R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: Acoustic foam and bass absorption  (Read 2968 times)

Sin x/x

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
Acoustic foam and bass absorption
« on: October 10, 2006, 07:17:34 am »

How is it possible that acoustic foam can absorb low frequency's when placed in a corner?

Of cause the thickness of the foam determines the lowest frequency, but when placed in a corner it seems to absorb much more lower frequency's.

How is that possible?

(please ignore if double posted)
Logged

Tom C

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 377
Re: Acoustic foam and bass absorption
« Reply #1 on: October 10, 2006, 09:46:41 am »

Sin x/x wrote on Tue, 10 October 2006 13:17

How is it possible that acoustic foam can absorb low frequency's when placed in a corner?

Of cause the thickness of the foam determines the lowest frequency, but when placed in a corner it seems to absorb much more lower frequency's.

How is that possible?

(please ignore if double posted)


When you place a sub-woofer in a corner the lower frequencies are
much louder because instead of emitting the sound in all directions
it's reflected from all 3 walls in your direction.
Placing an absorber in a corner is the same process backward
(okay, that picture isn't perfect but roughly explains the effect).
Logged
Tom

.signature failure

Sin x/x

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
Re: Acoustic foam and bass absorption
« Reply #2 on: October 11, 2006, 10:14:44 am »

Foam absorbs sound by reducing the velocity of the air molecules.

But in a corner the air molecules have hardly any velocity, so how the **** can it absorb sound/low frequency's?

Anyone?
Logged

maarvold

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 853
Re: Acoustic foam and bass absorption
« Reply #3 on: October 11, 2006, 11:03:54 am »

Sin x/x wrote on Wed, 11 October 2006 07:14

Foam absorbs sound by reducing the velocity of the air molecules.

But in a corner the air molecules have hardly any velocity, so how the **** can it absorb sound/low frequency's?

Anyone?


I always understood the principle to be that friction, as the sound wave contacts (and moves through/around) the foam, fiberglass insulation, etc, converts the motion of the air molecules into heat.  If what you say is true, maybe you need big dimensions for low frequency absorbers because--at that point--you want to make the molecules work harder (dissipate energy) to equalize air pressure, not motion.  
Logged
Michael Aarvold
Audio Engineer

Ethan Winer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 571
Re: Acoustic foam and bass absorption
« Reply #4 on: October 11, 2006, 11:31:01 am »

> But in a corner the air molecules have hardly any velocity, so how the **** can it absorb sound/low frequency's? <

You're correct - right at the corner boundaries there is more wave pressure and little wave velocity. But even a foot or two away the velocity is high enough for corner trapping to work well. This is one reason standard flat fiberglass bass traps can be effective straddling corners - a natural air space forms behind the panel.

--Ethan

JDNelson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 997
Re: Acoustic foam and bass absorption
« Reply #5 on: October 11, 2006, 05:46:26 pm »

I'm using stand-mounted Auralex LENRD traps positioned out from the corner six inches or so, and so far I'm satisfied with the improvement in my admittedly low-budget home studio...  I'm assuming the air space behind them improves their performance versus the glued-in-place LENRD's.

Ethan Winer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 571
Re: Acoustic foam and bass absorption
« Reply #6 on: October 12, 2006, 04:27:40 pm »

JD,

> I'm assuming the air space behind them improves their performance versus the glued-in-place LENRD's. <

It's hard to say without actually measuring the room's low frequency response both ways. LENRDs don't really get down that low anyway, so maybe a space would help them. Do you have a way to measure at high resolution to, say, a 1 Hz bandwidth?

--Ethan

franman

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 580
Re: Acoustic foam and bass absorption
« Reply #7 on: October 12, 2006, 10:28:23 pm »

Low (or zero) velocity right at the corner but plenty of motion just off the corner. Corners are great bass trap locations because:
a) LF energy is always high at boundaries(Max PRESSURE!!)
b) corners are connection of multiple boundaries
c) always lotsa modal action at (or right near) the corners
Logged
Francis Manzella - President, FM Design Ltd.
                 - Managing Director, Griffin Audio
fmdesign.com
griffinaudiousa.com

Sin x/x

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
Re: Acoustic foam and bass absorption
« Reply #8 on: October 13, 2006, 01:35:42 am »

franman wrote on Thu, 12 October 2006 21:28

Low (or zero) velocity right at the corner but plenty of motion just off the corner. Corners are great bass trap locations because:
a) LF energy is always high at boundaries(Max PRESSURE!!)
b) corners are connection of multiple boundaries
c) always lotsa modal action at (or right near) the corners


If this is true, and I think it is.
Then:
Panel absorbers must be much more effective than foam. (In corners at low frequency's)
Since they actually reduce the pressure energy of the sound wave.
And panel absorbers can be tuned to certain frequency's.
Foam can't do either.

If all this is true, I'm not sure.
Then:
Foam is a not such a smart way to eliminate acoustic problems.

Logged

jimmyjazz

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1885
Re: Acoustic foam and bass absorption
« Reply #9 on: October 13, 2006, 11:06:56 am »

Sin x/x wrote on Fri, 13 October 2006 01:35

Foam is a not such a smart way to eliminate acoustic problems.


Whoa . . . careful there.  We're talking about tools, here.  Would you use a screwdriver for a hammer?

Foam is a great acoustic absorber when you use enough of it in a given application.  There's nothing better, really.  (I'm including fiberglass, rock wool, faded blue jeans and whatever other flavor of the day is available for absorbing sound under the descriptor "foam".)

Do you need a lot of foam to achieve near-unity absorption down in the lowest octaves?  Well, yes.  In most situations, there comes a point where broadband bass-trapping probably represents a better use of space.  

But too many people over-simplify the activity of foam and get wrapped up in "thickness" and "wavelength".  Sound doesn't just hit walls in a perpindicular fashion.  For an oblique or near-tangential angle of incidence, the dimension of interest isn't the thickness . . . it's the width (or height) of foam.  Think about that and you might get a better feel for why a nominal thickness of foam can actually absorb some fairly low-frequency sound.
Logged

franman

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 580
Re: Acoustic foam and bass absorption
« Reply #10 on: October 13, 2006, 09:33:28 pm »

... and panel absorbers are by nature, tuned traps.. There are tricks to broaden bandwidth, but generally foam trapping is used as a more broadband solution, which is always required... We (FM Design) tend to rely on panel traps and other tuned "tools" when specific modal problems present themselves OR are predicted in our simulations..

Foam (and other porous absorption) has a very good use and what JJ points out about oblique activity is very valid as well... Keep using all your tools guys!! and ALWAYS remember, your ears are your best tool!!

... I know I sound like a broken record (anybody remember those?) but I will keep pounding it home,... it's all about what we hear and learning to hear better!
Logged
Francis Manzella - President, FM Design Ltd.
                 - Managing Director, Griffin Audio
fmdesign.com
griffinaudiousa.com

Sin x/x

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 72
Re: Acoustic foam and bass absorption
« Reply #11 on: October 14, 2006, 12:55:27 am »

Thanks for the discussion.

Keep up the good stuff.
Logged
Pages: [1]   Go Up