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Author Topic: Another Acoustics Question  (Read 1966 times)

Jonathan Heimberg

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Another Acoustics Question
« on: April 23, 2010, 10:57:29 am »

It seems the most common methods for mounting acoustic treatment is either directly attached or very close to the boundary surface. Ex: Attached acoustic panels or curtains to the walls, clouds hanging near the ceiling, etc.

It this merely an aesthetic or functional issue, or is it technical in nature, affecting the usefulness of the treatment?

Would it be acceptable for an acoustic treatment (panels or curtains) to by hung from the ceiling in free air? How would that affect (if at all) the absorption qualities of the material?

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Shane Ervin

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Re: Another Acoustics Question - link to product
« Reply #1 on: April 23, 2010, 11:45:00 am »

I've used baffles like these Acoustiguard baffles to good effect in both office and studio applications.  Both sides of the suspended panels are exposed to the incident sound.

It's surprising how well they can integrate into a space, visually speaking.  A certain height of ceiling is required, but in some spaces, it's an idea well worth considering.

Acceptable?  That's up to you and your client.  But it's certainly done often enough.  After all, Sabines are Sabines.

One client enjoyed the ability to adjust RT by the simple action of sliding some baffles along the guy wires - clustering some together, and leaving more open space.  He'd spread them out evenly for max absorption.
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Randy Pence

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Re: Another Acoustics Question
« Reply #2 on: April 23, 2010, 12:01:46 pm »

In some absorption coefficient charts you can already find your answer.  Offsetting an absorber from a barrier increases the lf effectiveness.  The soundwave passes through the material, bounces off the barrier, and then passes through the material again.  The transition between air and materials creates additional acoustic reduction.

Whether one can afford the space loss and aesthetics of offset absorbers is another story.
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Mac Kerr

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Re: Another Acoustics Question
« Reply #3 on: April 23, 2010, 01:26:59 pm »

Jonathan Heimberg wrote on Fri, 23 April 2010 10:57

It seems the most common methods for mounting acoustic treatment is either directly attached or very close to the boundary surface. Ex: Attached acoustic panels or curtains to the walls, clouds hanging near the ceiling, etc.

It this merely an aesthetic or functional issue, or is it technical in nature, affecting the usefulness of the treatment?

Would it be acceptable for an acoustic treatment (panels or curtains) to by hung from the ceiling in free air? How would that affect (if at all) the absorption qualities of the material?


Since the whole point of acoustic treatment is to keep sound from bouncing off the walls and other building surfaces it only seems logical that the treatment goes between the sound source and the walls and ceiling. By being close to the wall it can attenuate both the sound hitting the wall, and the reflection off the wall.

Mac
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Dick Rees

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Re: Another Acoustics Question
« Reply #4 on: April 23, 2010, 04:01:09 pm »

Jonathan Heimberg wrote on Fri, 23 April 2010 09:57

It seems the most common methods for mounting acoustic treatment is either directly attached or very close to the boundary surface. Ex: Attached acoustic panels or curtains to the walls, clouds hanging near the ceiling, etc.

It this merely an aesthetic or functional issue, or is it technical in nature, affecting the usefulness of the treatment?

Would it be acceptable for an acoustic treatment (panels or curtains) to by hung from the ceiling in free air? How would that affect (if at all) the absorption qualities of the material?





J....

What's the scenario?  Is this permanent treatment or do you need to be flexible in positioning a la studio space?

There's a great set of pics of DIY Gobo's in the Church side of the forum.  For non-floor mount stuff I've considered making some Gobo type panels and hanging them from the rafters as needed to divide the room into sections for recording purposes.

There is some good info on acoustic treatments in the Yamaha  Sound Reinforcement Handbook.  They discuss materials and depth/mounting as well as frequencies.

dR
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Jim Brown

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Re: Another Acoustics Question
« Reply #5 on: April 26, 2010, 02:14:01 pm »

Spacing an absorbent tile from the wall increases its effectiveness at lower frequencies.  

The absorbent tile an appropriate distance from the wall can be as effective as a thicker tile mounted directly on the wall.

Chapter 9 in the Master Handbook of Acoustics (Fourth Edition) by F. Alton Everest gives much more detail and explanation.   It's actually quite a readable book as technical matters go and well worth investing in.

Graphs of the absorption vs frequency vs space from the wall tell the story so much better than words.

Hope that helps.

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George Friedman-Jimenez

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Re: Another Acoustics Question
« Reply #6 on: April 26, 2010, 10:02:14 pm »

What is it you are trying to do?
If the point is to reduce room modes and uneven bass due to standing waves, you want your absorber to be 1/4 wavelength out from the surface where the air velocity max (and pressure min) occurs in the standing wave. You don't want the absorber at the surface where the pressure max and velocity min occur. If the absorber is placed closer or farther from the surface, absorbtion will be less effective. Obviously 1/4 wavelength is a different physical distance for each frequency, so an absorber at a given distance from the surface is more or less effective at different frequencies. If you want a "bass trap" to absorb best at say at 100 Hz, 1/4 wavelength would be 1130/100 x 1/4 = 2.8 feet out from the wall. To absorb 200 Hz, ideal spacing would be 1.4 feet out from the wall. Thicker absorber, like 6" or 12" fiberglass or rockwool batting will absorb more effectively than thinner, because it is close to 1/4 wavelength out for more different frequencies and also because it attenuates the sound more. Multiple layers of heavy drape stage curtains serve this broadband absorption purpose very well.

For a much more narrow purpose, for example to kill early reflections from sidewall or ceiling, as in a recording studio control room, you want to absorb mainly higher frequencies. Wavelengths are much shorter, so a 2" thick fiberglass panel placed 4" out from the wall or ceiling will be very effective for a broad range of higher frequencies, although it won't be a very effective bass trap. It all depends on what your goals are. Lately, more studios are using diffusors to reflect sound in many directions and let the room sound more live rather than absorbers that make the room sound more dead.

I also highly recommend F. Alton Everest's Master Handbook of Acoustics as a nonmathematical and very readable reference book on these topics.

If you are serious about acoustics and are investing significant money and time in a sound space, it would probably be worth your while to hire an acoustician to advise you or to design the acoustics. Low budget, check out Everest, or other books by Rod Gervais or Philip Newell. If you want to know all there is to know about diffusors and like math, check out Cox and D'Antonio's book on absorbers and diffusors.
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Jonathan Heimberg

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Re: Another Acoustics Question
« Reply #7 on: April 27, 2010, 06:28:16 am »

Thanks for all the replies and suggestions.

The space in question is a very diffuse space, about 60x80. Very few, if any, hard echoes or slap-backs. When walking the room, you don't get the impression that any particular surface or area of the room is causing problems.

The only issue is that the reverb time is too long. The hollow walls, carpet, and people are absorbing a good amount of the high frequency energy, so the characteristic sound is of a very 'warm' reverb.

The other issue is that we are very sensitive to changing the look of the space. Trying to find some solutions without it being obvious.

At the present, there are various flags hanging from the ceiling, in groups of three, making 5hx9w rectangles. One idea is to turn these into baffles. Build a 9x5 frame, fill w/rockwool or 703, and place the flags on either side. That way there isn't a very obvious visual change to the room. The two hangs we're looking at doing are about 15' from the back wall, in free space. The are also perpendicular to the direction the sound system is firing.

Would something along these lines be worth trying?

Thanks again for all your help,
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Mac Kerr

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Re: Another Acoustics Question
« Reply #8 on: April 27, 2010, 12:03:41 pm »

Jonathan Heimberg wrote on Tue, 27 April 2010 06:28

The space in question is a very diffuse space, about 60x80. Very few, if any, hard echoes or slap-backs. When walking the room, you don't get the impression that any particular surface or area of the room is causing problems.


If the flags will intercept the sound before it hits a wall or other reflective surface they may help. Reverb does not exist without reflections off surfaces. Keeping the sound off the walls, ceiling, and other reflective surfaces is how you reduce reverb.

Mac
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Weogo Reed

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Re: Another Acoustics Question
« Reply #9 on: April 28, 2010, 08:29:58 pm »

Hi Jonathan,

How many sources do you have?
Center cluster?
L/R hangs?
Front-fills?
Delay-Fills?
LOUD stage wedges bouncing off a wall?
Could your 'reverb' be arrivals from multiple sources?

Sound attenuation close to speakers, where the energy is concentrated, will be more effective than farther away.

Good health,  Weogo
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