Physical and Biological Bases of Hearing
Understanding hearing means to understand that pressure variations in the air are transformed into a code that the brain will recognize. In order to reach that goal, the reader is first introduced in this chapter to a description of the physical character
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Physical and Biological Bases of Hearing
Hearing relates to the sense responsible for translating a series of pressure variations in the air into an action potential, i.e., something that the brain can recognize. Before describing the biological bases of hearing, it is first necessary to understand what the brain needs to recognize.
2.1 Physical Characteristics of a Simple Sound Wave Sounds are produced because something vibrates in the environment. These vibrations are disturbances and their propagation is possible only because it happens in a material medium. This medium is usually air, but it could also be, for example, water or any other substance. If you are underwater and try to talk to someone, you will find that this is possible, but the carried message is far from being as clear as it is usually. In short, a body which vibrates produces sound, provided that the vibrations do not occur in a vacuum where nothing is transmitted. More specifically, the vibrations cause a series of compressions and rarefactions of the molecules in the environment. The normal pressure in the air is successively increased or decreased. As discussed below, the characteristics of these variations can be represented using a simple sine wave (for pure sound).
2.1.1 Frequency and Phase A key thing to consider in the analysis of sound is the speed of variations ranging from compressions to rarefactions to compressions and so on. These changes occur more or less rapidly. This speed of state changes is called the frequency, i.e., the number of cycles (“compression-rarefaction”) completed during a given period. It
© Springer International Publishing Switzerland 2016 S. Grondin, Psychology of Perception, DOI 10.1007/978-3-319-31791-5_2
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2 Physical and Biological Bases of Hearing
Fig. 2.1 Illustration of a sound (sinusoidal) wave for a pure tone of 1000 Hz (or 1 kHz)
was agreed to express this frequency in a number of cycles completed in 1 s. One cycle per second is 1 hertz (Hz), the unit used to express frequency and named after the German physicist Heinrich Hertz. The time taken to complete one cycle of the sine wave is called the period (Fig. 2.1). As for the circular motion, a period (or a complete cycle) involves 360° (360 degrees). The beginning of the cycle is 0°, whereas the maximum compression and the maximum rarefaction occur at 90° and 270°, respectively. Also, the relative position of two sounds over time is called phase. If two pure tones arrive at a given point in time with a difference of 1/8 of a cycle, they will be described as being 45° out of phase. If a sound has a frequency of 1 Hz when a cycle is completed in 1 s, a sound completing 500 cycles in 1 s has a 500-Hz frequency. If a cycle takes only 1 ms to be completed, that is to say, 1000 cycles are completed in a second, it will be a 1000-Hz, or 1-kHz, sound (pronounce kHz “kilohertz”). Sometimes, to express the idea of frequency, we use the notion of wavelength. This is denoted by the Greek letter lambda (λ) and consists of the linear distance bet
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