Environmental Engineering FOURTH EDITION

436 ENVIRONMENTAL ENGINEERING

As defined earlier, the symbol L refers to the percentage of time the noise is equal to
or greater than some value. The percentage is indicated by the subscript. ,510 is thus
the dB(A) level exceeded 10% of the time.
With reference to Fig. 22-9, Llo, L50, and Lgo are thus 80, 75, and 70dB(A),
respectively. These may then be substituted into the equation, and the NPL may be
calculated. It is always advisable to take as much data as possible. The 10 readings
in Table 22-2 are seldom sufficient for a thorough analysis. The percentages must
be calculated on the basis of the total number of readings taken: if 20 readings are
recorded, the lowest (Rank no. 1) corresponds to 95%, the second lowest, to 90%, etc.

THE ACOUSTIC ENVIRONMENT

The types of sound around us vary from a Beethoven concerto to the roar of a jet plane.

Noise, the subject of this chapter, is generally considered to be unwanted sound, or the

sound incidental to our civilization that we would just rather not have to endure. The

intensities of some typical environmental noises are shown in Fig. 22-10. Laws against

dB

50 HP Siren at 100 ft

Pneumatic rlveter

Jetliner 500 ft overhead

Air hammer

Air compressor

Heavy city traffic

if prolonged

70 Average traffic

60 Conversational speech, 3 ft

50 Business office

40

30

20 Motion picture studio

Average residence

Soft whisper, 5 ft

Room in a quiet house at night

'.f 0 +Threshold of hearing

Figure 22-10. Environmental noise.