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The design of a modular sound absorber for very low frequencies
Old 18th June 2009
  #1
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The design of a modular sound absorber for very low frequencies

i get the feeling that our method on this forum favours "porus absorbers" (as bbc refers to them) and a couple of types of diffusers exclusively... i am sure there are many, many possibilities we are not paying attention to...

I hope I am not violating any copyright laws... the title of my post is straight out of a bbc paper, pointed out to me by a member here (thanks avare...)...

if i am correct, it suggests that "porus absorbers" (exposed insulation materials... OC703, OC705, rockwool, whatever?) are most effective at frequencies over 120 Hz (or more)... to tackle deep bass in the range of 50-100 Hz, a membrane trap is suggested... bbc's working model seems to be 8" of "supawrap 600 loft insulation" stuffed into a box of 184 mm depth (slightly less than 8", so the insulation is compressed and directly in contact with the membrane...?), with sides and back of 9.5 mm plywood and front of 3.2 mm hardboard...

i hope this is of use to somebody... i invite discussions on whether and how this would work in reality...

jai shankar.
Old 18th June 2009
  #2


The thing about membrane traps is that they are a bit harder to build and tuning is a bit of an art. If you have decent facilities to test what you just built.

Also, in a lot of the world where exterior walls are typically stud-framed with gypsum board coverings on the inside, you already have some membrane absorbers anywhere there is an exterior wall.

Don't forget active bass traps either....



-tINY

Old 18th June 2009
  #3
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Lightbulb

Quote:
Originally Posted by audiothings View Post
it suggests that "porus absorbers" (exposed insulation materials... OC703, OC705, rockwool, whatever?) are most effective at frequencies over 120 Hz
I've seen "normal-size" porous absorbers have at least some affect as low as 30 Hz, and plenty of effect at 50 Hz.

Besides tINY's good comments, another problem with tuned absorbers is they are not broadband. You might get one octave out of them if you're lucky, but maximum absorption will be much narrower than that. And you still need to deal with the mid and high bass ranges which are even more important.

--Ethan
Old 18th June 2009
  #4
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You are welcome for the link to the BBC paper. The first question that comes to mind is what are you trying in achieve in what space?

Walls made of gypsum have high amounts of absorption at low frequencies to begin with. These walls are by definition membrane absorbers. This absorption is often ignored when calculating room absorption. As shown in the linked paper, the ~300 mm thick absorber goes down to 80 Hz with high absorption. Teh low frequency absorption can be increased by not partitioning the back of the absorber. 12" rockwool in standard reverb chamber testing shows flat absorption down to 59 Hz!

Keep teh questions coming. As you get clearer and more specific wioth your questions, the better the answers you will receive.

Andre
Old 19th June 2009
  #5
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Quote:
The thing about membrane traps is that they are a bit harder to build and tuning is a bit of an art.
agreed. but for the genuine enthusiast, it'd be fun... no? and besides, if you look at the the bbc document referred to, you may reconsider your perception of the extent of the risk.

Quote:
If you have decent facilities to test what you just built.
you are correct in principle. but we do have resources like the ones avare pointed us to, to start with... just like with any piece of gear, somebody has to take the plunge, and then share what they have learned... nobody gets to laboratory test every piece of gear... there are published specs and papers, and then there are user experiences and caveats... thats what these forums are about, isn't it?

Quote:
Also, in a lot of the world where exterior walls are typically stud-framed with gypsum board coverings on the inside, you already have some membrane absorbers anywhere there is an exterior wall.
I am in India and gypsum is almost never used in homes... its all brick and cement and concrete.

Quote:
Don't forget active bass traps either....

Whats that? new to me! any links?

Quote:
I've seen "normal-size" porous absorbers have at least some affect as low as 30 Hz, and plenty of effect at 50 Hz.
True, but everything I've seen seems to suggest that there are ways to do much better at the very low frequencies.

Quote:
another problem with tuned absorbers is they are not broadband
now here is the thing... i never asked about broadband absorbers... everything here seems to lead back to broadband absorbers, and thats what I commented on at the start of my previous post... agreed that under many circumstances, "porous" broadband absorbers are the best way to go... but my topic is about the very low frequencies, and what is the best way to absorb them.

Quote:
Keep teh questions coming. As you get clearer and more specific wioth your questions, the better the answers you will receive.
Thanks again avare. Mine was not exactly a question, it was an invitation to share opinions, experiences and known facts.

I don't look at this particular forum as merely a resource for building ones personal music space exclusively... i continue to be interested in these things even though i can perceive of no scope to treat my own space any further.

currently, i'm helping my friend treat his home theater, and i was thinking of using some of these deep bass traps... but i'll start another post about that...

cheers,

Old 19th June 2009
  #6
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You put an amp with a speaker and hook it up out-of-phase with a microphone...



-tINY

Old 19th June 2009
  #7
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I've never tested the Bag End E-Trap, but as far as I can tell it has some of the same problems as room EQ. For an absorber to be effective it must be suitably large. Room treatment is all about surface coverage, so if a room has, say, 800 square feet of surface area, you need to cover some meaningful percentage of that surface. If you stick a small loudspeaker-based bass trap in one corner, it won't do much even if its absorption is 100 percent because it's just too small.

To solve this, the active circuitry is set up to offer more than 100 percent absorption. A device like the Bag End works by playing a countering signal into the room. If 100 percent countering isn't enough, it can be set to more than 100 percent. But then the improvement becomes localized where the test microphone was placed. So a large resonating peak will be reduced at that location, but likely made worse elsewhere.

In my experience, bass peaks and nulls occupy highly localized areas in a room, especially if the room is small. So the response can vary significantly over spans as small as a few inches, even at very low frequencies.

Again, I haven't had the opportunity to actually test the Bag End trap, but simple physics suggests this is what will happen.

--Ethan
Old 22nd June 2009
  #8


My final project in college was similar to this trap. I added the complexity of sending a signal to the summing point, rather than just DC.

The thing is: You can get a near perfect null at the point of the mic. If placed properly, this will absolutely work miracles on standing waves that ring (Modal ringing). It also counteracts traffic noise.

What it won't do is fix boundary issues where path length differences cause problems.



-tINY

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