Page 161 - The Master Handbook Of Acoustics

136 CHAPTER SEVEN

100 100

Sound pressure level, dB 60 60 dB Sound pressure level, dB 60 60 dB
80
80

40
40

0 RT60 0 RT60
Time Time
A B

FIGURE 7-4

The length of the decay dependent on strength of the source and the noise level. (A)
Rarely do practical circumstances allow a full 60-dB decay. (B) The slope of the limited
decay is extrapolated to determine the reverberation time.

level is 30 dB (as in Fig. 7-4A), because source levels of 100 dB are
quite attainable. If, however, the noise level is near 60 dB as shown in
Fig. 7-4B, a source level greater than 120 dB is required. If a 100-watt
amplifier driving a certain loudspeaker gives a sound-pressure level
of 100 dB at the required distance, doubling the power of the source
increases the sound-pressure level only 3 dB, hence 200 watts gives
103 dB, 400 watts gives 106 dB, 800 watts gives 109 dB, etc. The lim-
itations of size and cost can set a ceiling on the maximum levels in a
practical case.
The situation of Fig. 7-4B is the one commonly encountered, a
usable trace less than the desired 60 dB. The solution is simply to
extrapolate the straight portion of the decay.
Actually, it is important to strive for the greatest decay range possi-
ble because we are vitally interested in both ends of the decay. It has
been demonstrated that in evaluating the quality of speech or music,
the first 20 or 30 dB of decay is the most important to the human ear.
On the other hand, the significance of double-slope phenomena is
revealed only near the end of the decay. In practice, the highest level
of sound source reasonably attainable is used, and filters are often
incorporated to improve the signal-to-noise ratio.