Wich frequencies will be reflected by thin multiplex?

[Gie-Sound]

Wich frequencies will be reflected when constructing a thin multiplex plate (or plywood) on the front of an absorber panel, and wich will get into the absorber?

I googled for an hour, but I could not find a table which shows what frequencies will be reflected by (for example) a 8mm multiplex plate, and wich ones will cut trhoug it, into the absorber.

[SAC]

For a start, I must admit to having to Google to determine just what “multiplex plate” refers, only to discover it is plywood.

There are no ‘tables’ that predict the exact frequencies that will be reflected! Such behavior is dependent upon many factors. Several of which in the broadest sense are - :

One, the acoustic impedance of boundary material itself.

Two, the dimensions of the incident surface area. The narrowest surface dimension will present a limit at which wavelengths larger than the dimension will simply diffract around it. Assuming a ‘high’ acoustic impedance of the boundary material, wavelengths smaller than the smallest surface dimension will be reflected.

Three, the incident angle of the boundary surface relative to the incident signal. Combined with the acoustical impedance, the lower the angle of incidence relative to the surface (with 90 degrees being normal/perpendicular and ranging to 0 degrees being parallel and non-incident to the surface), the greater the reflection and the lower the effective reflected frequency (limited by dimension and wavelength).

In the simplest model, and assuming a perfectly reflective surface, the frequencies whose wavelengths are longer than the smallest reflective surface dimension will pass around the surface to absorbent material behind and may potentially be absorbed.

[DanDan]

Here's is a spreadsheet which might help.
Porous Absorber Calculator V1.56

DD

[Gie-Sound]

interesting & difficult stuff. Thanks!

[SAC]

Chris' spreadsheet would be very useful IF the plywood is either porous, perforated or slotted.
(...porous plywood? OK... thin plywood to go with those "very small rocks" that float; or if one is working under the premise espoused in the The Adventures of Buckaroo Banzai! ...But you will need to supply your own oscillation over-thruster...)

But it is less useful for a solid boundary.

[DanDan]

You are welcome Gie. Here's a classic from way back
Build a Better Bass Trap
You might take a look at Modex and the Primacoustic Max also.
Broadband absorbers are a lot easier to make and the performance is more predictable. Lots here. studiotips - tips on studio design, acoustics, and wiring
DD

[SAC]

If you are speaking of a solid resonant panel absorber, you might check out the resonant panel calculator at: mh-audio.nl - Acoustic

But please note, none of these resourced listed here or above will provide information on what frequencies are reflected.