Page 213 - Master Handbook of Acoustics

FIGURE 11-4 Reverberatory decays produced by impulse excitation of a small studio. The upgoing
left side of each trace is instrumentation limited; the downgoing right side is the reverberatory decay.

In Fig. 11-4, the straight, upward traveling segment on the left side is the same slope for all decays
because it is a result of instrumentation limitation. The useful measure of reverberation is the

downward traveling, more irregular slope on the right. This slope yields a reverberation time after
the manner of Fig. 11-3. Note that the octave-band noise level is higher for the lower frequency
bands. The impulse is barely above the noise floor for the 250 Hz and lower octaves. This is often a
major limitation of the method.

Steady-State Sources

As noted, steady-state sources can be used to examine a room’s response; for example, as noted,
Sabine used a wind chest and organ pipes for his early measurements. However, care must be taken
to choose a steady-state source that provides accurate response data. Sine-wave sources providing
energy at a single frequency give highly irregular decays that are difficult to analyze. Warbling a tone,
which spreads its energy over a narrow band, is an improvement over the fixed tone, but random
noise sources are preferred. Bands of random (white or pink) noise give a steady and dependable
indication of the average acoustical effects taking place within that particular part of the spectrum.
Octave and 1/3-octave bands of random noise are commonly used. Steady-state sources are useful for

measuring the growth of sound in a room, as well as the decay response.

Measuring Equipment

The equipment layout shown in Fig. 11-5 can be used to obtain reverberation decays. In this example,
a wideband pink-noise signal is amplified and applied to a loudspeaker. A switch for interrupting the
noise excitation is provided. By aiming the loudspeaker into a corner of the room (especially in