REPEAT - Questions on Helmotz absorbers

[MarcoSan]

Sorry guys, but I'm still looking for some replies to my questions :

1) The formula for calculating the Helmotz absorber is based on the distance from the wall of the box. But in case of a corner unit, which distance should I consider ?

2) John suggest to alternate size of slats and of gaps on the front of the absorber, but is this really necessary ? I guess this will help to widen the bandwidth of the absorber, but how to use the formula to consider these variables ?

3) I am currently "playing" with a software called CARA (www.cara.de), that should help me with the design of the room from the acoustic point of view. For those who do not know this SW , it contains a sort of CAD for 3D objects to be later on placed in the room and each object is defined by its physical dimensions and its acoustical characteristics.
I wonder if it is possible to load the characteristics of the units as per John's design : of course size is not a problem as can be easily measured (or defined in advance) but what about the absorption (alpha) coefficient of these units at various frequencies ? Does anybody have some data for units he already built ?

Anybody out there wants to throw away some of his precious time to enlighten me ?

Thks in advance & have a nice day !

Marco

[barefoot]

Hi Marco.

1. Actually the fundamental formula for a Helmholtz resonator depends on the total area S of the slots and the volume V of the rear cavity.

f = (c/2π)sqrt[S/(V*SOd)]

where c is the speed of sound and SOd is the slot depth (slat thickness).

A rectangular slot resonator offers a special case. The slot area depends on the overall height H and width W of the box and the ratio of slot width SAw to slat width SOw.

S = H*W*[SAw/(SAw + SOw)]

Of course, the volume of the cavity is given by:

V = H*W*D

were D is the overall depth.

Substituting V and S back into the resonance frequency equation we get:

f = (c/2π)sqrt[H*W*[SAw/(SAw + SOw)] /( H*W*D*SOd)]

and H*W/ H*W = 1 so

f = (c/2π)sqrt[SAw/[(SAw + SOw)*D*SOd]

Which probably looks more familiar.

But if your rear cavity is oddly shaped, all you need to do is refer back to the original equation. Calculate the volume of the cavity and the overall slot area and you're good to go!

2. Varying the slot width can have a mild affect on bandwidth, but not as much as you might think. Notice that the resonance frequency depends on S, the overall slot area. To a certain degree it doesn't really care about variations in the individual slot areas. The only reason you do get some spread is because the whole resonator is usually bigger than a wavelength near the resonance frequency. So the slots are somewhat acoustically isolated from one another at opposite sides of the resonator. Make sense?

3. I don't have any experience with this software. Maybe somebody else can answer?

Thomas