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CHAPTER 1
Sound Waves
One of the main difficulties of studying speech is that sounds are so fleeting and transient. As each word is uttered it ceases to exist. We can, it is true, recall the sounds, either by repeating the words or by using some form of recording. But in both these cases it is another event that is happening. It is a copy of the original sound, not the sound itself.
Even during the brief existence of a sound it is curiously difficult to examine it. There is nothing that can be seen; there is no visible connecting link between a speaker and a listener. There is air around, but it is not normally possible to see any changes in the condition of the air when it is conveying a sound.
Because of these difficulties, it is perhaps best to begin our study of sound with a brief examination of the human ear. In this way we start with something tangible, for we know that the ear is the organ of hearing. Although there is still some uncertainty concerning the exact mechanism of the ear, we can nevertheless explain a number of facts about sound in terms of a simplified theory.
Figure 1.1 is a schematic diagram of the essential features of the ear. The first part to note is the eardrum, which is a thin membrane just over an inch down the narrow tube, or auditory passage, leading from the outer ear. When air is pushed down the auditory passage the eardrum tends to move with it; similarly it moves back as the air moves away. Connected to the eardrum is a chain of bones whose function is to transmit the movements of the eardrum to the liquid which is in the inner ear. Through the action of the bone chain the back-and-forth vibrations of the eardrum cause vibrations in the liquid. Closely linked with this liquid are the nerves which lead to the auditory sensation area of the brain. Movements of the liquid stimulate these nerves so that we experience the sensation of hearing. Bringing all these facts together, we may say that a sound is any disturbance of the air that could cause a displacement of the eardrum which, after transmission by the bone chain, could affect the liquid in the inner ear in such a way that the auditory nerves are stimulated. Our investigation into the nature of sound will be largely concerned with an examination of the disturbances in the air that can set off this process.
If we now turn to consider the origins of different sounds, we find that in every case some form of movement is involved. A noise occurs when a falling book hits the ground; a piano and a violin have strings that vibrate; and most speech sounds are caused by a movement of air from the lungs. It is these movements that set up the disturbances in the surrounding air.
The disturbances, however, do not occur instantaneously throughout all the air around the source of sound. They spread outward like ripples on a pond, so that there must be a short delay from the moment when the original movement caused the first disturbance to the instant when the disturbance reaches our ears. Sound travels very quickly, and consequently when we watch a person talking, we seem to hear the sounds at the same time as we see the movements that caused them. But in fact a small time has elapsed; and as we all know, in the case of a distant source of sound such as a thunderstorm, the flash of the lighting is often seen an appreciable time before the roll of thunder is heard.
In order to explain this phenomenon it is convenient to think of the air between our ears and a source of sound as being divided up into a number of particles. The source of sound causes movements of the air particles in its immediate neighborhood; these movements cause disturbances in the air a little farther away from the source; these air particles in their turn affect their neighbors which are still farther away from the source; and so the disturbance spreads outward.
We will begin our detailed examination of the production of sound by considering the note made by a tuning fork. If you look carefully at a tuning fork while it is sounding, you can see that the edges of the prongs are slightly blurred, because they are vibrating rapidly from side to side. This movement, which is shown in an exaggerated form in figure 1.2, makes a series of blows on the adjacent air. The diagram represents a moment when the right-hand prong of the fork has moved as far as possible to the right. At that moment the particle of air immediately alongside the fork has been moved so that it is now closer to the neighboring air particles. When the air particles are close together, the air is compressed; when they are farther apart than normal, there is what is called a region of rarefaction. A moment later, as the prongs of the fork spring together again, the air will be drawn back so that there is a region of rarefaction alongside the fork.
Thus, as the fork vibrates, the air alongside it will be alternately compressed and rarefied. This disturbance of the air alongside the fork will have an effect on the particles of air a little farther away. Small displacements of the air spread outward as indicated in the diagram; when they arrive at a listener's ear they will cause the eardrum to move, and this will result in their being perceived as sound.
To get a clearer picture of the behavior of the air we may consider the motion of a limited number of particles of air. In figure 1.3, the movements of thirteen air particles are represented (in a slightly simplified form). Each line of the diagram shows their positions a short interval of time after the moment represented in the preceding line. Line six, for instance, represents the positions these thirteen particles have assumed a moment after they were in the position indicated in line five. In this diagram stationary particles are indicated by a dash; when the particle is moving an arrow is used, the speed of movement being indicated by the boldness of the arrow. Positions of the tuning fork for corresponding times are shown on the left of the figure.
It is important to note that figure 1.3 is a kind of chart, and not, like figure 1.2, a diagrammatic picture of an event. It does not represent what happens to a whole body of air when a tuning fork sounds. Only thirteen particles are represented, the successive positions of these particles being shown in successive lines. Because each line represents a moment in time later than that of the line above, the diagram should be examined one line at a time. It is a good idea to begin by placing a sheet of paper on the diagram so that only the top line is visible. As you move the paper down the page, the areas of compression and rarefaction will appear to move to the right, although the individual air particles move only backward and forward.
This kind of phenomenon is known as a wave. It is typical of a wave movement that energy in the form of areas of compression and rarefaction should be transmitted considerable distances through a medium such as the air, although the individual parts of the medium are each only slightly displaced from their positions of rest.
In order to understand exactly how a wave motion is transmitted, we must make a more detailed examination of figure 1.3. When we examine the diagram line by line we see that in the first line the prongs of the tuning fork are moving rapidly outward through their positions of rest. All the particles are stationary except the first one, which is moving in sympathy with the tuning fork. In the second line, which represents the state of affairs a moment later, the first particle is slowing down slightly, as it has pushed against the second particle, which is now moving rapidly. In the third line (a moment later still) the first particle has come to rest, and the second particle is slowing down, having set the third particle in motion. In the fourth line the third particle is still moving outward, and has even set the fourth particle in motion. The second particle, however, has stopped, and the first particle is moving back toward the tuning fork, whose prongs are now moving toward one another. Each air particle is behaving like the bob of a pendulum. If you give a pendulum a push so that it moves to one side, it will move a certain distance and then start swinging back through its position of rest; similarly, each air particle is like a pendulum which has received its push from the particle next to it. Particle seven is set in motion by particle six, which in its turn owes its movement to the push given to it by particle five, and so on.
It is in this way that vibratory motion is transmitted through the air. The individual particles move backward and forward, while the waves of compression move steadily outward. Consequently a listening ear will experience moments of higher pressure followed by moments of lower pressure. This will affect the eardrum in the way we have already mentioned, so that the sensation of sound results.
Not all variations in air pressure are perceivable as sounds. For example, we can produce by means of a fan a movement of air accompanied by a pressure wave that can be felt but not heard. In this case there is definitely a disturbance of the air; but this kind of variation in air pressure cannot be sensed by the ear, because only very rapid fluctuations of air pressure affect the ear in such a way that sounds are perceived.
Anything which causes an appropriate variation in air pressure is a source of sound. As we have seen, the changes in air pressure are due to small but frequent movements of the air particles. These have arisen because the source of sound is making similar movements. Usually the movements are far too fast to be seen with the eye. But if you put your finger lightly against a sounding tuning fork, you can often feel the vibrations. The pressure of your finger will probably stop the movement, and hence the sound will cease. In the same way you can still a ringing glass by placing a hand upon it, and thus stop the glass from vibrating. Both a glass and a tuning fork are sources of sound only as long as they are vibrating.
Another comparatively simple source of sound is a stretched string. When this is plucked or pushed to one side and then released, it springs back through and beyond its original position, and starts vibrating. This is the basis of musical instruments such as the harp, guitar, and violin. A piano also uses stretched strings, or wires, but in this case they are hit with small hammers instead of being plucked or bowed to one side. In all stretched string instruments the vibrations of the string are transferred, often through a bridge, to a sounding board of some kind, which then becomes the source of sound.
Some sources of sound do not cause such regular vibrations of the air. When a falling book hits the ground, there is a noise, although there is nothing like a stretched string or a tuning fork vibrating. The sound is caused partly by the sudden compression of the air beneath the book, and partly by the diverse irregular movements set up in both the book and the floor.
The source of sound with which we are most concerned is the human voice. Here fluctuations in air pressure are caused by a variety of means. The most important of these is the rapid opening and closing of the vocal cords. Each time the vocal folds are closed pressure is built up, which is suddenly released when they are opened. Consequently the rapid opening and closing of the folds causes a series of sharp variations in air pressure. As we shall see later (chap. 7), these variations in air pressure affect the air in the throat and mouth in such a way that speech sounds are produced.
In our discussions of sounds it will be useful to have some means of representing them as visible shapes. This necessity leads us to a short consideration of the principles of drawing diagrams. So far we have been describing sounds in terms of the movements of the air particles, and also in terms of variations in air pressure. Our problem is to represent these movements and pressures in a suitable way. What we need is something that is sensitive to small changes in pressure or to movements of the air. A microphone is such a device in that it produces a variation in an electrical voltage that is exactly proportional to changes in the surrounding air pressure. With the aid of a microphone we can produce a graph (fig. 1.4) of the variations in air pressure which occur during the sounding of a tuning fork. In this case the changes in pressure occur at very great speeds. The pressure rises smoothly to a maximum and then falls away steadily to a minimum before rising again to repeat the cycle, all within a small fraction of a second. The height of any point on the curve above the center line represents the increase of air pressure at that time. Points below the line indicate air pressures below the normal level of the surrounding air.
From a diagram such as figure 1.4 we can see firstly the extent of the maximum increase of air pressure, secondly the rate at which maximum peaks of pressure occur (in this case one every one-hundredth of a second), and thirdly the way in which the pressure builds up and then decays. As these are the most important aspects of a sound wave, figure 1.4 is a useful form of diagram of a sound.
The variations in air pressure are directly related to the movements of the air particles. Peaks of pressure occur when they are close together, and moments of low pressure when they are furthest apart. Another way of representing a sound is to diagram these movements of air particles. As I stated earlier, the movement of the top of one of the prongs of a tuning fork corresponds to that of the neighboring air particles. Now it is conceptually fairly easy to make the movement of a tuning fork visible by attaching a sharp point to one prong and then drawing the vibrating fork over a sheet of paper at an even rate (fig. 1.5).
A more practical method of carrying out this experiment is to allow the vibrating fork to remain stationary above a sheet of paper wrapped round a drum which revolves at a constant speed. But in either case a curve of the form shown results.
If we now look again at figure 1.3, we can see how a curve of a similar shape can be built up from a consideration of the movement of an air particle. In figure 1.3 the position of each particle is shown at regular intervals of time. Consequently a curve drawn through the positions of any one particle will show how much it has been displaced from its position of rest at any particular time. This is one of the most common methods of representing a sound. An example using the arrows of figure 1.3, but with the time scale shown horizontally, is given in figure 1.6. When the curve is above the line, it means that the particle, at that time, is displaced from its position of rest away from the source of sound (i.e., to the right in fig. 1.3); when the curve is below the line it means that the particle is displaced toward the source (i.e., to the left). We should also note that the particles are stationary for a brief instant at the point of their maximum displacement, and that they are moving at their fastest as they pass their original position.
Generally speaking, in this book we shall be considering sounds as variations in air pressure. Consequently the most useful form of diagram will be one which shows how the air pressure at a given place varies over a period of time (as in fig. 1.4). We must remember, however, that it is also possible to draw a diagram of the same phenomenon by showing the movement of an individual air particle (as in fig. 1.6). These two forms of diagram are simply different ways of looking at the same event.
(Continues…)
Excerpted from "Elements of Acoustic Phonetics"
by .
Copyright © 1996 Peter Ladefoged.
Excerpted by permission of The University of Chicago Press.
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