Comparison of normal and impaired hearing. I. Loudness, localization

Impaired hearing is characterized by high thresholds and reduced loudness. Loudness, however, may quickly recover as it increases rapidly from an elevated threshold. This rapid growth, known as loudness recruitment, is a sign of cochlear impairment and is generally not seen in conductive or retrocochlear impairment. Loudness recruitment means that the hard-of-hearing person detects small changes in intensity near his elevated threshold but he probably does no better than a normal listener at the same SPLs. Recruitment is often accompanied by reduced loudness summation, which means that the loudness of a band of noise does not increase as much with increasing bandwidth as in normal hearing. This reduced summation of loudness is probably why the cochlearly impaired ear has nearly the same threshold for the acoustic reflex to pure tones as to wide-band noise, whereas the normal ear has a much lower threshold to wide-band noise. Corresponding differences between normal and impaired hearing are not found in auditory localization. Rather, the evidence suggests that persons with residual hearing learn to localize sounds reasonably well. Even the inability of many hearing impaired persons to understand a speaker in a noisy environment may result more from a failure of frequency analysis rather than of localization.