High Frequency Absorbers

High Frequency Absorbers

Well just about anything absorbs high frequencies so be careful in this area. If you suck out all the high frequencies in a room your room will become lifeless. The main consideration in room acoustics is to aim to make the reverberation time near to equal in all frequencies. So if you put walls and walls of high frequency absorbers you will not have enough wall space to lower the low frequencies proportionately.

If you look at the coefficient of absorption figures for the various products you will note that whilst some attenuate the highs some also attenuate the low mids as well. 100mm (4″) fibreglass for example not only absorbs high frequencies but it also works down into the low mids depending on how thick it is.

The other main factor is what are the highs in your room doing? Consider the fact that your high frequencies are coming from your speakers which have a directivity factor. In a standard multi – speaker system the highs are coming from the tweeters or horns. Both these units have a fan shaped dispersion of around 30 degrees. And create what is referred to as the on axis off axis effect. Stand in front of a speaker and you hear all the highs but go 30 degrees off axis and the highs start to reduce to the point that if you are 90 degrees off axis the highs are eliminated completely (apart fro highs that reach you my reflection from some other surface.)

Take a look at this plan of a control room:

Control Room Plan

The dotted lines indicate the axis of the high frequency projection. Note that the engineer is sitting on axis to the speakers yet someone sitting to the right of the console is off axis to the right speaker but still on axis to the left speaker. The high frequencies are reflected by the opposing walls (in this case glass doors). The idea of this control room design is make sure (by angling the walls) that the high frequencies from the right speaker are not reflected back into your left ear.

Once the sound passes the engineer the rear of the control room absorbs the sound and it doesn’t come back to the engineer.

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