Environmental Engineering FOURTH EDITION

438 ENVIRONMENTAL ENGINEEFUNG

micircular canal

Auditory

nerves

-_,I--I-I

Outerear I I Innerear I

I I

I

Middle ear

Figure 22-11. Cut-away drawing of the human ear.

0

50

Teenage band \ \

members

playing rock

and roll music

b. \ -.- - -0

M Preexposure

+ -a Postexposure

60 I I I I I I

1000 3000 5000

Frequency (Hz)

Figure 22-12. Temporary threshold shift for rock band performers. (Source: Data by

the U.S. Public Health Service. See Still (1990).)

and the latter is known aspemanent thmhddshzft. Literally, your threshold of hearing

changes, so you are not able to hear some sounds.

Temporary threshold shift is generally not damaging to your ear unless exposure

to the sound is prolonged. People who work in noisy environments commonly find

that they hear worse at the end of the day. Performers in rock bands are subjected to

very loud noises (substantially above the allowable OSHA levels) and commonly are

victims of temporary threshold shift. In one study, the results of which are shown in Fig.

22-12, the players suffered as much as 15 dl3 temporary threshold shift after a concert.

Repeated noise over a long time leads to permanent threshold shift. This is espe-

cially true in industrial applications in which people are subjected to noises of a certain