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Let's discuss membranes touching the absorber
Old 3 days ago
  #31
Quote:
Originally Posted by Hannes_F View Post
I can add that my interest in limp membrane absorbers began in 2008 when I read some remarks by the venerated Eric Desart in this very forum.

This is what he wrote then:
"By NOT sealing a membrane trap it goes deeper in frequency since you weaken the spring of the mass-spring system. You only get a lower Q and somewhat extended bandwidth.
A membrane damper does NOT assume a sealed cavity, it's only easier to calculate."

limp mass membrane

That is what I do, I do not seal them.
Hey just curious what the method would be for calculating for a non-sealed membrane trap? Thanks.
Old 3 days ago
  #32
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Quote:
Originally Posted by Kyle P. Gushue View Post
Hey just curious what the method would be for calculating for a non-sealed membrane trap? Thanks.
Yes Kyle, that is what I am also interested in, that is why I opened this thread.
The problem is of course that since the closed cavity is not dominating the process, other parameters become more dominant: Internal stiffness of the membrane, pressure on the damping material, stiffness of the absorber.

Empirically I can say that I found a 1 x 1.5 m² Fermacell plate (1 cm strong, 11.8 kg/m²) with 6 cm Sonorock (6000 Pa.s/m²) glued behind it (and then comes the wall), no side shelves, making a bass trap with a center frequency around 50 Hz. The Multi-Layer Absorber Calculator shows a center frequency around 60 Hz, so in that case it is not too far off. The more pressure you apply, the higher the pitch and the lower the Q.

http://www.acousticmodelling.com/mli...14=60&v14=6000
Old 3 days ago
  #33
Quote:
Originally Posted by Hannes_F View Post
Yes Kyle, that is what I am also interested in, that is why I opened this thread.
The problem is of course that since the closed cavity is not dominating the process, other parameters become more dominant: Internal stiffness of the membrane, pressure on the damping material, stiffness of the absorber.

Empirically I can say that I found a 1 x 1.5 m² Fermacell plate (1 cm strong, 11.8 kg/m²) with 6 cm Sonorock (6000 Pa.s/m²) glued behind it (and then comes the wall), no side shelves, making a bass trap with a center frequency around 50 Hz. The Multi-Layer Absorber Calculator shows a center frequency around 60 Hz, so in that case it is not too far off. The more pressure you apply, the higher the pitch and the lower the Q.

http://www.acousticmodelling.com/mli...14=60&v14=6000
Im wondering if something more like a helmholtz calculator would be useful. A vented membrane trap seems akin to a perforated panel. Im thinking the internal pressure/release might vary depending on if the vent is say a single hole vs say the perimeter of the front panel being left attached, but unsealed.

Maybe that would affect the velocity of the air flow, and the way the panel vibrates?

I think this topic is interesting since we use sealed cavities in isolation assemblies, and windows/doors, where resonance is am important factor.
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