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.

  • What are you measuring all this with, & from how many points in the room? I'm wondering how you get a 35Hz standing wave between two parallel walls 11m apart. It should be closer to 31. I'm also a bit puzzled at why you went with a huge range of rectangular & parallel devices, rather than first just getting a heavy carpet down, then getting a slope into the ceiling & back wall & seeing how much improvement that would give. It would have been heavier work but would have immediately broken up the main standing waves [ringing] & shortened the decay times massively.
    – Tetsujin
    Oct 2 '21 at 12:14
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    I’m voting to close this question because (although interesting) it's really not about "DIY Home Improvement".
    – brhans
    Oct 2 '21 at 12:45
  • @brhans I checked the questions here first and there are several about room acoustic like: diy.stackexchange.com/questions/158850/…. And my home theater is 100% DIY and I need help for the next DIY work. Oct 2 '21 at 12:56
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    OK, 35Hz sounds much closer to 9.5m resonance. I'm still puzzled why in a dedicated room you didn't first remove the parallelism. Low frequency standing waves are notoriously difficult to trap, it is far, far easier to take them out of the equation early on, before you start to do the stray-frequency trapping. Making the ceiling & back wall sloped rather than square would have removed so much of the issue in one fell swoop, leaving only a few stray frequencies to clean up, rather than an entire room full of various traps.
    – Tetsujin
    Oct 2 '21 at 16:32
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    If you're convinced something sounds horrible to you, it will, no matter what your wife, friends, or even Julian Hirsch says; audio is funny like that. If echo or standing waves impedes dialog that's one thing, but if you ever get the room flat, it will sound soulless and unemotional/detached; great for cutting movies, terrible for enjoying them. Fix the notch in EQ if you can, that's exactly what it's designed for; adjusting to the environment, nothing indecent about doing it correctly versus an imaginary perfection that will never be good enough.
    – dandavis
    Oct 3 '21 at 18:52

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.

  • Thanks. +1 :-) I could cover 13% of the wall with my material, so that 13% of the wave would be absorbed. But since the resonance is caused by multiple reflections, it is not possible to predict how this will affect the overall result, correct? Oct 3 '21 at 7:05
  • One more note: I've checked many different measures to optimize the room. I currently have 2.4 m3 absorbers with different flow resistivities (5 - 15 kPa * s / m2). I have mattresses on the floor as a test, as even thick carpets either didn't do anything or only absorbed the higher frequencies. Oct 3 '21 at 7:19
  • In terms of resonance, what matters are standing waves, and they need reflections on both ends. So if you 100% absorb on one end, there is theoretically no more resonance. But reflection also affects spectrum flatness (as I am sure you know) since waves do not need to "stand" to constructively add at the listener's ear, so it will be tough to get rid of a final ripple if not all walls are dead. Nice question by the way!
    – P2000
    Oct 3 '21 at 15:36

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