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Acoustic phonetics Acoustic phonetics deals with the properties of sound as represented in variations of air pressure. A sound, whether its source is articulation of a word or an exploding cannon ball, disturbs the surrounding air molecules at equilibrium, much as a shove by a person in a crowded bus disturbs the standing passengers. The sensation of these air pressure variations as picked up by our hearing mechanisms and decoded in the brain constitutes what we call sound [see also AUDITORY PHONETICS]. The question whether there was a sound when a tree fell in a jungle is therefore a moot one; there definitely were airmolecule variations generated by the fall of the tree but, unless there was an ear to register them, there was no sound. The analogy between air molecules and bus

passengers above is rather misleading, though, since the movements of the molecules are rapid and regular: rapid in the sense that they oscillate at the rate of hundreds and thousands of times per second, and regular in the sense that the oscillation takes the form of a swing or a pendulum. That is, a disturbed air molecule oscillates much as a pushed pendulum swings back and forth. Let us now compare air molecules to a pen-

dulum. Due to gravity, a pushed pendulum will stop after travelling a certain distance, depending on the force of the push; it will then begin to return to the original rest position, but, instead of stopping at this position, it will pass it to the opposite direction due to inertia; it will stop after travelling about the same distance as the initial

initial rest position; but it will again pass this point to the other direction, etc., until the original energy completely dissipates and the pendulum comes to a full stop. Imagine now that attached at the end of the

pendulum is a pencil and that a strip of paper in contact with the pencil is being pulled at a uniform speed. One can imagine that the pendulum will draw a wavy line on the paper, a line that is very regular in its ups and downs. If we disregard for the moment the effect of gravity, each cycle, one complete back-and-forth movement of the pendulum, would be exactly the same as the next cycle. Now if we plot the position of the pendulum, the distance of displacement from the original rest position, against time, then we will have Figure 1, in which the y-ordinate represents the distance of displacement and the x-abscissa the time, both units representing arbitrary units. Since a wave form such as the one given in Figure 1 is generatable with the sine function in trigonometry, it is called a sine wave or a sinusoidal wave. Such a wave can tell us several things. First, the shorter the duration of a cycle, the

greater (the more frequent) the number of such cycles in a given unit of time. For example, a cycle having the duration of one hundredth of a second would have a frequency of 100 cycles per second (cps). This unit is now represented as Hz (named after a German physicist, Heinrich Hertz, 1857-94). A male speaking voice has on average 100-50 Hz, while a woman’s voice is twice as high. The note A above middle C is fixed at 440 Hz. Second, since the y-axis represents the dis-

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rest position, the higher the peak of the wave, the greater the displacement. This is called amplitude, and translates into the degree of loudness of a sound. The unit here is dB (decibel, in honour of Alexander Graham Bell, 1847-1922). A normal conversation has a value of 50-60 dB, a whisper half this value, and rock music about twice the value (110-20 dB). However, since the dB scale is logarithmic, doubling a dB value represents sound intensity which is ten times greater. In nature, sounds that generate sinusoidal

waves are not common. Well-designed tuning forks, whistles, and sirens are some examples. Most sounds in nature have complex wave forms. This can be illustrated in the following way. Suppose that we add three waves together having the frequencies of 100 Hz, 200 Hz and 300 Hz, with the amplitude of x, y and z, respectively, as in Figure 2. What would be the resulting wave form? If we liken the situation to

direction, the first person pushing it with the force z at every beat, the second person with the force y at every second beat, and the third person with the force x at every third beat, then the position of the pendulum at any given moment would be equal to the displacement, which is the sum of the forces x, y and z. This is also what happens when the simultaneous wave forms having different frequencies and amplitudes are added together. In Figure 2, the dark unbroken line is the resulting complex wave. Again, there are a few things to be noted here.