Rear wall absorption

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jamikl

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Joined 2005
Yesterday, at a local cafe, I was reading a New Scientist Magazine which had an article about sound absorption for apartment dwellers. There was mention of a latex film arranged like small square tiles with a heavy ball in the centre of each tile. The tiles were made into panels which when stacked one against the other made a panel which from memory was between 2 and four inches thick. It supposedly gave a 40 dB reduction over the frequency range 75Hz to 500 Hz. This would appear to have some use in our hobby. Has anybody else heard of it or knows whether it is production. I think the magazine may have been about two years old. When I mention in the heading rear wall I actually meant the wall directly behind the speakers which I realise is usually referred to as the front wall.
jamikl
 
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NATDBERG

Member
Joined 2006
These noise-cancelling panels consist of a latex rubber membrane stretched over a 3-millimetre-thick rigid plastic grid of 1-centimetre-wide squares. In the middle of each square is a small, weighted, plastic button.

When sound waves hit the panel, the membrane and weighted buttons resonate at difference frequencies. "The inner part of the membrane vibrates in opposite phase to the outer region," says Yang. That means the sound waves cancel each other out and no sound gets through.

Each weighted membrane only cancels out sound waves within a small band of frequencies. But changing the weight of the buttons alters the operational frequency, says Yang. By stacking five membranes together, each tuned to a specific band, you can create a soundproof panel that works in the range from 70 to 550 hertz.
Click to expand...

Sounds cool and not too difficult a thing to replicate..
 
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sreten

R.I.P.
Joined 2003
Hi,

The Hong Kong University of Science and Technology has shown experimentally that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies. The small decay length of the evanescent waves at these frequencies implies that several membrane panels can be stacked to achieve broad-frequency effectiveness. We report the realization of acoustic metamaterial panels with thickness 15mm (0.7 inches) and weight 3 kg/m^2 (about 6 pounds per square yard) demonstrating 19.5 dB of internal sound transmission loss (STL) at around 200 Hz, and stacked panels with thickness 60 mm and weight 15 kg/m^2 demonstrating an average STL of >40 dB over a broad range from 50 to 1000 Hz.

These noise-cancelling panels consist of a latex rubber membrane stretched over a 3-millimetre-thick rigid plastic grid of 1-centimetre-wide squares. In the middle of each square is a small, weighted, plastic button.

When sound waves hit the panel, the membrane and weighted buttons resonate at difference frequencies. "The inner part of the membrane vibrates in opposite phase to the outer region," says Yang. That means the sound waves cancel each other out and no sound gets through.

Each weighted membrane only cancels out sound waves within a small band of frequencies. But changing the weight of the buttons alters the operational frequency, says Yang. By stacking five membranes together, each tuned to a specific band, you can create a soundproof panel that works in the range from 70 to 550 hertz.
Click to expand...

rgds, sreten.
 
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gedlee

gedlee

Member
Joined 2004
gedlee.com
Remember that STL - sound transmission loss - is not the same thing as absorption. For example a sheet of steel will have a very high STL but very low absorption. Al of the incident sound get reflected. Only if the sound energy is dissipated internally is the STL going to be high while the absorption is low. With resonant systems this is usually not the case, but can be in some situations. Clearly this design is meant to maximize STL not absorption. And behind the speakers what we want is absorption. Only your neighbors care about the STL.
 
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layhzer

Member
Joined 2009
I have an application for a panel where either STL or absorption would probably work. I sought out and read the original article today. I must be missing something. It says that in each "cell" the area around the button moves in opposite phase to the portion of the cell where the button is located. My assumption is that the button is attached to the latex membrane and that the membrane is elastic. If so, wouldn't the membrane and the button form a spring/mass resonator? And wouldn't the whole cell move as one (all in phase)?
Or is this somehow analogous to the breakup mode of a driver?
 
gedlee

gedlee

Member
Joined 2004
gedlee.com
Thanks Frank

Interesting, but all of those application result in a very narrow band of effect. In acoustics we general want a wide band effect although there are cases when a narrow band is desirable.

Similar techniques have been used in the design of submarine internal structure to help to "stealth" them to sonar.
 
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