Sound Absorption Measurement And Other Absorbing Tails

Where’s The Point? Part 5

Over the past couple of months, I have discussed reverberation time (RT) and how reverberation is measured. However, it is often not realized (or is forgotten) that, when you measure the reverberation time of a space, you are really measuring the sound absorption, or at least the amount present at the time of the measurement.

It is far more convenient to discuss reverberation time when wanting to describe the acoustic “liveliness” of a space rather than the area of absorption present. Equally, to be meaningful, in the latter case, we would also need to know the volume of the room because 100m2 of absorption might be a lot of absorption in some rooms but not very much at all in others. We are generally interested in the reverberation time to provide an indication as to the likely clarity of speech or a room’s general acceptance for music, but reverberation time, although it is our oldest and primary measure of acoustics, is pretty crude in terms of what it can actually tell us.

As already noted, although RT is a measure of the amount of sound absorption present in a space, the parameter knows nothing about where the absorption is located, but merely how good (or bad) the space is at absorbing sound. The way in which sound is absorbed opens up a whole new area of acoustics, and understanding how materials absorb sound enables us to acoustically “tune” spaces for optimal acoustic performance. The sound-absorbing properties of a material can be measured in two ways. The first is by measuring the difference in reverberation time that occurs when a known amount of material is placed in a test room or chamber. The second way is by measuring the acoustic impedance of a material by placing a small sample of the material at the end of a plane wave tube; the “standing wave ratio” is measured with and without the sample present (or, nowadays, the transfer function between two microphones located along the length of the tube is measured and the reflection coefficient/sound absorption is inferred).

Although useful for quick “desktop” measurements, the technique can only deal with small samples (around 1.1in to 4in or 29mm to 100mm in diameter, depending on the frequency of interest) and so is limited to fairly simple compositions. On the other hand, although the reverberation room technique can deal with large samples (typically around 10m2) and so is ideal for measuring typical building materials, this does require the construction of a purpose-built and massive test chamber, typically having a volume of about 200m3 to 500m3 (about 7000ft3 to 17,500ft3).

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