Acoustic Treatment for Small Listening Rooms: Absorption vs. Diffusion

tmallin

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May 19, 2010
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I've been using acoustical treatments for my listening rooms since about 1984 which was when my listening area went from a shared space to a dedicated space. Then, as now and at all times in between, my audio rooms have been rather small, especially in terms of the width dimension, meaning that the speakers and listening position usually ended up within no more than a few feet (often closer) from the walls. I have never had an audio room whose dimensions allowed the speakers to be placed ten feet or more from the walls beside and behind them. Such distances would probably allow any reflections of sound from the room surfaces to take the subjective form of pleasing reverberation, rather than annoying echo, brightness, or brittleness. And unless you're dealing with mid-air mounting of speakers in a massive ballroom or auditorium, there is no speaker position which will place the speakers ten feet away from both the floor and ceiling.

My earliest efforts at treating room acoustics involved pinning unused blankets and towels to room surfaces in an effort to cut down on the audible "slap echo" one hears in a small room when critically listening to an audio system. While I have a few pictures of those early efforts, I won't bother posting them. Trust me, the visual effects were, as you can imagine, rather patch work in nature—not appealing at all. Sonically, yes, it was an improvement in terms of reducing high-frequency glare and a bit of echo, but I could clearly tell that something more effective was still needed.

After blankets, I moved on to using Sonex acoustic foam as my primary acoustic treatment, first in three-inch thicknesses and later in the four-inch-thick version. My use of Sonex was influenced strongly by an article Bert Whyte wrote in Audio magazine about using Sonex to create an LEDE (Live-End, Dead-End) listening room. Because of its futuristic/space-age pattern (the wedge pattern was often part of Star Trek episode decor, for example) Sonex, even in its thicker versions, could be easily mounted to drywall surfaces using an electric staple gun (as Bert Whyte recommended) rather than adhesives. It could thus be moved around with comparative ease without damaging the room surfaces too much. Usually touch up paint could cover the staple holes even without any spackling, assuming your room, like mine, was painted with latex flat paint in a forgiving color such as the classic construction favorite, Glidden Bone.

I used Sonex in my listening rooms for decades. It was only upon my purchase of the Janszen Valentina Active speakers that David Janszen suggested to me that I might want to try acoustic diffusers in place of some of my sound absorbing Sonex pads.

Thus, for the past few years I've been leaning heavily on dispersive room treatment, primarily in the form of the P. I. Audio Group AQD1 Quadratic Residue Diffusers. These are basically 47.5" x 23.5" x 6" panels made of polystyrene, the type of "hard" foam material used my some manufacturers of electronic equipment for custom packing moldings inside the shipping box. The price is certainly right: a box of two male/female mated pairs of these costs only $160 in unpainted white, which is what I've used recently. Before that, with the Janszen speakers I also owned a set of grey panels which P.I. Audio custom painted before shipment to me; painted, they cost $200 for two pairs.

These P.I. Audio Group diffusers have proved fine for treating my small room acoustics as long as I use enough of them. They excel with speakers which have more controlled mid- and high-frequency dispersion. They are at their best where the speakers sound quite "closed in" or even "dull" outside the sweet spot, as is definitely the case with the hyper-directional Janszen Valentina Active speakers I had in my room. These speakers used a side-firing "Air Layer" tweeter to help open up the sound of the speakers (make them "airier" sounding, if you will) when heard from outside the sweet spot. Placing the AQD Diffusers on the side walls and wall behind the speakers in the areas where, from the listening position, one could see any part of the speaker reflected in a mirror placed flat against the wall, eliminated any harsh reflections of treble energy from the flat walls, breaking up the reflection by scattering it in multiple directions—dispersing the reflection rather than allowing it to reflect—as from a mirror—off the smooth surfaces of my room.

In other words, in my small room, in situations where I do not want the speakers to sound any duller in terms of room sound than they already do without room treatment, these diffusers and other diffusive treatments, such as CDs on the variably angled shelves of my Stor-a-Disc CD racks, or wooden venetian blinds covering windows, work well.

These diffusers also have proved pleasing in their effect with speakers with much broader dispersion, including Harbeth M40.2, Gradient 1.4, and now my Dutch & Dutch 8c models. Granted, all of these have more "controlled" dispersion than speakers which are billed as "wide dispersion," much less "omnidirectional." I strongly suspect that with such speakers, especially in a small room such as mine, a good deal of absorptive room treatment would be beneficial.

With the Harbeths, as with the Janszens, I used a combination of absorbing Sonex foam and the P.I. Audio diffusers. But with both the Gradient 1.4s. and now my Dutch & Dutch 8c speakers, I've had great success with banishing Sonex from the room entirely and relying on using a lot of the diffusion panels to break up reflections and keep the room quite "live" sounding without any echo or harshness audible on music material.

I also strongly suspect that the subjectively pleasing effects I've heard from using all-dispersive room treatment with the Gradients and D&Ds relate to the uncharacteristic smoothness of the off-axis response of both speakers. Gradient speakers have off-axis frequency response which is quite smooth below 8 kHz or so. See, for example, John Atkinson's lateral measurements of the Gradient Revolution in Figure 4 on this page. The D&Ds are even better, with only a smooth gentle rolloff in the measured response to the sides of the primary axis of the speaker; see Atkinson's measurements in Figure 3 on this page. Most speakers are not nearly so smooth in their off-axis response. The smoothness probably decreases the necessity of absorbing the off-axis sound which would otherwise reflect off room surfaces since the off-axis response from these speakers is not as colored as it is with most speakers compared to the response on the design axis.

With the Dutch & Dutch speakers especially, even an untreated room does not sound obnoxious in the way it does with most speakers. Yes, all program material sounds livelier than it probably should. This is most evident when critically listening to the sound of radio announcers in a studio (which is almost universally a small dead-sounding space) or the Clap Track test tone on the Sheffield/XLO Test & Burn-In CD. But on most music, the added reverb creates a more open sound, a bit more liveliness, but rarely sounds obnoxious. Diffusive room treatment reduces the added liveliness, but even with large portions of the room surface treated does not make the sound dull or closed in. Diffusive room treatment also improves all aspects of the spatial performance of loudspeakers in my small room compared to untreated room surfaces.
 
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tmallin

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Still, correct is correct. Recent discussions on REG's forum have led me to reconsider my choice of room treatment for my small (13' x 11' x 8.5') audio room. Ideally, the Clap Track should sound the same way through your speakers as it does through headphones: a single clap transient with absolutely no following echo or ambient trail. Announcers in studio should sound like they are speaking in a small acoustically absorptive room, not a normal house room. There is no way that this happens with just dispersive room treatment, at least in my small listening room.

I have also noticed that since using primarily or exclusively diffusive room treatment, program material does not seem to have as much depth of imaging as I once conjured in this same room. Further, while my reproduction is eye-poppingly open and large for such a small room, I have sensed a bit too much similarity in the spatial presentation from one recording to another.

Thus, on the theory that, particularly in a small room, the listening room's second-venue effect should be maximally suppressed since one's small-room acoustics can add nothing helpful or meaningful to the reproduction of well-recorded program material, I decided to try switching back to a combination of absorption and diffusion to treat the acoustics of my small room.

Going back to my LEDE roots, I'm now using absorptive room treatment at the speaker end of the room (the Dead End), while the listening end relies entirely on dispersive room treatment (the Live End). The theory is to use absorptive room treatment to suppress the mid-and high frequency early reflections since such reflections occur quite early/fast in a small room and thus add nothing helpful to the sonic presentation. They blur imaging and staging, produce colorations in the frequency balance, as well as adding obnoxious echo, brightness, grit, and grunge. Such reflections also seem to fill in the silences in between and around notes, presenting a less quite/"noisier" presentation. Further down, such reflections add to the "room roar" effect many have talked about.

Reflections that take more time (at least ten milliseconds longer) to get to your ears than the direct sound from the speaker can be helpful, at least in larger rooms, by increasing the apparent size of the listening space. But in a small room like mine, even the areas of the room behind me are just on the border of being "late" enough to be beneficial. Thus, in my experience it helps to at least break up even the reflections from behind the listening seat.

Thus, behind the listening seat I'm retaining the six P. I. Audio Group AQD1 diffuser panels. These panels cover the general areas where I can see the speaker reflections from the listening seat if I place a mirror flat against the wall behind the listening seat. Additional dispersion in the listening end of the room is provided by my large Stor-a-Disc CD racks with variably angled shelves and a bookcase stuffed with audio magazines.

For the first reflection areas on the walls behind the speakers, the side walls, and floor, I'm now using Alphasorb Flat Foam, the version which is two-foot by four-foot, four-inch-thick white melamine foam. These panels are flat on all sides; they have no sculpturing and thus are four inches thick at all spots of the panel. I purchased these from Acoustical Solutions; see this link. Each shipping box of this foam contains three sheets and covers 24 square feet. I'm using six boxes of this foam, for a total coverage of 144 square feet.

As the specs for this 4-inch-thick foam indicate, the foam has a fairly consistent absorption of at least the same as an open window (defined as an absorption coefficient of 1) from about 250 Hz up through the treble range. Like most foam of less than a foot or so thickness, the absorption falls off greatly in the bass range. For the 4-inch foam, the absorption coefficient is down to 0.58 by 125 Hz and I'm sure falls off steeply below that.

I should state that I have never attempted to use foam or other acoustical products to construct a "bass trap" for the low bass ranges. Most speakers are bass deficient anyway, at least in the bottom octave (20 to 40 Hz) or even the bottom two octaves (80 Hz down). Why get rid of the bass your speaker is cranking out? It seems better, in my judgment, to use a combination of careful speaker and listening position placement, combined with electronic equalization to smooth the range below 200 Hz. That is exactly what the design and recommended placement of the Dutch & Dutch 8c speakers does.

The amount of absorbing acoustical foam I'm using may be overkill; the worksheet accompanying this product suggests that for my room size I would only need half this much material to reduce the reverb time to below half a second. But I can always reduce the amount of foam coverage. I'm using the 144 square feet of foam to treat 8' high x 6' wide areas on each side wall at the speaker end of the room, two 8' high x 2' wide areas on the wall behind the speakers toward the room center, and two 4' x 2' areas on the floor. These treated areas are the general areas where reflections of some part of the speakers would be visible from the listening position from a mirror mounted flat against the room surfaces.

I currently am not treating the ceiling reflection. Previous experience with placing foam on or near the ceiling indicates to me that I will only need 2' x 2' areas treated. I have additional Alphasorb Linear Foam in 2' x 2', four-inch thickness to treat this area if desired. My prior experience suggests that is very easy to "overdamp" the ceiling reflection with the result being reduced soundstage height and a "dead" sound. A little foam goes a long way on the ceiling, in other words, in my experience.

Subjectively, compared to the all-dispersion room treatment, this new absorbtive foam/dispersive panels combination provides a lot quieter/less reverberant result. This took just a bit of getting used to, but after half an hour or so the virtues of this arrangement were already strongly shining through.

The Clap Track test track I mentioned now sounds just like it does on headphones--no slap echo audible at all from the listening seat. Radio announcers now sound like they are talking from a small very padded, non-reverberant room, as they should. Center imaging is yet more focused and monophonic recordings image at a much smaller center spot. Stereo recordings which play with phase are quite a bit more expansive. Depth of field is greatly expanded.

Tonal balance is at least as good as before with additional reductions in any residual harshness when I turn up the playback SPL. It is not as though the highs are reduced in level compared to what they were before. The tonal balance seems similar. However, the lack of bouncing sound seems to clarify and clean up the upper range in such a way as to make the treble generally more natural sounding, more a part of the instrumental sound, not tacked on as a separate element. The treble is both cleaner and more filigreed. There is more stick sound with struck cymbals and the cymbal shimmer is both airier and cleaner, with less of that dreaded "escaping steam from a valve"/hissy sound.

The imaging and staging remain utterly consistent and stable regardless of playback volume, with no tendency to smear a bit as levels get high as there usually is without absorptive room treatment. There are now considerably greater differences in the spatial presentation from track to track and disc to disc than was present with all-dispersive room treatment. In my book, that's a good thing—a very good thing—if one is aiming to reproduce what is actually on the recording, instead of moving all recorded material a bit to the more "user friendly" side. Recordings where the space is handled poorly—e.g., a lot of mono recordings and ping-pong stereo recordings—sound poorer now, but good/great ones sound stunningly better now.

Some practical considerations: Like the diffusers, this 4-inch-thick foam is quite stiff enough to stand up on its own, even when stacked vertically or horizontally for an 8-foot-tall array, and thus, at least in my one-person listening room with no pets and no kids allowed, it doesn't have to be physically attached to the walls. That makes changing position of the foam so much easier.

My old Sonex foam panels, even in the stiffest four-inch-thick female wedge pattern, were never quite stiff enough to easily stand on end unless leaned against a wall and not stacked. Thus the need for stapling Sonex to the walls.

The polystyrene diffusion panels were quite light in weight—about two pounds each—especially compared to diffusers made from wood slats. This Alphasorb Flat Foam is much lighter yet, probably no more than half a pound per panel. In this respect it is much lighter than Sonex foam.

As with all new acoustical foam I've bought over the years, there is a bit of an odor which, if true to form, will totally dissipate in a week or two.

It's early yet, but I'm already fairly certain that this new LEDE arrangement is a real acoustic-treatment winner for music listening in my small-room situation. The absorption treatment at the speaker end of the room lets the recorded ambiance shine through with minimal contribution from my small room acoustics while the dispersion treatment at the listening end of the room keeps the room from sounding closed in, small, and dead as can too easily happen when the room is uniformly treated with absorption.
 
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tmallin

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Here are a few wide-angle pictures of the newly configured speaker end of my listening room.


IMG_7882.jpg


IMG_7883.jpg IMG_7885.jpg


And here is a wide-angle picture of the Live End (dispersion-treated) listening end of the room:

IMG_7886.jpg
 
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MadFloyd

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Thank you for taking the time to write this and share it. Happy new year!
 

Direct Drive

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Very interesting and thank you for the explanation.
Interestingly I have over the holiday changed my room around, moving the speakers from the livelier end to the deader end. The outcome has not been as successful as I hoped.
I have had to move damping out of the dead end. Still got work to do and will get some diffusers to try.
I have Magnepan speakers so they are different in behaviour.
 

tmallin

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Yes, Maggies--or any dipole radiator speaker for that matter--are certainly different beasts from a dispersion perspective as well as a room treatment perspective. While dipoles put out little sound to the sides and toward the floor or ceiling, much of the sound signature you hear from such speakers (especially full-ranged dipoles like the Maggies) results from bouncing the sound off the wall behind them since just as much sound comes out the back as comes out the front of the speaker.

Plus, if you toe in the Maggies more than just a bit, you will end up engaging the side walls in this reflection as well. Thus, if you aim the Maggies at your ears, the speakers will be using not only the wall behind the speakers, but also the side walls, to create their characteristic sound.

Damping the walls behind and to the sides of dipole speakers will thus change the sound of those speakers you hear from the listening position A LOT. Regardless of whether you hear this difference as an improvement or not, it certainly will be a major difference.

After decades of experience with direct radiators, omnis, wide dispersion speakers, controlled dispersion speakers, and dipole speakers, I've concluded that dipoles just don't work very well in small rooms like I've used. To get the best results from such speakers, you need a large room where the panels can be placed at least 8 feet from the wall behind them and ideally that far from the side walls to allow the strong rear radiation to be heard as pleasing ambiance rather than annoying early reflections.

And even in a very large listening room the sound of a dipole speaker will never produce the correct result on the Clap Track test track I mentioned unless the rear radiation is very heavily damped. But once you so damp the rear radiation, the dipole speaker will lose much of its "magic." This is a conundrum I've chosen to avoid by limiting my speaker purchases in recent years to non-dipole speakers and concentrating on speakers with much more "controlled" dispersion.
 
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tmallin

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For those who are a bit uncertain, about how to find the places where the absorbers or diffusers should be placed in the room, here is a link to my tutorial on this from years back: "How to Find and Damp Specular Mirror-Like Room Surface Reflections"

Today I would emphasize that the "spots" I refer to as where damping/diffusion should be placed are best understood as "areas" since the reflections are not perfectly mirror-like. It helps to treat areas around the spots where, when viewed from the listening position, you can see any part of the speaker reflected in the mirror.

I like to use a 4" x 6" flat mirror with a smooth flat backing so that it lays flat against a wall, floor, or ceiling. I use a single piece of masking tape attached to the top of this mirror to hang it on the walls and two pieces--one each at each short end of the mirror--to attach it to the ceiling. No tape is necessary for the floor, obviously. One of the following currently available mirrors should be suitable if you can't find one these days with a plain cardboard backing like the one I bought decades ago:

Plano Glass Mirror

Plastic Mirror

Magnetic Locker Mirror
 
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BruceD

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Indeed basic and user friendly technique-- can be helpful in most setups

-but overly enthusiastic treatments in some cases can be to the detriment I agree.

Therefore careful listening and appraisal is mandatory --

All as tm above posts --myself I use a BOSCH DR130 laser at Tweeter height to the Reflective to the ear level at listening seat.

My ASC Tube traps -prudently dispersed I may add! --have been with me since 1990 and still going strong :) !

In spite of the disdain shown on the site for these products --if treated properly with placement I've found improvements

that are successful.

YVMV

BruceD
 
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MTB Vince

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I love my Tube Traps too but wish they weren't so expensive @BruceD. Treating just the four corners of a moderate sized room floor-to-ceiling with 16" or 20" traps is a $7k-$8k USD undertaking- here in Canada with my 11' ceiling it was almost $14k CAD.
 
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BruceD

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Hmmm-Yes they are not the cheapest out there but I've tried a couple of the alternatives and guess just decided to stick with those --I've moved

houses fair bit as well since the early 90's and it's handy just to slide em back into their cartons and they are off to their new abode-ha!

Re the pricing I get Arts newsletters--they have good deals on 2 packs at present --did you not qualify for any of those?

I can quantify my original cost in1990--they are still with me after 31years--I use 8.

I'm sure yours will still be part of your setup --maybe 30 odd years as well-- as Gucci once said

"The quality remains long after the price is forgotten" ;)

Good listening

BruceD
 

sbnx

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Looks very nice and I am sure it sounds great.

Personally, FWIW, I think that an RT60 of 0.5 is too high. At this level it seems that we are listening at least as much to our room rather than the speakers. Even 0.35 to seems a little "acoustically noisy" to me. But some people like it.

Something to try that is pretty cheap if you still have the need. Put 3 skyline diffusers on the ceiling just behind your listening position. This scatters the sound and gives a little more envelopment. (These can be the polystyrene type) This guy sells the bigfuser 1 in black or white for $87 ea. (I have no affiliation with this company)

 
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tmallin

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As I mentioned, the instructions for the foam I'm using indicated that to get an RT60 of 0.5, I would need 74 square feet of the foam for a room my size. I'm using twice that much foam, so my RT60 should be considerably below 0.5. The only reason I mentioned the RT60 of 0.5 is that this figure was the lowest RT60 provided in the manufacturer's chart.
 
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tmallin

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I experimented with less foam on my side walls--I temporarily eliminated the four panels which were furthest from the speakers, closest to the listening seat. The effect of this change was not bad and added back some "life," but lost some imaging/staging specificity and resulted in a tiny bit of ring on the clap track. With my mirror placed against the side walls, I could see just a bit of reflection on the side walls of the opposite side's speaker from the listening seat. I put the foam back.

Then I tried adding some of the prior white foam I had (AlphaSorb Linear Foam, 4 inches thick, two 2-foot by 2-foot panels) into the very center right over the equipment rack. I turned the triangular cut out side toward the wall to match the smooth-surface appearance of the other foam. As luck would have it, these pieces wedged nicely into the gap between the existing foam "towers" on either side of the equipment rack.

This produced a shockingly HUGE improvement in three dimensionality! I've never had this happen before with other speakers since the very center of the wall behind the speakers is not a reflection point of the speakers as viewed from the listening position. Maybe it has to do with how close the D&D speakers are to that wall or the fact that the subs bounce sound off that wall. In all my prior set ups I've kept the speakers a lot further from the wall behind them.

But, in any event, adding that foam (an area of 2' x 4' high) greatly increases three dimensionality (depth of imaging beyond the wall, depth of close miked soloists more out in the room toward me, solidity of those images), specificity of imaging AND, for some reason, adds back the "life" I heard when subtracting the side wall foam. Now I know why a lot of pictures of audiophile systems show room treatment right in the center. But, as I said, this had never worked for me before.

With this new arrangement, the depth of field even with sighted listening is fully the equal of other set ups I've had with prior speakers in this room with those prior speakers placed way out into the room. And in all other ways, the D&Ds remain the best I've heard in this room by a significant margin.
 
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