why fiberglass panels absorb lower frequencies than they theoritically should?

[jacko]

In theory to absorb a wave we need a porous absorber that is 1/4 wave length deep. Why fiberglass panels absorb lower frequencies? My panel is 8" deep (6" fiberglass spaced 2" from the wall). It should absorb frequencies down to 425Hz (8" is 1/4 of a 425Hz wave length) but in fact it is very effective down to 60Hz and still effective at 50Hz. Could someone point me to some documents where it is explained?

[PaulP]

Originally Posted by jacko

In theory to absorb a wave we need a porous absorber that is 1/4 wave length deep. Why fiberglass panels absorb lower frequencies? My panel is 8" deep (6" fiberglass spaced 2" from the wall). It should absorb frequencies down to 425Hz (8" is 1/4 of a 425Hz wave length) but in fact it is very effective down to 60Hz. Could someone point me to some documents where it is explained?

It's not that an absorber should be 1/4 wavelength thick but that it should be
positioned at 1/4 wavelength from the wall.

What's important is for the wave to pass through the absorber, which can be
of any thickness. The 1/4 wave bit is just that at that distance from the
wall the velocity of the wave will be at its highest so it's the best place to
stick your absorber.

Paul P

[nosebleedaudio]

The BBC testing came up with more like 4% of the wavelength,depending on design of the box ect.. which is the same as other groups..Bob Gold for example..
I just finshed a room that has 8" thick panels on the ceiling Plus a 4" space..should be good below 50Hz..Or in the ball park..

[avare]

Originally Posted by jacko

In theory to absorb a wave we need a porous absorber that is 1/4 wave length deep. Could someone point me to some documents where it is explained?

The 1/4 wavelength is valid for very thin porous absorbers. Unfortunately there is no web page that I am aware that explains the loss mechanism well. Fortunately it is like most of acoustics in that it is not intuitive. Trying to put it in succinct terms, there are three main factors involved.

1. As sound strikes a porous absorber at greater angles than normal incident, the sound wave goes goes through a longer and longer path, increasing low end absorption.

2. Sound in a porous absorber travels at ~70% of the speed in open air, increasing low end absorption.

3. In a porous abosrber the insulation conducts heat away from the compressed zones, causing #2 above, and a phase incoherence at the material - open air boundary, again increasing low end absorption.

For the physicists looking for an explanation of the speed of sound change, normal sound waves are adiabatic, with localized zones of increased and decreased temperature. In porous absorbers the material conducts heat from the high to low temperature areas. Taking a variation on the gas law, adiabatic sound and isothermal sound speed are related by the square root of the ratio of constant presure specific heat to constant volume specific heat of air.

The net result, as nosebleedaudio wrote, is that porous absorbers are effective down to around 5% of the wavelength of the sound in random incidence absorption, and 10% in normal incidence.

Phillip Newell has a good explanation in Recording Studio Design.

Andre

[Bubbagump]

Also, the 1/4 wave length deal is for optimal placement at a given frequency (see boundary cancellation in subwoofer placement). However, you will never be able to place a panel at the 1/4 wave length of every frequency. 1/4 wave length placement is a more a theoretical ideal than a practical possibility.

[jacko]

Originally Posted by Bubbagump

Also, the 1/4 wave length deal is for optimal placement at a given frequency (see boundary cancellation in subwoofer placement). However, you will never be able to place a panel at the 1/4 wave length of every frequency. 1/4 wave length placement is a more a theoretical ideal than a practical possibility.

No absorbers:
Attachment 161907

22 absorbers:
Attachment 161908

depth of these absorbers is 1/4 wave length of 425Hz

[SaSi_SiDi]

I think that Avare's response pins down the issue quite elegantly. Only thing I'd add is that absorbtion is not all or nothing. Even in a frequency range where the panels won't give 80% effect, enough panels can do a difference. But then facing needs to be applied so that the panels won't deaden the higher frequency range.

[Nordenstam]

Originally Posted by avare

In porous absorbers the material conducts heat from the high to low temperature areas.

IIRC, Cox and D'Antonio claims that heat exchange is the secondary source of absorption with friction losses (viscous boundary layer losses) being the primary source of absorption in such devices.

[avare]

Originally Posted by Lupo

IIRC, Cox and D'Antonio claims that heat exchange is the secondary source of absorption with friction losses (viscous boundary layer losses) being the primary source of absorption in such devices.

Uhm, I do not understand the reference. They are of course correct. It is a matter of physics. The heat exchange within the porous material between the areas of compression and rarefaction is what causes the speed of sound to decrease compared to open air. It is not a mode of sound absorption.

Andre

[DanDan]

Thanks Andre. Nicely put as always. You should write that web page!
Speaking of which, do you have a website?
Happy St. Patrick's Day
DD

[Weasel9992]

Originally Posted by Lupo

IIRC, Cox and D'Antonio claims that heat exchange is the secondary source of absorption with friction losses (viscous boundary layer losses) being the primary source of absorption in such devices.

Like Andre, that confused me a little...they're two sides of the same coin. Thermal dissipation is a function of friction loss isn't it?

Frank

[avare]

Originally Posted by DanDan

Thanks Andre. Nicely put as always. You should write that web page!

Thanks Dan. That is 2 items I should write now. The other being an introduction to gas flow resistance and absorption.

Speaking of which, do you have a website?

No.

Happy St. Patrick's Day

Thanks.

Despite popular belief, the Irish only drink one day a year, St. Patrick's Day. They practice the other 364.

Andre

[Nordenstam]

Originally Posted by avare

The heat exchange within the porous material between the areas of compression and rarefaction is what causes the speed of sound to decrease compared to open air. It is not a mode of sound absorption.

Thanks for the clarification!

Cox and D'Antonio writes: [blahblah viscous boundary layer losses] "As well as viscous effects, there will be losses due to thermal conduction".

I read it as both friction and thermal conduction dissapates energy, with the former being the primary source of loss and the latter a secondary source of loss. Another way to see it as that a material with lots of friction and zero heat conduction would still have significant absorption.

Semantics... But you know, details matter! Was just being curious.

[avare]

Originally Posted by Lupo

Thanks for the clarification!...

Semantics... But you know, details matter! Was just being curious.

You are welcome. If it was intuitive, it would not be acoustics!

Andre