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Trying to understand the limitations of Helmholtz resonators in LF absorption

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Delrin

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Here's an old but very informative thread on a successful Helmholtz absorber project that gives lots of build details, great references, and testing results. It meets pretty much all the criteria I listed earlier.

It's worth reading the thread from start to finish if you're interested in Helmholtz absorption.

A few highlights:
  • the project uses plywood boxes with lengths of 2" ABS pipe as necks in the resonator
  • the only absorption added is speaker fabric (or equivalent) across the mouth of the absorber (no insulation inside the cavity)
  • numerous measurements and photos are shown, and the absorbers appear effective at improving the targeted room modes
The poster also found Ingard 1953 to be one of the best references on Helmholtz absorbers.
 
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BR52

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I am really not an Acoustic specialist and see here some deeper knowledge.
In general, from my understanding an acoustic resonance absorber is divided in 2 portions, one Frequency selective part and second the absorbing part.
First one is made by geometry, second is made by something that changes the acoustic energy to heat. (Damping wool, pressure changing etc.)
Please let me know if my understanding is on the right way because if the answer is yes I would like to discuss something more.
Thanks in advance
 

mczx

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Agreed.

In order to make a Helmholtz resonator absorb sound, instead amplifying, you need to slow the air oscillating in the opening by friction. I glued a thin fleece material to the rear of the opening with an additional layer of wool behind it.

Similar but opposite in action of subwoofers, the amount of energy to absorbed can only be generated with equally large amount of air displaced, I.E. with large opening area.
What thickness of Roxul, 703 fiberglass, recycled denim or felt would be too THICK and defeat the purpose?
 

mczx

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Edit: There is a somewhat well-known example of a Helmholtz resonator in Chapter 12 of "Master Handbook of Acoustics" by Everest and Pohlmann. The example is purported to address a mode at 47Hz and they show waterfall plots that demonstrate greatly improved ringing. The resonator is based on a large concrete forming tube (commonly branded Sonotube in North America) with a single port at the bottom. I don't really consider this to meet the standard laid out above, as the implementaiton details are somewhat sketchy as is information about the room. Moreover, there is at least one online account from someone who did their best to replicate this design and found no effect on room acoustics.
Interesting experiments here, all measured.
 

mczx

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I enjoyed a person call Master M over at the AG forums. I named him. Look at his room. Seriously LOOK at his 2-3 year experiment.
You will be amazed. He took Helmholtz, 500.00 dollars and really became a happy fellow. The room is something.

He never measured a single thing. The plates and wires and passive materials he used were from the trash.

AG forums?
 

ChrisG

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Interestingly, as the SPL of the sweep is decreased, we can see the emergence of some small acoustic reactivity at the resonant frequency. This fits with the notion of non-linear effects, and further suggests that some neck geometries are just "bad" even if they do support resonance.

I just wanted to pick this up. The approach I'll outline below isn't a particularly scientific approach, but it is a handy way of getting an order-of-magnitude idea of what might be going on.

Let's say we have a 12" driver in a sealed box. To produce 100dB@40Hz, 4.8mm of excursion is required: http://www.baudline.com/erik/bass/xmaxer.html

Now, let's imagine for a moment that we wish to absorb that 100dB@40Hz, using a bunch of tiny drivers distributed across the opposite wall. Qty 1000x 0.25" (approx - there's a note on the website regarding radiating diameters) drivers would each require an Xmax of 11mm to produce an identical SPL. NB - Xmax is a one-way measurement.

Given that the perforated board is often only a few mm thick, the "slug" of air inside the perforations is being pushed very much further than the thickness of the board.

Consider, for a moment, an imaginary ported box where the air is being shoved that far through the port and out the other side, and it becomes intuitively obvious that small-diameter holes will quickly become overloaded. The solutions might include larger-diameter holes, or simply using many more of them.


Further notes:
- The grid of holes could be simulated in something like Hornresp, a loudspeaker simulator, to calculate the Helmholtz resonant frequency. Imagine virtual cabinets stacked in a large grid. Each is a ported box. Volume of air within the imaginary cabinet is your simulated volume, and you have the "port" dimensions already. Hornresp can also simulate chamfers (both sides), peak air velocity, etc etc. You'll need a generic loudspeaker input so that the simulator can have something to "drive" the acoustic chamber, but keeping that generic loudspeaker constant while varying other parameters (depth/area of cabinet, port dimensions) will allow comparisons.
- As hole diameters tend towards the very small and numerous, the surface area to volume ratio of the slug(s) of air will change, in the direction which provides more resistance to the flow of air through the hole. We've seen above that this will make them non-linear in their absorption as incident SPL changes.
- Staying in the world of loudspeaker design for a moment, there's a sliding scale between Helmholtz resonators and quarter-wave resonators. I'd suggest that when the aspect ratio of an absorber passes about 5:1, there's likely to be quarter-wave (and harmonic) resonances happening as well. Putting a "port" on the end turns it into a mass-loaded quarter-wave resonator, where the "slug" of air at the end will act as an additional mass on the end of the spring.


Apologies for the brain-dump. I read the first few pages and was excited to contribute.

Chris
 

BDWoody

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