I'm wondering if your 32Hz issue might actually be a structural resonance, rather than a mode... or maybe a combination of the two. What are the dimensions of your room? (Bare inner-leaf surfaces, before you installed any treatment).

- Stuart -

No, this are my own experience after building some studios and around 60-70 Helmholtz. The literature is not very good defined today and often do not correlate to 100% with reality.

You have to notice, that there are two kinds of Helmholtz absorbers:
1. plates with holes, slots – a lot of them
2. an definite enclosure with only 1 single (variable) slot

Sure, you can tune a slot-hh to 39Hz but it needs a lot of depth/place and the porous material inside have to be the right one.
You can also use porous absorber below 200Hz – if you have the place for it. Ok, maybe 120-150Hz would possible mostly.

I know, I also find the range of 90-200Hz the most complicated in acoustics. To thread a 30Hz mode i find much more easy.


No, I wrote "single-slot" which means a enclosure with a hole in it

I haven't read the thread but:


Low end response is fine but low mids are horroble. Why why why?

It has to be thick if you want to target a bigger region (wider Q).
But for a specific frequency like 40 Hz, it doesn't need to be thick.

Good call!

Dimensions are 5.35 (h) x 2.1 (w) x 2.35 (h), which gives a fairly strong mode at 32 Hz. But waterfall plots also show other resonances/slow decay around and beneath that frequency, which I've always assumed were caused by more general room reverberations (e.g. drywall).

So likely a combination of the two, as you rightly say.

Here's a copy of the room modes plot for ref:

... which, by definition, is a slot resonator... It it has a single slot, then it is a slot resonator. If it has a single hole, then it is a perforated panel resonator. If that's not what you meant, then you should have used the correct term.

There's no difference. They are all Helmholtz resonators, with the only difference being variations in the equations to account for the difference in geometry of the air masses, and perhaps also for boundary layer effects if the slot(s) or hole(s) are very small.

If it has one slot or a hundred, it is still a slot resonator and still uses the same equations as all other slot resonators. If it has one hole or a thousand, it is still a perforated panel resonator, and still follows the same equations as all other perforated panel resonators. One hole works the same as any other hole. (Of course, for a single hole or slot, you do have to take into account the correction factors to account for the distance to the side walls of the cavity).

And if there is no sealed enclosure behind your "plate", then there is no Helmholtz effect: it's just a plate with holes (or slots) in it. It's the sealed cavity that creates the resonance.

Not necessarily. You can have slot resonators, or perforated panels, or any other form of Helmholtz resonator tuned to 39 hz without excessive depth. And the porous material is most effective when it is IN the moving air mass (inside the slots or holes), or right next to it. That's where the velocity is at it's peak, and even a thin layer of light insulation can be very effective if placed there, without detuning the resonance much. Placing thick absorption in the cavity isn't as effective at damping the resonance, but does broaden the Q, as well as lowering the frequency slightly.

As Andre has pointed out repeatedly (but few people seem to have noticed), there is a usable effect from porous absorber whose thickness is just 3.5% of the wavelength. Thus, an 80 Hz wave, whose wavelength is 430 cm, could see significant absorption from insulation that is 15cm thick. That's about 6". I have seen this demonstrated convincingly several times in rooms that I have designed. For example, when the insulation goes up on the ceiling (often 6"), there's a general smoothing out in the low end, easily down to 80 Hz, and lower.

The effect is real, and usable. If it wasn't, my clients would not be very happy about the acoustic results of their rooms! Porous absorption works down low. It isn't extremely effective, and is not what I use primarily for low modes, but it does have an effect.

Strange... most people have more trouble in the lowest end, less as you go higher. In many rooms, the 200 Hz region is over-treated, because it's so easy to hit with most types of treatment.

True! Thick can make it easier, but isn't necessarily required.

- Stuart -

One quick and somewhat silly test that might help: Run very slow sweeps at loud volume in REW (with ear protection, of course), but not to take measurements with a mic: to take measurements with your fingertips! Run your sweeps up through that region (say 20 Hz to 50 Hz), and rest your fingertips gently on the walls, floor, ceiling, and any other large possibly resonant surface in the room, to see if you can feel the vibrations at specific frequencies. If so, it's possible that you found a structural resonance. If you don't feel anything it doesn't mean that there is no structural resonance! It just means that it's not large enough to detect with fingertips. In which case a doctor's stethoscope can sometimes help.

Yeah, it all sounds weird, I know, but a couple of times this type of strange stuff has helped my find resonant problems in surfaces. Just a few weeks ago, I found an issue caused by an HVAC compressor like this: the 50 Hz vibration sound was visible in the graphs, and audible in only some places around the building (long wavelength), but hard to identify where it was coming from. The walls gave it away though: fingertips on the walls lead to the bathroom, and guess what was attached to the outside wall of that bathroom, with no vibration mounts?

Not 100% sure that this is your problem, but it's worth trying out. The wall might be vibrating anyway from the modal stuff, so run the sweeps slowly, and feel carefully for a noticeable increase in vibration at frequencies other than the modal one.

- Stuart -

That's a great idea! Beautifully simple too. Love it

I'm going to do just that and see what I can, well... feel. I'd wager it's almost certainly the drywall vibrating. The room is a converted internal garage: solid brickwork walls with drywall sheets fitted inside using standard 'dot and dab'. So it would make sense that the drywall is vibrating due to LF sound pressure.

I'll check.

EDIT: ok, so I tried this test and sure enough I can feel a very slight vibration of the drywall at the front of the room (i.e. nearest the speakers/sub) when playing low frequency sweeps from 20-60Hz. Not significant, but noticeable. Didn't check the ceiling, but I assume that's also feeling the vibes.

So that would appear to be a likely culprit for some if not all of the very low end resonance in my room. Good call Stuart