OOF! wrote on Thu, 22 March 2007 10:41 |
one more question- i've found the info for calculating the depth etc. but no one mentions the dimensions of the box for either a Helmholtz or membrane resonator. what about length and width of the box? thanks again, David
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There are several kinds of traps based on Helmholtz resonance. These can include slat absorbers, perforated panel absorbers (a personal favorite of mine), masonry slot absorbers etc. At its most basic, Helmholtz resonance doesn't care about the dimensions of the box as much as it cares about the volume of the cavity. Of course the other variables are the surface area of the opening, and length (or depth, like the length of the neck of a bottle) of opening, adjusted for effective length rather than physical length (having to do with the air column's interaction with air on either side of the opening since the air column is a gas and not a solid).
To figure out the dimensions, you must do the calculations, and then take other things into consideration. You could make the cavity shallow, but you would need more area to attain the proper volume, thus limiting how many resonators you could fit in a space, ultimately affecting how much bass you could trap. You can't just put up a single Helmholtz absorber. You need to cover some area with them to make a significant difference. If you have plenty of depth, you can fit more per sq. ft. of surface area, and have more absorption.
In the case of slat absorbers and perforated panel absorbers, there are formulaic examples for reasonable depths, such as 8", where you can figure spacing of openings, size (area) of openings, and depth of openings for the box depth (hence volume). The ultimate dimension depends on how large an area you wish to cover with this (for example) 8" deep box with the suface openings as calculated for your target frequency.
Other things to keep in mind with Helmholtz are that you want your box to be air tight (apart from the required openings in front of course) and rigid. If it flexes easily, or air can escape from the sides or back, the spring of the cavity volume will change and the as-built response will not match your calculations.