Helmholtz again;)

Hello,... i´ve often see Helmholtzresonators with more than one whole. Say for example with 8 or more to reach a specific frequency. Is a helmholtz with more than one whole (and more volume) more effective for reducing axial roommodes or reflections? kind regards

If you're talking about perforated panels (with absorptive material behind), the size and spacing of the holes, and the thickness of the perforated board dictate the performance. Generally, they are a better approach than a simple "cavity resonator" because it's easier to cover large areas of walls and ceiling with perforated board and absorber material. "Slat" absorbers are another alternative. (As are panel absorbers with no holes or slats.) John (et al) can probably provide some good "how-to" links. Also, the necessary formula(s) can be found in most acoustics textbooks (e.g., Everest's Master Handbook of Acoustics). Also, if you're good with Excel, Chris Whealy's Porous Absorber spreadsheet has a page for calculating the response of a perforated panel absorber. All the best,

by the way is it possible to use a board with random holes diameter ?

Hello Lovecow,... yes i read the article in Everests Handbook. :D Actually you are right, i mean a perforated absorber but with only a few holes. Say for example only 8 or something like that to reach such a low frequency like 40 to 100 Hz. Actually a Cavity Resonator with more than one hole (and therefore with more volume) to increase its effect. I have often seen such resonators in rectengual form in corner nears mounted flat to a wall. Is it more effective for a special known sidewall reflections or an axial mode in contrast to a cavity resonator placed at the floor? thx kind regards :)

psycho: Random hole diameters will result in random absorption. The overall effect is likely to be a broader frequency range, but with less overall absorption. If you were to do, say, 4 ft² areas, each with a different set of hole diameters and spacings, it will probably be a little more predictable. Of course, it also depends on how many holes - see my response to Micha below. Micha: What you're talking about is more along the lines of a cavity resonator. For example, a 4 ft² panel with several holes will have a very narrow frequency range with high absorption. The depth of the holes would, presumably, be fixed. The overall area of the holes could then be summed as the "hole area" that is included in the Helmholtz equation for a cavity resonator. I have done something similar with seating risers; to use the joist cavities as Helmholtz resonators, I determined the volume of the joist cavity and used one or more holes on the face of the riser to get it to resonate at the desired frequency. Using different hole sizes for different joist cavities - say A1 total area for odd joist cavities and A2 total area for even joist cavities - can address two frequencies. (And so on.) Additionally (or alternatively), the volume of the joist cavities can be varied by changing the joist spacing. One thing to be aware of is the more frequencies one tries to address, the less overall absorption one will have at each frequency. Additionally, the larger the absorber and the more holes in the face, the closer you will get to a perforated panel resonator. I don't have a really good idea about where you would transition from one to the other - it would depend on the number of holes relative to the size of the cavity behind. Please also keep in mind that a perforated absorber does not require a sealed cavity behind. A cavity resonator - even in the form of a 2'x2' "panel" has to have a sealed cavity in order to have the desired (calculated) performance. Without a sealed cavity, you're likely to wind up with a perforated absorber of low efficiency. As for placement, any resonant absorber is best placed against the wall at a place where the modal frequency of interest is at maximum pressure. For a side-to-side axial mode, this could theoretically be anywhere on either side wall. In practice, I have found that they are more effective when they are placed so that they are centered with your ears on the wall, give or take. Finally, do keep in mind that resonant absorbers are very sensitive to design parameters. Required tolerances (for panel thickness, cavity depth, hole spacing, etc.) are typically 1/8" to 1/16" (1.5 to 3 mm) (even smaller for hole diameters) in order to ensure the correct frequency range is being addressed. In other words, be very sure of the frequency range (measure the room response at multiple points, if possible), check your calculations multiple times, and measure two or three times before firing up the saw / drill. :wink: HTH. :)

thnx man

@lovecow: wow...thx for your fast help. Great informations from you :D Should those resonators have a minimum depth behind the holes for example for 300l volume to work correctly? I´ve heard or read that once. I know its possible to build them a little bit more broadband thus i could fill them with a little more insulation. Say perhaps the half or a third?! The problem i have is a very very deep dip @52Hz. The reason for this is a single side wall reflection and also the first axial mode in lenght. Do i also have a chance to absorb this single surface reflection with such kind of absorber? Panel Traps would be to thick and perhaps have not of enough effectiveness. I´m just a lilte bit unsure if a cavity absorbers itself didn´t resonate a little bit to long for one single reflection causing more trouble.

Michawave wrote:

Should those resonators have a minimum depth behind the holes for example for 300l volume to work correctly? I´ve heard or read that once.

I hadn't heard that exact number, but that approach makes sense.

I know its possible to build them a little bit more broadband thus i could fill them with a little more insulation. Say perhaps the half or a third?!

I generally advise at least 50-75% full to adequately widen Q without taking too much away from the absorption. Less (or none) could work fine, depending on how close the design is to reality. However, since reality is, well, reality, at least some absorbent fill is encouraged. Of course, it shouldn't be too complicated to design the absorber so that fill material can be added / removed easily in situ.

The problem i have is a very very deep dip @52Hz. The reason for this is a single side wall reflection and also the first axial mode in lenght. Do i also have a chance to absorb this single surface reflection with such kind of absorber? Panel Traps would be to thick and perhaps have not of enough effectiveness. I´m just a lilte bit unsure if a cavity absorbers itself didn´t resonate a little bit to long for one single reflection causing more trouble.

Resonating absorbers will work for the target frequency range regardless of the nature of the problem - modal or otherwise.