Auditoryphysiologychapter.doc

Auditory Physiology The primary aim of this chapter will be to review the physiological mechanisms that are involved in two basic and extraordinarily important functions of the auditory system: (1) conversion of the vibratory energy that reaches the ear drum into a series of neural impulses on the auditory nerve (this is called transduction), and (2) the spectrum analysis function of the auditory system; that is, the ability of the auditory system to break a complex sound wave into its individual frequency components. Before getting into the details, it might be useful to consider some of the fundamental capabilities of the auditory system which, from any point of view, are nothing short of awe inspiring. A brief and by no means exhaustive list appears below. The faintest sound that can be detected by the human ear is so weak that it moves the ear drum a distance that is equivalent to one-tenth the diameter of a hydrogen molecule. If the ear were slightly more sensitive we would hear the random particle oscillations known as Brownian motion. The most intense sound that can be heard without causing pain is approximately 140 dB more intense than a barely detectable sound. This means that that the dynamic range of the ear – the ratio of the most intense sound that can be heard without pain to the intensity of a barely audible sound – is an astounding 100 trillion to 1. The frequency range of human hearing runs from approximately 20 Hz to 20,000 Hz, a range of about 10 octaves. For signal levels approximating conversational speech, the ear can detect frequency differences that are on the order of 0.1%, or approximately 1 Hz for a 1,000 Hz test signal (Wier, Jestaedt, Green, 1977). Later in the chapter we will see that the auditory system utilizes an elegant mechanism that delivers sounds of different frequencies to different physical locations along the cochlea; i.e., a sound of one frequency will produce the greatest neural activity at one physical location whi