Page 244 - Master Handbook of Acoustics

is, the more accurate the measurement. A standard sample of the material to be tested, usually
measuring 8 × 9 ft, is mounted on the floor and the reverberation time measured. Comparing this time
with the known reverberation time of the empty room yields the number of absorption units the sample
2
adds to the room. From this, the absorption attributed to 1 ft of material is determined, giving the
2
equivalent of the absorption coefficient in sabins (1 m of material gives absorption units in metric
sabins).
The construction of the chamber is important to ensure a large number of modal frequencies and to
equalize mode spacing as much as possible. The position of the sound source and the number and

position of the measuring microphones must be considered. Large rotating vanes are used to ensure
adequate diffusion of sound. Absorption coefficients supplied by manufacturers for use in
architectural acoustical calculations may be measured by the reverberation chamber method.
A square-foot open window is the perfect absorber with coefficient equal to 1.0, but some

chamber measurements can show absorption coefficients greater than 1. This is because the
diffraction of sound from the edges of the standard sample makes the sample appear, acoustically, of
greater area than it really is. There is no standard method of making adjustments for this artifact.
Some manufacturers publish the actual measured values if greater than unity; others arbitrarily adjust
the values down to unity or to 0.99.

Impedance Tube Method

The impedance tube (also called a standing-wave tube or Kundt tube) has been applied to the

measurement of the absorption coefficient of materials. For this application, it can quickly and
accurately determine coefficient values. It also has the advantage of small size, modest demands in
terms of supporting equipment, and it requires only a small sample. This method is primarily used for
porous absorbers because it is not suited to those absorbers that depend on area for their effect such
as vibrating panels and large slat absorbers.

The construction and operation of the impedance tube are illustrated in Fig. 12-2. The tube usually
has a circular cross section with rigid walls. The sample to be tested is cut to fit snugly into the tube.
If the sample is intended to be used while mounted on a solid surface, it is placed in contact with the
heavy backing plate. If the material is to be used with a space behind it, it is mounted an appropriate
distance from the backing plate.