Chapter 13 - The Auditory System

Chapter 13 - The Auditory System - Chapter 13 The Auditory...

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Overview The auditory system is one of the engineering masterpieces of the human body. At the heart of the system is an array of miniature acoustical detectors packed into a space no larger than a pea. These detectors can faithfully transduce vibrations as small as the diameter of an atom, and they can respond a thousand times faster than visual photoreceptors. Such rapid auditory responses to acoustical cues facilitate the initial orientation of the head and body to novel stimuli, especially those that are not initially within the field of view. Although humans are highly visual creatures, much human communication is mediated by the auditory system; indeed, loss of hearing can be more socially debilitating than blindness. From a cultural perspective, the auditory system is essential not only to language, but also to music, one of the most aesthetically sophisticated forms of human expres- sion. For these and other reasons, audition represents a fascinating and espe- cially important aspect of sensation, and more generally of brain function. Sound When people speak of sound, they are usually referring to pressure waves generated by vibrating air molecules. Sound waves are much like the rip- ples that radiate outward when a rock is thrown in a pool of water. How- ever, instead of occurring across a two-dimensional surface, sound waves propagate in three dimensions, creating spherical shells of alternating com- pression and rarefaction. Like all wave phenomena, sound waves have four major features: waveform , phase , amplitude (usually expressed in log units known as decibels, abbreviated dB), and frequency (expressed in cycles per second or Hertz, abbreviated Hz). For the human listener, the amplitude and frequency of a sound roughly correspond to loudness and pitch , respectively. The waveform of a sound is its amplitude plotted against time. It helps to begin by visualizing an acoustical waveform as a sine wave. At the same time, it must be kept in mind that sounds composed of single sine waves (i.e., pure tones) are extremely rare in nature; most sounds in speech, for example, consist of acoustically complex waveforms. Interestingly, such complex waveforms can often be modeled as the sum of sinusoidal waves of varying amplitudes, frequencies, and phases. In engineering applications, an algorithm called the Fourier transform decomposes a complex signal into its sinusoidal components. In the auditory system, as will be apparent later in the chapter, the inner ear acts as a sort of acoustical prism, decomposing complex sounds into a myriad of constituent tones. The Auditory System Chapter 13 275
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276 Chapter Thirteen Figure 13.1 diagrams the behavior of air molecules near a tuning fork that vibrates sinusoidally when struck. The vibrating tines of the tuning fork pro- duce local displacements of the surrounding molecules, such that when the tine moves in one direction, there is molecular condensation; when it moves in the other direction, there is rarefaction. These changes in density of the air molecules are equivalent to local changes in air pressure.
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This note was uploaded on 02/10/2010 for the course BIO NPB taught by Professor Furlow during the Spring '10 term at UC Davis.

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Chapter 13 - The Auditory System - Chapter 13 The Auditory...

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