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Soundproof a Room Stage 1 Part 5

Absorbency

The acoustic absorbency of a material determines the proportion of energy removed from a sound wave when it reflects off that material.

Absorbency values are almost always quoted as a number between 0 and 1. So at the bottom end of the scale, a material with an absorbency of 0 is a perfect reflector (which, in reality, does not exist), and a material with an absorbency value of 1 indicates a perfect absorber (think of an open window; the sound travels right through and you get absolutely no reflection).


If this was all there was to sound absorbency, then life would be very simple. You'd just figure out how much sound energy you want to absorb, calculate the resulting RT, and there you are!

Unfortunately, it doesn't work that way. The reason is that the acoustic absorbency of a material changes depending on the pitch of the note. Typically, you will find that high notes will be absorbed more completely than low notes.

For example, a material that is able to absorb 60% of at note at 1KHz, may very well only be able to absorb 15% of a note three octaves lower at 125Hz. The technical way of describing the way absorbency behaves is to say that it is frequency specific.

Below is a table showing the typical absorbency values for 9.5mm plasterboard mounted on a stud frame at 600mm centres behind which is a 120mm air gap. (Notice that in order for absorbency figures to be quoted accurately, you must be very specific about the construction details).

 

125 Hz 250 Hz 500 Hz 1 KHz 2 KHz 4 KHz
9.5mm Plasterboard 0.22 0.08 0.11 0.07 0.09 0.22

 

It should be fairly clear from this table that plasterboard mounted on this type of stud frame is a pretty lousy acoustic absorber!





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