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I would like to fight a resonance with a Helmholtz absorber in a room with a volume of 97 cubic meters.

I'm building a home theater in a room that's 9.2m long (11 x 3.3 x 3.2).

enter image description here

In the beginning I had a disastrous reverberation. With self-built broadband absorbers, I have achieved a linear reverberation of approx. 250 ms - 350 ms in the range from 150hz to 20khz.

Broadband absorbers and bass traps (Bass traps and broadband absorbers)

But, since the broadband absorbers are hanging, I have now a resonance peak at around 35hz.

REW EQ resonance peak

(The reverb is clearly audible playing a sine tone in the frequency.)

My idea was to set up an additional bass trap in the other corners of the room or to build a Helmholz absorber.

Here is my calculation for a Helmholz absorber (actually a multi-layer absorber)

Helmholz absorber calculation

(This is no standard helmholz resonator with a defined volume and one hole!)

Question: What volume or area should such an absorber have?

I have a massive old beech wood panel with about 150cm x 80cm that I could use as a back. The volume would then be 0.48 cubic meters. (180 x 80 x 40)

If I can believe the little information that can be found on the subject, then that could already be too much.

marsh-wiggle
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1 Answers1

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As to your question: "what volume or area should such an absorber have", the volume is the depth times the area, of course, and the depth in this calculation is given by the absorber "thickness" behind the perforated front. Some models call this the "construction depth", if the absorber material thickness is less than the distance to the backing.

As for area, it would be proportionate to the desired peak absorption. So to attain a peak of 0.5 * 1 (1 from the resonator's performance), you'd cover 50% of the wall on one reflective side, e.g. the back. That's your area, in theory.

Since you have a complex set-up of broad band absorbers, with reflective floors and ceilings and future furniture and human(s), the ultimate listening quality will not be easy to model and predict. Moreover, modelling and measuring small rooms is exceptionally fickle.

Also, you seem to be correctly concerned about T60 at 35Hz, which is important in critical music listening, but less so with movies which are mostly full of distracting wide band noise just when the 35Hz matters. Looking at your curves, the T20 or T30 at 35Hz seem to be less problematic, and might better predict what you'll experience in a movie.

In your room it's quite possible that after putting down carpet and furniture, you could probably do with equalizing the 35Hz down for music listening & movie watching, depending on the content.

P2000
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