Environmental Engineering FOURTH EDITION

434 ENVIRONMENTAL ENGINEERING

from people. A plethora of parameters have been devised to respond to this discrep-
ancy in reactions, all supposedly “the best” means of quantitatively measuring human
response to noise. Among these are:

0 Traffic noise index (TNI),

0 Sones,

0 Perceived noise level (PNdB),

0 Noise and number index (NNI),

0 Effective perceived noise level (EPNdB), and

0 Speech interference level (SIL).

Measuring methods have proliferated because the physical phenomenon, a pres-
sure wave, is related to a physiological human response (hearing) and then to a
psychological response (pleasure or irritation). Along the way, the science becomes
progressively more subjective.
Measuring some noises, particularly those commonly called communi@ noise,
such as traffic and loud parties, is complicated further. Although we have thus far
treated noise as a constant in intensity and frequency with time, this is obviously not
true for transient noises such as trucks moving past a sound level meter or a loud party
near the measurement location. An unusual noise that may occur during a period of
general quiet is an intennittent noise and has a still different effect on people.

MEASURING TRANSIENT NOISE

Transient noise is still measured with a sound level meter, but the results must be
reported in statistical terms. The common parameter is the percent of time a sound level
is exceeded, denoted by the letter L with a subscript. For example, Llo = 70dB(A)
means that 10% of the time the noise is louder than 70 dB as measured on the A scale.
Transient noise data are gathered by reading the SL at regular intervals. These numbers
are then ranked and plotted, and the L values are read off the graph.

EXAMPLE 22.3. Suppose the traffic noise data in Table 22-2 are gathered at 10-s

intervals. These numbers are then ranked as indicated in the table and plotted as in

Fig. 22-5. Note that since^10 readings are taken, the lowest reading (Rank no. 1)

corresponds to a SL that is equaled or exceeded 90% of the time. Hence, 70dB(A)

is plotted versus 90% in Fig. 22-9.2 Similarly, 71dB(A) is exceeded 80% of the

time.

20ne might argue that since 70dB(A) is the lowest value, it is the SL exceeded 100% of the time and

one should plot 70dB(A) vs 100%. The second value, 71 dB(A), is exceeded 90% of the time, etc. Either

method is correct, and the error diminishes as the data increase.