Acoustic Treatment for Small Listening Rooms: Absorption vs. Diffusion

[ACHiPo]

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.

MTB Vince,
I understand the passage depth advantage when absorbers are against a wall. My point is that a frictional absorber is much more effective/efficient when velocity is max which happens away from a room boundary and the room midpoint is usually a null (lowest pressure) for the lowest room modes which means the velocity is highest.

Generally these flag type of installations at room modes are impractical, but it seems they could be the most efficient placement of absorbers for bass.

Now how they act for first reflections (mids/highs) is a different story.

Evan

 

[MTB Vince]

MTB Vince,
I understand the passage depth advantage when absorbers are against a wall. My point is that a frictional absorber is much more effective/efficient when velocity is max which happens away from a room boundary and the room midpoint is usually a null (lowest pressure) for the lowest room modes which means the velocity is highest.

Generally these flag type of installations at room modes are impractical, but it seems they could be the most efficient placement of absorbers for bass.

Now how they act for first reflections (mids/highs) is a different story.

Evan

While you are correct that bass velocities are higher out away from rigid room boundaries, we are still dealing with the physics of how huge a simple fibrous velocity based trap must be in order to absorb bass frequencies. Dimensionally these sorts of traps need to be a minimum of 1/4 of a wavelength deep in order to absorb that frequency. A 56Hz wavelength is 20 feet long. So the required depth of our freestanding "flag trap" would need to be 5 feet! Not terribly practical. Pressure based resonant traps at room boundaries or ASC's Tube Trap (hybrid design) in the corners are decidedly more practical.

 

[ACHiPo]

MTB Vince,
The 1/4 wavelength thickness makes sense against the wall (thus why tube traps are effective even though they are significantly less than 1/4 wavelength in diameter as there are parts away from the wall that benefit from the sound wave traveling through twice and air). If you look at the section depth of a tube trap, however, it is relatively thin, yet they still tame longer wavelengths because they are away from the wall. I'm struggling to explain what's in my head, but I'm pretty sure that absorbers don't need to be 1/4 wavelength thick when positioned at the point of max velocity. Absolutely agree with the benefits of air/thickness/dual-pass when it comes to bass waves in high pressure areas.

Evan

 

[MTB Vince]

MTB Vince,
The 1/4 wavelength thickness makes sense against the wall (thus why tube traps are effective even though they are significantly less than 1/4 wavelength in diameter as there are parts away from the wall that benefit from the sound wave traveling through twice and air). If you look at the section depth of a tube trap, however, it is relatively thin, yet they still tame longer wavelengths because they are away from the wall. I'm struggling to explain what's in my head, but I'm pretty sure that absorbers don't need to be 1/4 wavelength thick when positioned at the point of max velocity. Absolutely agree with the benefits of air/thickness/dual-pass when it comes to bass waves in high pressure areas.

Evan

Hey Evan,

I am intimately familiar with Tube Traps. 20+ years ago, I made my own 14"D x 6'H DIY versions from 2" wall rigid fiberglass pipe insulation with plywood end caps. They worked remarkably well and were relatively cost effective but were challenging to make well. Then over the past 12 years I gradually replaced the DIY versions with larger diameter, more effective and much more costly genuine ASC versions. The real-deal ASC Isothermal Tube Trap isn't strictly a simple velocity based absorber when it comes to the lowest frequencies. It's construction is decidedly more complex than simply placing blocks of fibrous insulation in room corners, flat against a wall, or free-standing out in the room as we've been discussing here in tmalin's thread.

As for the 1/4 wavelength depth requirement for the simple velocity based absorbers we have been discussing here, well that is straight forward and well understood physics and so not up for conjecture.

 

[christoph]

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Hey Evan,

I am intimately familiar with Tube Traps. 20+ years ago, I made my own 14"D x 6'H DIY versions from 2" wall rigid fiberglass pipe insulation with plywood end caps. They worked remarkably well and were relatively cost effective but were challenging to make well. Then over the past 12 years I gradually replaced the DIY versions with larger diameter, more effective and much more costly genuine ASC versions. The real-deal ASC Isothermal Tube Trap isn't strictly a simple velocity based absorber when it comes to the lowest frequencies. It's construction is decidedly more complex than simply placing blocks of fibrous insulation in room corners, flat against a wall, or free-standing out in the room as we've been discussing here in tmalin's thread.

As for the 1/4 wavelength depth requirement for the simple velocity based absorbers we have been discussing here, well that is straight forward and well understood physics and so not up for conjecture.

I love your dual purpose room

 

[hemiutut]

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:

View attachment 82333

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:

View attachment 82334

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:

View attachment 82335

Hello
Does this room have measurements made with Room Eq Wizard for example?
Just curious to see the acoustics of it.

Written with translator.

Greetings

 

[ACHiPo]

I love your dual purpose room

Yes that is a great room!

 

[ACHiPo]

View attachment 82339 View attachment 82340

View attachment 82341 View attachment 82342

Hey Evan,

I am intimately familiar with Tube Traps. 20+ years ago, I made my own 14"D x 6'H DIY versions from 2" wall rigid fiberglass pipe insulation with plywood end caps. They worked remarkably well and were relatively cost effective but were challenging to make well. Then over the past 12 years I gradually replaced the DIY versions with larger diameter, more effective and much more costly genuine ASC versions. The real-deal ASC Isothermal Tube Trap isn't strictly a simple velocity based absorber when it comes to the lowest frequencies. It's construction is decidedly more complex than simply placing blocks of fibrous insulation in room corners, flat against a wall, or free-standing out in the room as we've been discussing here in tmalin's thread.

As for the 1/4 wavelength depth requirement for the simple velocity based absorbers we have been discussing here, well that is straight forward and well understood physics and so not up for conjecture.

Vince,
I'm jealous of your listening/screening room! Obviously hit a lot of the right stuff--big bass absorption/traps in the front and corners, first reflection absorbers and polycylindrical diffusers along the sidewalls with QRD diffusers behind the primary listening position far enough away to avoid artifacts. Very nice indeed! I'm guessing the room measures pretty well, especially decay time. The multiple subs surely help smooth out the lowest octaves.

What is the big round thing hanging from the ceiling on the left side facing the front?

Evan

 

[MTB Vince]

Vince,
I'm jealous of your listening/screening room! Obviously hit a lot of the right stuff--big bass absorption/traps in the front and corners, first reflection absorbers and polycylindrical diffusers along the sidewalls with QRD diffusers behind the primary listening position far enough away to avoid artifacts. Very nice indeed! I'm guessing the room measures pretty well, especially decay time. The multiple subs surely help smooth out the lowest octaves.

What is the big round thing hanging from the ceiling on the left side facing the front?

Evan

Holy hawkeyes @ACHiPo, you miss nothing- LOL! Well, according to the last round of REW measurements which my REW wiz buddy and I performed about a year ago, the room's reverberation time falls almost perfectly within the range considered ideal for small-to-medium size listening rooms. Better yet, it sounds great- quiet, clean, and open with just a touch of warmth. Conversation, stereo or multi-channel music, and movies all seem to be treated equally well. Exactly what I was going for with this latest revision of the dedicated room I originally built 24 years ago for two channel. I've actually tweaked the acoustic treatments a little more since those pics were taken back in March. The pair of 4'x2'x5" broadband panels on the front wall have been replaced with 4'x2'x7" modified RPG BAD ARC panels. I've also added a pair of 2'x2'x7" BAD ARC panels higher on the rear wall to either side of the projector. and a few weeks ago I replaced my remaining pair of DIY Tube Traps flanking the L&R loudspeakers with identically sized real ASC Tube Traps. These minor changes created a little more depth of soundstage and tightened/cleaned up the mid-bass a little more without messing up the room's pleasing spectral balance.

Oh and that big round thing above and to the left in the foreground of the forward facing pic? My huge (as big as your head!) Isco 3L anamorphic lens. It rides in and out of the projector's light path on a motorized sled when required. Along with our motorized AT screen, the Isco lens is used for viewing 2.35-2.4:1 Superscope movies optimally. In the pics it was resting in it's "stowed" position.

 

[hemiutut]

Holy hawkeyes @ACHiPo, you miss nothing- LOL! Well, according to the last round of REW measurements which my REW wiz buddy and I performed about a year ago, the room's reverberation time falls almost perfectly within the range considered ideal for small-to-medium size listening rooms. Better yet, it sounds great- quiet, clean, and open with just a touch of warmth. Conversation, stereo or multi-channel music, and movies all seem to be treated equally well. Exactly what I was going for with this latest revision of the dedicated room I originally built 24 years ago for two channel. I've actually tweaked the acoustic treatments a little more since those pics were taken back in March. The pair of 4'x2'x5" broadband panels on the front wall have been replaced with 4'x2'x7" modified RPG BAD ARC panels. I've also added a pair of 2'x2'x7" BAD ARC panels higher on the rear wall to either side of the projector. and a few weeks ago I replaced my remaining pair of DIY Tube Traps flanking the L&R loudspeakers with identically sized real ASC Tube Traps. These minor changes created a little more depth of soundstage and tightened/cleaned up the mid-bass a little more without messing up the room's pleasing spectral balance.

Oh and that big round thing above and to the left in the foreground of the forward facing pic? My huge (as big as your head!) Isco 3L anamorphic lens. It rides in and out of the projector's light path on a motorized sled when required. Along with our motorized AT screen, the Isco lens is used for viewing 2.35-2.4:1 Superscope movies optimally. In the pics it was resting in it's "stowed" position.

View attachment 82397 View attachment 82396

Hello
Can you put some measurements with the Room Eq Wizard or your .mdat file ?.

Just curious to see the acoustics of the room.

Do you have a multi-subwoofer setup?

And if you have a specific thread where your dedicated room appears and its measurements can be seen, if it seems right you could put the link, so as not to do offtopic in this particular thread.

Written with translator.

Greetings

 

[MTB Vince]

Hello
Can you put some measurements with the Room Eq Wizard or your .mdat file ?.

Just curious to see the acoustics of the room.

Do you have a multi-subwoofer setup?

And if you have a specific thread where your dedicated room appears and its measurements can be seen, if it seems right you could put the link, so as not to do offtopic in this particular thread.

Written with translator.

Greetings

Sorry @hemiutut, we ran all the REW measurements on my buddy, Jim the REW expert's Macbook. At one point I had the final waterfall measurement set and the series of band limited impulse and frequency response measurements we used to fine tune my subwoofer array integration in an email. Unfortunately I appear to have deleted it.

I do run a (ridiculously over-specified) multi-subwoofer set-up in the interest of smoother, effortless bass. Four Seaton Sound Submersive HP subwoofers to be exact. Each subwoofer employs a pair of 15" paper cone drivers on opposite ends of a 24"x25"x18" 120lb extensively braced sealed enclosure powered by 2.4kW US built plate amp. I have them arranged in symmetrical pairs snugged up against the front and rear walls at .25W and .75W. I run the subwoofer array in mono and it is shared between both the two channel and multi-channel systems which co-exist in the room. I use a DSPeaker AntiMode2.0 in it's mono multi-sub configuration to provide a 6ms time alignment delay for the nearer rear subs and surgical parametric EQ solely on the subwoofer channel to tidy up modal misbehavior in the subwoofers' 8-to-80Hz passband.

 

[hemiutut]

Thanks MTB Vince for the deference in answering.
I'm sure the room will be acoustically fine tuned and with the inclusion of a multi-subwoofer setup,
it will have a very good waterfall.

Greetings