Acoustic Treatment for Small Listening Rooms: Absorption vs. Diffusion

Cellcbern

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You can't break down low frequency waves or avoid room nodes with such little mass.

As for the Mandalorian, sure, you can do that for voices as they don't contain low frequency information. Big difference between reflections of vocals and acoustic bass, drums etc.
The ZR technology was developed for and is used most extensively in recording studios where there is no scarcity of drums and bass. Did you look at the "Portfolio" link I posted which lists their large and growing number of pro audio customers? What I see over and over when I read pro audio reviews like this one (see link below) is references to improved bass from deployment of these thin panels. And in my listening room the bass heft and clarity improved significantly so I gave away my ASC tube traps. Mass matters with conventional acoustical treatments. It is irrelevant to the ZR panels which appear to represent a disruptive technology with respect to conventional acoustical room treatment. See the comment on improved bass response in:

 

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Beautiful!
 

Cellcbern

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tmallin

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Acoustical research suggests that the listening room's floor reflection is the one most damaging to imaging and staging. For years I have treated the floor reflection by laying flat foam over the carpet on the floor between the listening position and the speakers. The problem with such foam treatment is that it usually must extend fairly close to your feet (at least if you listen in the near field as I do) and thus creates a trip hazard in the room, especially for getting in and out of the listening chair.

Robert E. Greene (REG of TAS) recently suggested placing the sound absorbing material in a vertical orientation rather than covering the floor with it as I have done. This is an excellent suggestion. It may have advantages over the foam-flat-on-the-floor method I've used for decades.

First, it keeps the floor clearer of trip hazards. Second, foam or fiberglass is more effective in absorption if it is standing in free air. Third, you'll need less sound absorbing material this way, or at least you should use less to keep the top of the "barrier" low enough to keep it from interfering with the direct sound from the speakers.

Of course, you could drape heavy cloth material over a stand as REG describes for free. That may well work just fine. But without trying it, I would worry that the stand structure might have some residual reflectivity even if covered by the heavy material.

The 4-inch thick flat foam I use is ideal for this experiment since it is square on the edges and thus is quite stable for free standing a, say, two-foot-by-two-foot slab on edge on the floor.

To figure out where to put the foam, place a small rectangular flat mirror (I use a 4" x 6" version) on the carpet between you and the speaker with the long axis aligned between the speaker and the listening position. Then arrange the mirror so that when you are sitting in the listening position you see the tweeter and midrange reflected symmetrically along the long axis of the mirror. Then place your acoustic foam/fiberglass slab between the listening position and the speaker so that it is normal (at a right angle to) to the mirror at a spot which totally hides the mirror from your gaze when you are sitting in your listening position. Repeat for the other speaker.

To do this as I describe you need acoustic foam or fiberglass which is stable enough to be free standing on carpet. I know that traditional Sonex wedge pattern foam is not stable enough to do this. But the Alphasorb Flat Foam I currently use definitely is stable enough, at least in the four-inch thickness. This foam is both very light and very stiff and that makes it stable standing on edge in a two-foot-by-two-foot slab. See: https://acousticalsolutions.com/product/flat-foam/ This costs $300 for a box of six sheets. You'll only need two sheets to treat the floor. You could use the rest on the ceiling or walls, or share with a friend or two.
 
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ACHiPo

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Acoustical research suggests that the listening room's floor reflection is the one most damaging to imaging and staging. For years I have treated the floor reflection by laying flat foam over the carpet on the floor between the listening position and the speakers. The problem with such foam treatment is that it usually must extend fairly close to your feet (at least if you listen in the near field as I do) and thus creates a trip hazard in the room, especially for getting in and out of the listening chair.

Robert E. Greene (REG of TAS) recently suggested placing the sound absorbing material in a vertical orientation rather than covering the floor with it as I have done. This is an excellent suggestion. It may have advantages over the foam-flat-on-the-floor method I've used for decades.

First, it keeps the floor clearer of trip hazards. Second, foam or fiberglass is more effective in absorption if it is standing in free air. Third, you'll need less sound absorbing material this way, or at least you should use less to keep the top of the "barrier" low enough to keep it from interfering with the direct sound from the speakers.

Of course, you could drape heavy cloth material over a stand as REG describes for free. That may well work just fine. But without trying it, I would worry that the stand structure might have some residual reflectivity even if covered by the heavy material.

The 4-inch thick flat foam I use is ideal for this experiment since it is square on the edges and thus is quite stable for free standing a, say, two-foot-by-two-foot slab on edge on the floor.

To figure out where to put the foam, place a small rectangular flat mirror (I use a 4" x 6" version) on the carpet between you and the speaker with the long axis aligned between the speaker and the listening position. Then arrange the mirror so that when you are sitting in the listening position you see the tweeter and midrange reflected symmetrically along the long axis of the mirror. Then place your acoustic foam/fiberglass slab between the listening position and the speaker so that it is normal (at a right angle to) to the mirror at a spot which totally hides the mirror from your gaze when you are sitting in your listening position. Repeat for the other speaker.

To do this as I describe you need acoustic foam or fiberglass which is stable enough to be free standing on carpet. I know that traditional Sonex wedge pattern foam is not stable enough to do this. But the Alphasorb Flat Foam I currently use definitely is stable enough, at least in the four-inch thickness. This foam is both very light and very stiff and that makes it stable standing on edge in a two-foot-by-two-foot slab. See: https://acousticalsolutions.com/product/flat-foam/ This costs $300 for a box of six sheets. You'll only need two sheets to treat the floor. You could use the rest on the ceiling or walls, or share with a friend or two.
Do you have a pic? I’m struggling to understand the approach.
 

BlueFox

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What type of carpet do you have? I don’t have any floor reflections with my carpet.

A5BF7DE6-E31A-4604-9D42-E19AEA16A788.jpeg
 

tmallin

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Carpet on the floor does provide some absorption of reflected sound. However, even a one-inch thick heavy pile carpet will not absorb more than the top couple of octaves (5000 Hz up) with any degree of effectiveness. You need absorptive foam or fiberglass of significant thickness to absorb well down through the midrange. With most speakers, adding 4-inch foam flat atop any carpet in the right areas will yield an obvious improvement. My experiments over the years indicate that adding additional layers up to a foot thick keeps improving things. This vertical positioning of foam in the right spots I'm describing may be yet more effective for the reasons I stated.
 

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Cellcbern

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Acoustical research suggests that the listening room's floor reflection is the one most damaging to imaging and staging. For years I have treated the floor reflection by laying flat foam over the carpet on the floor between the listening position and the speakers. The problem with such foam treatment is that it usually must extend fairly close to your feet (at least if you listen in the near field as I do) and thus creates a trip hazard in the room, especially for getting in and out of the listening chair.

Robert E. Greene (REG of TAS) recently suggested placing the sound absorbing material in a vertical orientation rather than covering the floor with it as I have done. This is an excellent suggestion. It may have advantages over the foam-flat-on-the-floor method I've used for decades.

First, it keeps the floor clearer of trip hazards. Second, foam or fiberglass is more effective in absorption if it is standing in free air. Third, you'll need less sound absorbing material this way, or at least you should use less to keep the top of the "barrier" low enough to keep it from interfering with the direct sound from the speakers.

Of course, you could drape heavy cloth material over a stand as REG describes for free. That may well work just fine. But without trying it, I would worry that the stand structure might have some residual reflectivity even if covered by the heavy material.

The 4-inch thick flat foam I use is ideal for this experiment since it is square on the edges and thus is quite stable for free standing a, say, two-foot-by-two-foot slab on edge on the floor.

To figure out where to put the foam, place a small rectangular flat mirror (I use a 4" x 6" version) on the carpet between you and the speaker with the long axis aligned between the speaker and the listening position. Then arrange the mirror so that when you are sitting in the listening position you see the tweeter and midrange reflected symmetrically along the long axis of the mirror. Then place your acoustic foam/fiberglass slab between the listening position and the speaker so that it is normal (at a right angle to) to the mirror at a spot which totally hides the mirror from your gaze when you are sitting in your listening position. Repeat for the other speaker.

To do this as I describe you need acoustic foam or fiberglass which is stable enough to be free standing on carpet. I know that traditional Sonex wedge pattern foam is not stable enough to do this. But the Alphasorb Flat Foam I currently use definitely is stable enough, at least in the four-inch thickness. This foam is both very light and very stiff and that makes it stable standing on edge in a two-foot-by-two-foot slab. See: https://acousticalsolutions.com/product/flat-foam/ This costs $300 for a box of six sheets. You'll only need two sheets to treat the floor. You could use the rest on the ceiling or walls, or share with a friend or two.
DHDI's research concluded that the wall behind the speakers was the most important to treat when it comes to improving imaging and overall clarity. My deployment of their ZR Acoustics panels confirms that. With 56 sf of ZR panels deployed on my walls imaging is superb and clarity has improved dramatically over when I had only conventional panels behind the speakers and on the opposite wall. I am experimenting now with absorption on the floor but so far even 6" of fiberglass at the floor first reflection points makes little if any audible difference. Note that I do have RPG BAD panels at the ceiling first reflection points.

Dedicated 16d x 9.5w x 8h music only room acoustically treated with DHDI's Zero Reflection panels behind the speakers and listening position augmented by a mix of RPG BAD and Gik 242 and 244 panels on the side walls and ceiling.
 
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MTB Vince

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Carpet on the floor does provide some absorption of reflected sound. However, even a one-inch thick heavy pile carpet will not absorb more than the top couple of octaves (5000 Hz up) with any degree of effectiveness. You need absorptive foam or fiberglass of significant thickness to absorb well down through the midrange. With most speakers, adding 4-inch foam flat atop any carpet in the right areas will yield an obvious improvement. My experiments over the years indicate that adding additional layers up to a foot thick keeps improving things. This vertical positioning of foam in the right spots I'm describing may be yet more effective for the reasons I stated.

It is not quiite as simple as standing the 4" slabs on their side @tmalin as this orientation results in a reduced absorption bandwidth. The lower frequency absorption roll-off point of a fibrous (velocity based) broadband absorber like we are discussing is based upon the total depth of absorption material which the incident sound waves must pass through. To absorb a given frequency wavelength, our absorber must force the sound wave to travel through a depth of absorptive material equivalent to a quarter of its wavelength.

When the absorptive panel is laying flat against a boundary surface addressing primary reflection points on the sidewalls, floor, or ceiling we get to add the distance the wavelength must travel as it enters the panel at an angle to the distance it must again travel after it bounces off the boundary and travels back out through the fibrous panel. So for a wavelength approaching the panel at 45 degrees, the incident sound wave actually travels 5.66" passing inwards through your current 4" foam slabs and another 5.66" passing outwards again after reflecting off of the rigid wall/floor/ceiling boundary. That 10.33" total distance is the 1/4 wavelength of 298Hz. Your 4" fibrous panel laid flat against the boundary should absorb efficiently all the way down to 298Hz, so within spitting distance of absorbing the entirety of both the midrange and high frequencies.

Now when we stand that 4" slab on edge between the sound source and MLP, the incident sound wave will still be approaching at an angle but we only benefit from the soundwave passing through the absorber a single time. So in this ""flag" orientation your 4" slab only absorbs down to 596Hz. You have missed out on over an octave of the midrange. When orienting the absorber like a flag vs flat against the wall you will need to double up your 4" slabs to provide an 8" total depth in order to obtain the same absorption bandwidth as when oriented laying flat against the boundary.

To me the most practical and stealthy variation on an effective broadband absorber for the primary floor reflections would be a pair of 18"x18"x30" blocks of the foam you currently use (or a glass/mineral fiber product of appropriate density) upholstered in fabric so it appeared to be an ottoman. With a pair of 18"x30" surfaces (one side and the top providing 1080 sq inches) oriented to face towards both the loudspeaker and the listener, the ottoman shaped "sound stool" ought to intercept the primary floor reflection every bit as well as the ackwardly sized 2'x4' flat panel (with it's comparable 1152 sq inches of surface area) laid flat on the floor. Better still, the much greater depth of the ottoman shaped trap will offer absorption all the way into the upper bass.
 
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tmallin

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Good discussion and idea, MTB Vince! I agree that thicker would be better and the "ottoman" would be more effective yet.

Cellcbern, the research I'm referring to is that done by the Danish Archimedes Project (a collaboration of a Danish acoustical institute, KEF, and B&O) back in the 1990s. I personally would agree that damping the walls is more important for reducing obnoxious high frequency reflections, but Archimedes seemed to find that in terms of imaging and staging precision the floor reflection was basically the only one that counted. I personally treat all the reflection areas, finding all such treatment sonically beneficial.

As to pictures which will clarify what I'm talking about: I'm not actually using this "flag" method, just reporting it. However, below are three pictures which will help illustrate the point. Understand that in the method I'm describing, the white foam flat on the floor would not be there.

The first picture shows my flat mirror atop the foam reflecting the tweeter and woofer of my Dutch & Dutch 8c speakers. I carefully positioned my iPhone camera to basically see what my right eye sees when I'm sitting in my listening chair:

IMG_8269.jpeg

The next picture, taken from the same perspective, shows what my right eye sees when I place a small cardboard box cover on edge in the "flag" orientation between the listening position and the mirror. The box cover on edge represents a piece of acoustical foam on edge. As you can see, even this small box cover blocks my view of the reflecting area shown by the mirror:

IMG_8270.jpeg

Finally, the third picture is a wide-angle shot from the side showing the orientation of the mirror, box cover on edge, speaker, and listening chair:

IMG_8271.jpeg
 
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tmallin

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I just realized that this thread does not contain my discussion of how I'm treating the ceiling reflection these days. See the following posts from my Dutch & Dutch 8c speaker discussion:


 

MTB Vince

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@tmalin has used his iphone to represent the small mirror we typically use to assist in identifying the center point of a primary boundary reflection. Alternatively an acoustician may use "ray tracing" in a computer simulation of your room, loudspeaker, and listener locations. But to clarify, the actual surface area of the wall/floor/ceiling boundary being impacted by that reflection is far larger than the small reflected image seen in a mirror which we are using to determine the center points for treatment. Hence the "industry standard" 2'x4' acoustic panel.

And a second observation in response to the the Danish Archimedes Project findings with regards to the primary reflection from the floor being the most sonically critical. In the '80s into the early '90s Kef was still manufacturing conventional tweeter over midrange and MTM loudspeaker designs as were B&O. The vertical offset of the tweeter above or below the midrange driver creates interference problems and lobing in the vertical response curves which are not present in the horizontal response measurements. This is why the specified optimal vertical listening window is much narrower on conventional loudspeakers than the horizontal window. And why excepting tall line source and true dual concentric or full range point source designs, most loudspeakers sound considerably worse when standing vs seated. I would imagine what the Archimedes Project postulated and proved was that the more ragged frequency response impulse of a reflection bounced off the floor or ceiling is sonically more problematic than is the relatively smooth measured response which is bounced off of a sidewall. The spectral balance of the sound we hear in our rooms is determined by a combination of the direct sound and the reflected sound arriving at our ears. If there is a lot of reflected sound and the spectral content of those reflections is measurably much more ragged, then the harsher more ragged sound dominates and is what our ears perceive. Which brings me to a final observation...

Room treatment deniers will almost always trot out this oft heard complaint, "Room treatments made my(or my friend's) room sound dead". In 100% of the occasions I've heard this, additional questioning reveals the following: The complainant had employed poor performing, narrow band treatments (cheap poor performing thin sponge "acoustic foam" treatments, 1-2" depth fiberglass, or makeshift rugs hung on walls as treatments. All of which only absorb high frequencies. So the resulting combined direct and reflected sound was lacking in high frequency content making it sound as though a treble control knob had been dialed all the way down. This explains why effective absorptive treatments must be truly broadband in nature. If you do not absorb 100% of the midrange and high frequency content in that unwanted reflection, you are tilting the spectral balance of your room.

For sidewall, floor, and ceiling treatment of primary reflections, 4-5" deep panels will get the job done due to the boost we get from the incident sound approaching the panel from a significant angle. Broadband panels on the front or rear wall need to be deeper as the sound reflections we are intercepting there are aimed nearly directly at the panel. These front and rear wall reflections pass through less fibrous material on their way in and out of the trap requiring 25-30% additional depth to compensate.
 
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tmallin

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Yes, of course, MTB Vince. The area that should be treated is far larger than the small spots where you actually see the drivers reflected. I typically use the spot where the drivers are reflected in the mirror to center my placement of 2' x 4" pieces foam. And then I experiment with more coverage if I still hear problems or less coverage if the room sounds too "dead."
 

ACHiPo

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It is not quiite as simple as standing the 4" slabs on their side @tmalin as this orientation results in a reduced absorption bandwidth. The lower frequency absorption roll-off point of a fibrous (velocity based) broadband absorber like we are discussing is based upon the total depth of absorption material which the incident sound waves must pass through. To absorb a given frequency wavelength, our absorber must force the sound wave to travel through a depth of absorptive material equivalent to a quarter of its wavelength.

When the absorptive panel is laying flat against a boundary surface addressing primary reflection points on the sidewalls, floor, or ceiling we get to add the distance the wavelength must travel as it enters the panel at an angle to the distance it must again travel after it bounces off the boundary and travels back out through the fibrous panel. So for a wavelength approaching the panel at 45 degrees, the incident sound wave actually travels 5.66" passing inwards through your current 4" foam slabs and another 5.66" passing outwards again after reflecting off of the rigid wall/floor/ceiling boundary. That 10.33" total distance is the 1/4 wavelength of 298Hz. Your 4" fibrous panel laid flat against the boundary should absorb efficiently all the way down to 298Hz, so within spitting distance of absorbing the entirety of both the midrange and high frequencies.

Now when we stand that 4" slab on edge between the sound source and MLP, the incident sound wave will still be approaching at an angle but we only benefit from the soundwave passing through the absorber a single time. So in this ""flag" orientation your 4" slab only absorbs down to 596Hz. You have missed out on over an octave of the midrange. When orienting the absorber like a flag vs flat against the wall you will need to double up your 4" slabs to provide an 8" total depth in order to obtain the same absorption bandwidth as when oriented laying flat against the boundary.

To me the most practical and stealthy variation on an effective broadband absorber for the primary floor reflections would be a pair of 18"x18"x30" blocks of the foam you currently use (or a glass/mineral fiber product of appropriate density) upholstered in fabric so it appeared to be an ottoman. With a pair of 18"x30" surfaces (one side and the top providing 1080 sq inches) oriented to face towards both the loudspeaker and the listener, the ottoman shaped "sound stool" ought to intercept the primary floor reflection every bit as well as the ackwardly sized 2'x4' flat panel (with it's comparable 1152 sq inches of surface area) laid flat on the floor. Better still, the much greater depth of the ottoman shaped trap will offer absorption all the way into the upper bass.
Interesting. I'm starting to get it. While the "flag" will be higher Q in the side modes, it will remain a good broad absorber front to back. Also, the mid room flag should have high velocity in both directions (depending on frequency of course) compared to the high pressure zone when lying on the floor. Finally, a 4' or so length of 4" foam should absorb pretty low frequencies, although the relatively small cross section will limit the amount of absorption.

Here's my ottoman/absorber. I've thought about building something similar out of rigid fiberglass panels, but haven't gotten around to it yet. This is a leather covered storage bench (hollow) that is padded and covered with a blanket. It's mostly there because I like having a table there. The blanket should help with some high frequency absorption first reflections, though.
 

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ACHiPo

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Feb 22, 2015
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I just realized that this thread does not contain my discussion of how I'm treating the ceiling reflection these days. See the following posts from my Dutch & Dutch 8c speaker discussion:


Wow that's a lot of absorption. Does the room sound overly damped on the highs?

I was actually thinking the "flag" configuration would be standing the 4"x4' foam vertical down the middle of the room. Thus my thought that it could be pretty effective at dealing with the mid-room nulls (high velocity) bass.
 

MTB Vince

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Interesting. I'm starting to get it. While the "flag" will be higher Q in the side modes, it will remain a good broad absorber front to back. Also, the mid room flag should have high velocity in both directions (depending on frequency of course) compared to the high pressure zone when lying on the floor. Finally, a 4' or so length of 4" foam should absorb pretty low frequencies, although the relatively small cross section will limit the amount of absorption.

Here's my ottoman/absorber. I've thought about building something similar out of rigid fiberglass panels, but haven't gotten around to it yet. This is a leather covered storage bench (hollow) that is padded and covered with a blanket. It's mostly there because I like having a table there. The blanket should help with some high frequency absorption first reflections, though.
As I already pointed out in my two prior posts, a 4" deep fibrous velocity trap mounted flat to the wall so the reflected soundwaves pass through it twice, will be an effective absorber down to about 300Hz. That same 4" deep 2'x4' trap stood on edge in your room will only offer absorption to about 600Hz. When used freestanding without a reflective boundary, sound waves only pass through the fiber once. And to achieve the low frequency cut-offs, both these calculations assumed the sound waves approach the panel at a 45 degree angle. So nowhere near being a literal bass trap in either case.

As for your current makeshift ottoman absorber, covering the reflective leather surface with a blanket isn't really any different than placing a blanket over a flat panel TV. It does practically nothing. The 1/4 wavelength requirement for effective velocity based absorption means that even with a 45 degree incident angle for the soundwave, a 1/4" thick blanket will only fully absorb frequencies above 10kHz. If you folded the blanket over several times to achieve a thickness of 1/2" you still only have absorption down 5kHz.
 

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