by Patrick Wolfe
At the October 15, 1997 meeting of the Chicago section, Professor C. Daniel Geisler from the University of Wisconsin-Madison spoke on the topic of auditory physiology. Dr. Geisler's presentation, entitled "The Wonders of the Ear", addressed physiological mechanisms of the ear and theories accounting for their behavior. In Dr. Geisler's view, one of the most wondrous feats of the mammalian ear is its ability to successfully cope with a large input dynamic range (up to 140 dB at some frequencies) while outputting a range of values several orders of magnitude less (approximately 1-200 nerve pulses per second). Researchers now understand that the ear is able to accomplish this task through varying degrees of amplitude compression at almost every stage of the hearing process. The process begins as sound waves propagate along the ear canal and cause vibration of the tympanic membrane. The eardrum then vibrates the three bones of the middle ear (the incus, malleus, and stapes) which in turn cause a pressure differential at the oval window leading to the cochlea. Although the middle ear's operation is nearly linear for 0-130 dB, it can act to reduce sound pressure levels as a preventative action. When we speak the brain triggers contraction of middle ear muscles; thus, humming before a loud impulse such as a gunshot actually helps reduce the sound pressure level (SPL) seen by the cochlea. In the inner ear, pressure waves propagate along the fluid-filled cochlea until reaching a portion "tuned" to the particular frequency of the wave (low frequencies propagate the farthest). At this point a resonance occurs and the vibrations are damped. Observation of living cochleas indicate that the response of the cochlea at resonance is highly compressive for all but low frequencies. The Organ of Corti, located inside the cochlea, is responsible for the final physiological stage of the auditory process. It contains two sets of hair cells, inner and outer, which are connected to auditory nerve fibers. Dr. Geisler made the observation that although there are 3 times as many outer hair cells (OHC) as inner hair cells (IHC), 90-95% of nerve fibers are connected to the IHC. Further study into this area led him to propose the theory that the OHC also serve as an amplifier by acting as synchronized anti-friction devices. As part of a complex feedback system, they allow the IHC greater displacement for low SPLs, while at high SPLs the physical force limit on the hair cells limits their response. As his final topic Dr. Geisler touched briefly on the topic of hearing loss. As part of the aging process, humans begin to lose hair cells (mostly OHC) and even some of the 300,000 auditory nerve fibers leading to the brain. Exposure to high SPLs over extended periods of time accelerates this process, and as of yet it is irreversible. Dr. Geisler concluded by presenting some data comparing hearing loss in peoples of non-industrialized and industrialized parts of the world. Interestingly, the data indicate that the increased noise level of our industrial society does in fact cause a slight acceleration in the rate of age-induced hearing loss. Dr. Geisler's presentation drew a large and varied audience, including audio hobbyists, hearing aid manufacturers, and pyschoacousticians, to name a few. He remained afterwards to answer questions and to discuss further his most current theories. The Chicago section would like to thank Dr. Geisler sincerely for his time and for his clear, concise presentation of the complex concepts of auditory physiology.
