'x4'x4", 2'x4'x6", 2'x4'x2", etc. better achieve this?

That's why I would vary the thickness, and the spacing depth. Varying the panel sizes just means that you are absorbing more or less of the exact same frequency range at each location, but varying the depth changes the frequency range, to absorb down lower, and so does leaving empty air behind a panel.

Rather than plastering the wall with panels, would I be better off mixing diffusion and absorption techniques (bearing in mind that the primary goal is overall sound dampening)?

You could do diffusion too, it would work nicely for that room, but your budget might not extend far enough to do that.

Finally, given the dimensions of the room, what would you recommend for speaker placement (2 speakers)?

It's a rehearsal room, so the placement of the speakers isn't critical, like it is for a control room. I would just suggest keeping them away from walls, putting them in heavy stands so they are at or above head height of the students when they are rehearsing (either seated or standing, as the case may be), and angling them so that they do not point head-on at the end walls. I would probably start by trying one next to the entrance door on the left, and the other to the right of the window on the same wall, both facing down the long axis, of the room, and pointing more or less towards the opposite corner of the far end of the room. That might work, but I'd also try a few other positions, and see what makes the most sense, and sounds best.

P.S. My apologies to Soundman2020 for missing a few posting rules. Hopefully these recent post edits (and the inclusion of my location in my profile) rectify these issues. Looking forward to your reply!

No problem! It happens frequently around here.... And you fixed it very well, for sure!!!


- Stuart -

[TWU_Chapel]

Stuart, thanks for such a detailed and helpful reply!

When you say "sound damping", do you mean in order to isolate the room ( so that sound cannot get out and disturb other classes), or do you mean treatment (so that the room can be made to sound good for rehearsals)? That's two very different things, but they are often confused.

I mean treatment. Sound "escaping" the rehearsal space is not a concern. Good clarification.

Because we use acoustic drums and electric guitar, everything needs to be amplified.

OK; but I do see some mics on stands in there: how can you record vocals with mics in the same room as the drums, bass and guitars?

This is a live practice space, not a recording space.

To decrease the overall dB level within the rehearsal space as much as possible

There might be a misunderstanding here, about what acoustic treatment does. Even if you lined all the walls, floor and ceiling with a foot of absorption, the peak level inside the room would not change: If the drums produce 115 dBC, then they produce 115 dBC, and no amount of treatment will make them produce less. What it WILL do, is to damp reflections, resonance, and reverberation of the room, to a certain extent. So the room might sound quieter, simply because the sound only reaches you once at a loud level, instead of multiple times, but in reality the peak level in the room will not have changed. The only way to reduce the dB level within the room is to teach the musicians to play more quietly, and turn down their amps.

This is a really helpful clarification, and makes perfect sense. Thanks!

Floor: Loop pile carpet (commercial grade)

I guess there's no chance that you can take the carpet out?

Unfortunately not. It's a leased portable so we cannot make these sorts of modifications.

Width: 15 feet - Length: 24 feet - Height: 8 feet

There's a problem, right there. The length is exactly three times the height, and the width is very close to twice the height. So you will have some modal issues going on in there, especially around 71 Hz, 80 Hz, 103 Hz, and 113 Hz, plus their harmonics. I would guess that drums sound very ugly in there: boomy, muddy, unclear?

Yup. Drums sound terrible in the space. I hadn't considered the room dimensions as being problematic. That's interesting indeed (in the worst way possible ).

I am hoping to get this room treated for $1500 CAD or so. If this is impossible, I'd like to know that too.

You can probably do some pretty good stuff with that, to get a rather noticeable difference inside the room, in terms of resonance, flutter, and overall balance, but it's probably not enough to deal with the ore serious modal issues.

Good to know. Hypothetically, should the treatment of modal issues be prioritized over dealing with resonance, balance, etc., or the other way around? We may be able to do the project in several phases to deal with our budget constraints. What's the best way to address modal problems around the frequencies you've mentioned?

3 "super chunk" corner bass traps

Those are great, but from the diagram they don't appear to be big enough. The front face should be 36" across, for a room like that.

Check.

7 2'x4' acoustic panels on the right wall
6 2'x4' acoustic panels on the left wall.
2 2'x4' acoustic panels on the back wall.

Theoretically, you need about 600 sabins of absorption in there, to get that room under control. I would go for a bit less than that for a rehearsal room: say around 500. That implies about 500 square feet of perfect absorber. Your panels total around 110 square feet, so you are coming up a bit short there. And you are missing out on the single most important surface in the entire room: the ceiling. That needs treating.

Youch. So we need 5 times more absorption that currently proposed? I'll look into our options for attaching stuff to the ceiling. Small hooks may be permitted, I just know we are not authorized to put nails in the ceiling or other big holes.

Wall panels would be mounted 9" apart in a vertically "zig-zagged" pattern (the first panel 1 foot from the ceiling, the second panel 1 foot from the floor, alternating down the length of the wall).

Great, but also "stagger" them across the room, so that each panel on one side is facing a piece of empty wall on the other side, and vice-versa. "Checkerboard" is another good option for that type of room. I would also vary the panel thickness: make some 2", some 4", and some 6", and also vary the spacing away from the wall: some right against the wall, some with a couple of inches of empty air behind them. Another thing I would do, is to make sure that there are thick panels at the 25%, 50% and 75% locations along each wall.

Superb. This is gold.

Or for those who prefer Sketchup: practice-space.skp (apologies for the external link, but it was too big to upload directly to the forum). Measurements are an inch or so off, but pretty close.

I tired to download it, but there doesn't seem to be anything there?

Apologies. The link must have expired. You can download the file here.

I would vary the thickness, and the spacing depth. Varying the panel sizes just means that you are absorbing more or less of the exact same frequency range at each location, but varying the depth changes the frequency range, to absorb down lower, and so does leaving empty air behind a panel.

I listened to a discussion about absorption vs. membrane panels, and the effectiveness (or ineffectiveness) of placing traps directly against the wall (in this interview on Pensado's Place). Should all of my panels be off the wall by a few inches?

Rather than plastering the wall with panels, would I be better off mixing diffusion and absorption techniques (bearing in mind that the primary goal is overall sound dampening)?

You could do diffusion too, it would work nicely for that room, but your budget might not extend far enough to do that.

What about simply putting in a bookshelf or two, or taking some time to do a DIY diffuser like this? Would the improvement be marginal, or significant?

Thanks again,

Jared

[Soundman2020]

Yup. Drums sound terrible in the space. I hadn't considered the room dimensions as being problematic. That's interesting indeed (in the worst way possible)

That's what I figured. But all is not lost! It's not such a big issue overall, and your superchunks will go a long way to damping the modal issues. You might also want to make a coupled of gobos on wheels, and try maneuvering those around the room, to see if you get lucky and can break up the modal stuff a bit. They would have to be pretty large, and rather massive on the hard side, but they could help a lot. And even if they don't do anything for the modes, they are still very useful to have in a place like that, to separate instruments a bit better, or to get a certain "sound".

Hypothetically, should the treatment of modal issues be prioritized over dealing with resonance, balance, etc., or the other way around? ... What's the best way to address modal problems around the frequencies you've mentioned?

There's not a lot you can do to direct change the modal spread, since the modes are simply a consequence of the dimensions of the room: for every frequency where a half-wavelength fits in exactly between two walls (or floor and ceiling), you have an axial mode. For every frequency where a half-wave fits in between four walls (floor/ceiling), you have a tangential mode, and for every half-wave that fits in between all six surfaces, you have an oblique mode. The only way to move a mode to a different frequency, is to move the hard, solid boundaries of the room. Gobos (and some other devices) help to break up the modal action a bit, but the mode is still there: just subdued.

So unless you can move the walls and ceiling ( ), there's not much you can do about changing the modes. What you CAN do is to treat them, by damping them: Superchunks are good at that, and so are other devices, to a certain extent. But what I would suggest you do before getting into all of that, is to run an analysis on the room to see how it is responding right now, and see which modes are giving you the biggest problems, so you can hit those harder. For example, if you 0,0,1 mode is a problem (the first axial mode related to the height of the room), then putting treatment on the walls is not going to do much: you would have to treat the ceiling to deal with that. So it helps if you know which guys are causing the biggest problems, in order to give them more attention, and treat them in the right place. You can do this analysis by downloading REW from the Home Theater Shack web site (its free!), and running it on your room, then posting the data file here so we can analyze it for you, and tell you what is wrong, and what needs most attention.

Youch. So we need 5 times more absorption that currently proposed?

For a rehearsal room or live room, the actual amount is relative. The calculation I did to come up with that 600 figure is meant more for control rooms, where the requirements are very tight, but for a live room / rehearsal room you can have whatever sound you happen to like. That's why I suggested 500 ft2, since 600 would probably be a bit too dead for your type of room. You might even happen to like how 400 ft2 sounds, or even 300: I would do it in stages, adding say 50 or a hundred ft2 at a time, and stop when you like it. But I would venture to say that 100 is not going to be enough ... And regardless, you DO need something on the ceiling: That's a huge solid unbroken reflective surface up there: not good. It needs treating, if you hope to get the room under control.

Apologies. The link must have expired. You can download the file here.

OK, that one works! I'll take a look, and let you know if I see any red flags...

I listened to a discussion about absorption vs. membrane panels, and the effectiveness (or ineffectiveness) of placing traps directly against the wall (in this interview on Pensado's Place).

Unfortunately, I couldn't watch that:

video-not-avail.jpg

I'm not sure what he said, but there's a lot of confusion and misinformation about what absorbers do against walls, when in reality that's the way they are tested in acoustic laboratories! The ASTM C423 standard method for testing absorption is to lay it on the floor of the test chamber, and compare the response of the room with and without the specimen in place (known as "A" mounting). If you are interested, you can download the full specs here: http://www.astm.org/Standards/C423.htm . All the specs for absorption are actually for that exact situation: up tight against the wall.

So Regardless of what the "internet opinionators" say about it working or not working, the standard testing method show that it does in fact work, and it works very well: Absorption against a wall, ceiling or floor is effective, and down to reasonably low frequencies. (C-423 does allow for different mounting methods, including leaving air gaps behind the sample, but the usual way of testing is still flat against the surface).

Why does it work? The reason is simple: many people think that a 2" piece of mineral wall cannot possible have any effect on a wave ten feet long, since 2" is such a tiny percentage of ten feet, but in reality that's not what it is about at all. The wave only "sees" 2" of insulation if it approaches head-on ("normal incidence"), but for a wave that arrives at a glancing angle, it can indeed "see" many inches, even several feet, of insulation, and thus will be affected greatly. It all depends on the angle of incidence. And since the vast majority of sound in a room does NOT hit the walls exactly head on, it is obvious that "the vast majority of sound" does indeed see a substantially larger depth of insualtion than just the simple thickness of the panel.

But even for the case of normal incident waves that only see 2", that's still plenty to have some effect on the wave, since the wave must still pass through the entire thickness, hit the wall, then pass through it again on the way back out. Even though there isn't a whole lot of effect, some is better than none.

But "the proof of the pudding is in the eating", as they say, and the tests clearly show that it works, as does the empirical evidence of numerous real-world studios with absorption on the walls: they sound great! Regardless of the opinions expressed on YouTube videos, real life shows that absorption works.

That said, the ASTM test suite also shows that by placing the absorption away from the walls, it can work down to even lower frequencies. That's why I suggested leaving varying depth gaps: so that you get coverage at higher frequencies from some panels, extending down lower for others. So you cover more of the spectrum.

If you are interested, here's the publish specs for OC-700 series panels:

703-specs.jpg

There you can see the results for "A" mounting (flat against the floor) as compared to E-405 mounting (16 inch air gap behind), for each frequency band from 125 Hz to 4kHz. As you can see, 2" of 703 against the wall has a coefficient of .17 at 125 Hz. Put it 16" away, and that jumps to .40.

But here's another curve-ball for you: Remember that I said that your room needs 600 sabins of absorption, which implies 600 ft2 of perfect absorber? Notice the "perfect" part.... There's no such thing in the real world. "perfect" absorption is just a mathematical abstraction that imagines some magical material that absorbs exactly 100% of every single frequency, at all angles of incidence and all power levels. As you can see from the above table, even the venerable OC-703, which is a darn good absorber, is still far from "perfect". So in reality you need MORE than 600 ft2 of "imperfect" absorption, and you need to adjust some of it to better cover frequencies that it doesn't cover naturally. Another reason for spacing some of it, not all of it.

Ain't acoustics simple and intuitive? (not!).

Should all of my panels be off the wall by a few inches?

No. For the same reasons as above.

What about simply putting in a bookshelf or two,

That can have some effect, but unpredictable. Considering your modal issues, I would consider something like large poly-cylindrical diffusers against some walls, combined with absorption on the ceiling.

or taking some time to do a DIY diffuser like this? Would the improvement be marginal, or significant?

I watched that video, even though I was tempted to quite with the opening titles! They author claims that diffusers work on "standing waves" (ie "modes". Sorry, but they just don't. In small rooms ("small" with respect to the size of low frequency sound waves), the real modal or "standing wave" issues are in the low end of the spectrum, below about 200 Hz. In order to build a QRD diffuser that can deal with such waves, the wells and columns would have to be many inches wide, and they would have to extend out many feet into the room... Notice that video even mentions that the low cut-off frequency for that devices is 1500 Hz.? Just imagine that to make it work at 150 Hz, each part would have to be ten times bigger.... I think you get the picture!

The rule is: "The maximum depth of the wells determines the effective low frequency limit of the
diffusers. The well depth should be 1½ times the wavelength at the lowest frequency." The wavelength for your 103 Hz mode is about 11 feet.... 'nuf said...

Diffusers can be useful, and they normally are tuned to frequencies in the mid range, such as that one, but they will do absolutely nothing for the real standing wave issues in the average room, since the are tuned way, way to high, and are far, far too small to have even the slightest effect on modal issues.

QRD diffusers are also a lot of work to build, not cheap, and very heavy: they have to be made of solid wood to be useful.

I would not suggest using numeric-based diffusers in your room. Angular or poly-cylindrical would be good though.


- Stuart -

[TWU_Chapel]

This is gold, Stuart. Many thanks.

Gobos (and some other devices) help to break up the modal action a bit, but the mode is still there: just subdued.

Got it. I had someone else suggest gobos, to place around louder instruments like the drums. If they help to mask the modal issues a bit, all the better.

it helps if you know which guys are causing the biggest problems, in order to give them more attention, and treat them in the right place. You can do this analysis by downloading REW from the Home Theater Shack web site (its free!), and running it on your room, then posting the data file here so we can analyze it for you, and tell you what is wrong, and what needs most attention.

I'm looking into REW now but discovered my SPL meter doesn't have a line out, so I'll see what I can dig up.

the standard testing method show that it does in fact work, and it works very well: Absorption against a wall, ceiling or floor is effective, and down to reasonably low frequencies.

This is great news for me and anyone else who has a small-ish working space. Thanks for clearing this up.

in reality you need MORE than 600 ft2 of "imperfect" absorption, and you need to adjust some of it to better cover frequencies that it doesn't cover naturally. Another reason for spacing some of it, not all of it.

I'll go with checkerboard, or at the very least make sure wherever I place a panel there's some empty space on the opposite wall. Thanks for the advice.

Diffusers can be useful, and they normally are tuned to frequencies in the mid range, such as that one, but they will do absolutely nothing for the real standing wave issues in the average room

Got it. I'll focus on absorption for now and if I end up getting a bit more budget (or time on my hands), I can look into diffusers. This helps me prioritize. Thanks.

I would not suggest using numeric-based diffusers in your room. Angular or poly-cylindrical would be good though.

I'm not familiar with the math behind diffusion. Is numeric-based diffusers simply not as effective, or does it have more to do with my room dimensions?

Thanks again for your insights, Stuart. This is a treasure trove of info, and will help me in future projects as well.

Best,

Jared

[Soundman2020]

I'm looking into REW now but discovered my SPL meter doesn't have a line out, so I'll see what I can dig up.

Any decent omnidirectional mic will work fine.

I'm not familiar with the math behind diffusion. Is numeric-based diffusers simply not as effective, or does it have more to do with my room dimensions?

Diffusion, at its simplest, is just reflecting sound in random directions, instead of normal "mirror" type reflection. So instead of the sound coming back at you as a hard "echo", it sort of breaks up and scatters "blurrily". That's useful, acoustically.

There are different ways of doing that. One very simple way is to have several surfaces arranged at different angles, so different parts of the wave are reflected in different directions, depending on which surface they happened to hit, and the angle they happened to hit it at. So a flattish "pyramid" shaped thing works well for that. Another approach is to used curved convex surfaces: they do the same thing, only better, since the curve changes continuously. Some people take large "sonotubes" (cardboard tubes used in casting concrete pillars and columns), cut them in half lengthwise, and put them up against the walls. Another method is to bend thin wooden panels to a curved shape. A more refined approach is the "poly-cylindrical" diffuser, which is a bit different in shape. With a cylinder, the curve remains constant everywhere (same radius) but with a poly-cylinder, the curve itself changes, flatter in some places and rounder in others. If you put a sheet of paper flat on the table, put your hands over the left and right edges, and slide your hands towards each other a bit, then the "lump" shape that rises in the middle is "poly-cylindrical". Do the same thing on a larger scale with a full sheet of thin plywood, and you have a really good poly-diffuser.

Those are simple diffusers, but they can still be very effective, and they are not really "tuned" to any specific frequencies: Bigger ones affect down to lower frequencies, but apart from that, they are not really tuned.

There's another class of diffuser that is tuned: numeric-sequence diffusers. There are several variations, but the basic concept is that there is a series of sharp-edged "wells" cut into some substrata, and the well depths are calculated using some form of pseudo-random mathematical equation. So as sound waves hit the wells, they travel down the well, hit the bottom, bounce back up, and emerge again: But due to the mathematics, they leave at a different angle, and also with a different phase and different timing. Since each part of the wave hits a different "well" that has a different depth, it takes a slightly longer or shorter time for it's journey down the well and back, so it emerges slightly earlier or later than its neighbors, and heads off in a slightly different direction. So these devices scatter waves not just in space, but also in time and in phase-relationships. Each part of the wave still retains its original frequency, but is all muddled up in angles, directions, timing and phase, with respect to the other parts of the wave. So this spreads the energy around the room "diffusely", which is good.

These numeric-sequence based diffusers come in various types, but there are basically three types 1-D slats, 2-D skylines and QRDs, and more complex 3-D fractal-like things, where there are "wells within wells". The basic theory is the same.

The interesting thing about them, is that they can be tuned to affect only certain frequency ranges, by changing the dimensions of the wells: The depth of the wells sets the low frequency limit, and the width of the wells sets the high frequency limit. For frequencies higher than the upper limit, it just acts like a flat plate, and for frequencies lower than the lower limit, it does nothing useful.

That's the theory, anyway...

So those are your choices for diffusion. Polys are the easiest to build, by far, and are very effective. And cheap. They also look pretty nice, too.

- Stuart -