Page 75 - Master Handbook of Acoustics

basilar membrane are stimulated by characteristic frequencies corresponding to that location. The
inner hair cells can be thought of as microphones, transducers that convert mechanical vibration to
electrical signals that initiate neural discharges to the auditory nerve and the brain. The outer hair
cells provide additional gain or attenuation to more sharply tune the output of the inner hair cells and

to make the hearing system more sensitive.
Bending of the stereocilia triggers the nerve impulses that are carried by the auditory nerve to the
brain. A single nerve fiber is either firing or not firing, in binary fashion. When a nerve fires, it
causes an adjoining one to fire, and so on. Physiologists liken the process to a burning gunpowder
fuse. The rate of travel bears no relationship to how the fuse was lighted. Presumably the loudness of
the sound is related to the number of nerve fibers excited and the repetition rates of such excitation.

When all the nerve fibers are excited, this is the maximum loudness that can be perceived. The
threshold sensitivity would be represented by a single fiber firing. The sensitivity of the system is
remarkable; at the threshold of hearing, the faintest sound we can hear, tiny filaments associated with
the stereocilia move about 0.04 nm—about the radius of a hydrogen atom.
A well-accepted theory of how the inner ear and the brain function has not yet been formulated.

The presentation here is a highly simplified explanation of a very complex mechanism. Some of the
theories discussed here are not universally accepted.

Loudness versus Frequency

The seminal work on loudness was done by Fletcher and Munson at Bell Laboratories, and reported
in 1933. Since that time, refinements have been added by others. The family of equal-loudness
contours shown in Fig. 4-6, more recent work by Robinson and Dadson, has been adopted as an
international standard (ISO 226).