THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

10Physiology of Hearingand Psychoacoustics10.1 Human HearingThe mechanism of the human ear has been a source of much wonder for physiologists,who are progressing well beyond the fragmentary knowledge of the pastby continually uncovering layers of new marvels of how the human ear reallyfunctions. Our hearing mechanism is a complex system that consists of many subsystems.The mechanism of the brain’s processing of auditory stimuli is probablybeginning to be understood, with the relatively recent discovery that hearing ismetabolic in nature. Much of the pioneering work was performed by Georg vonBékésy (1899–1972) who received the Nobel Prize in Medicine or Physiologyfor his investigations, particularly those entailing the frequency selectivity of theinner ear. His work indicated that the frequency selectivity ranked far poorer thanthe ear actually exhibits, but William Rhode found much greater selectivity in hiswork with live animals (von Békésy worked with dead animals, which most likelyaccounted for the difference). It is now realized that frequency selectivity of theinner ear fades within minutes after the metabolism ceases.A young, healthy human is capable of hearing sounds over the frequency rangeof 20 Hz–20 kHz, with a frequency resolution as small as 0.2%, Thus, we candiscern the difference between a tone of 1000 Hz and one of 1002 Hz. With normalhearing, a sound at 1 kHz that displaces the eardrum less than 1 Å (angstrom) canbe detected, in fact, less than the diameter of a hydrogen atom! The intensity rangeof the ear spans extremes from threshold at which softest sounds can be detected tothe roar of a fighter jet taking off, thus covering an intensity range of approximately100,000,000–1. The ear acts as a microphone in the process of collecting acousticsignals and relaying them through the nervous system into the brain. The ear(cf. Figure 10.1) subdivides into three principal areas: the outer, middle, and innerear.The outer ear consists of a pinna that serves as a sound-collecting horn and theauditory canal that leads to the inner ear. The collected sound enters the ear throughthe opening (the meatus) into the auditory canal that forms a tube approximately0.75 cm in diameter and 2.5 cm length. The canal terminates at the tympanicmembrane (eardrum). Under the impetus of the sound the eardrum vibrates, causing213