ABSTRACT
The A network is now used when measuring sound to estimate the risk of noiseinduced hearing loss, because it represents more accurately the ear's response to loud noise. It is not possible to describe a noise's damaging effect on hearing simply by stating its intensity. For instance, if one noise has a spectrum similar to that shown in curve A in Fig. 2.11, with most of its energy in the low frequencies, it may have little or no effect on hearing. Another noise of the same overall intensity, having most of its sound energy in the higher frequencies (curve C), could produce substantial hearing damage after years of exposure. Examples of low-frequency noises are motors, fans, and trains. High-frequency noises are produced by sheet metal work, boiler making, and air pressure hoses. Although the human ear is more sensitive in the frequency range 1000-3000 Hz than it is in the range <500 and >4000Hz (Fig. 2.13), this frequency-specific differential sensitivity does not explain fully the ear's vulnerability to high-frequency sounds. Although various explanations have been proposed involving everything from teleology to redundancy of low-frequency loci on the cochlea to cochlear shearing mechanics, the phenomenon is not completely understood. Mechanisms of noiseinduced hearing loss are discussed in Chapter 14.
