resonance
Acoustics is the branch of physics dealing with the properties of sound.
Acoustic phonetics is the sub-branch which focuses on the sounds used in human language. As we know from physics, the sounds we hear are caused by vibrations in the air. These vibrations are conveyed through the air in the form of waves. This chapter examines the basic properties of these sound waves. Chapter 9 will apply this knowledge in an acoustic descrip-tion of the sounds of English.
In this chapter you will learn about:
l sine waves and complex waves;
l sound spectra;
l resonance.
Sound waves
In this section we will look at two questions. What are the measurable properties of vibrations in the air caused by speech? And how do these vibrations travel through air?
When we speak, we push a stream of air out of our body. This air stream has a constantly varying pressure. The variations are caused by the many individual actions in the vocal tract. They are conveyed through the air to the listener and are interpreted as speech. To understand these complex variations, we first need to examine simple vibrations, such as those pro-duced by a tuning fork.
A tuning fork is a device constructed with two arms which oscillate, that is, swing back and forth in a regular fashion, alternately pushing and pull-ing the neighbourpull-ing air. Figure 7.1 (overleaf) shows the effect of the vibra-tions of the tuning fork on the air. The dot to the right of the fork represents a molecule of air. At point (a), the fork is at rest. Striking it sets it in motion with the arms moving in opposite directions; when the arm of the fork moves to the right (b), the pressure between the arm and the air molecule is increased and the air molecule is pushed to the right (c). Eventually, the
arm of the tuning fork stops moving to the right and begins to move leftwards (d), reducing the pressure between it and the molecule of air. This pulls the mol-ecule of air towards the left. The leftward movement continues until the tuning fork arm reaches the limit of its range (g). Then the movement starts to the right again (h). We can see that the movement of the tuning fork causes a similar movement in the molecule of air.
If we were to fix a pen on to the mol-ecule of air and pull a sheet of paper past the pen, the motion of the air molecule would be recorded as shown in Figure 7.2.
The horizontal dimension shows time going from left to right, and the vertical dimension shows the displacement of the molecule of air from its resting place, which is indicated by the horizontal line.
The resulting shape is a waveform which shows the movement of the air. As the tuning fork moves, the air pressure sur-rounding the air molecule increases and decreases proportionally to the
move-ment. Since it is relatively easy to measure changes in air pressure and rather hard to observe the movement of a molecule of air, in the laboratory we use variation in air pressure to describe a sound.
The waveform of the pressure variation has the same shape as that of the movement of the air molecule. This pattern of movement is shown in Figure 7.2; it is called a sine or sinusoidal wave because of certain math-ematical properties it has. Physicists can make a number of measurements of such a wave. Sound waves of actual speech are more complex than a sine
Time
Figure 7.1 Movement of a tuning fork
wave, but it is easier to understand the basic principles if we start with simple sine waves.
Speech is fairly rapid. We average around seven segments a second. To measure such short units, phoneticians use milliseconds (1 ms = 0.001 sec-ond). In Figure 7.2 we see that one complete cycle of oscillation takes 10 ms.
Propagation
So far we have been talking about a single molecule of air vibrating as a result of speech. Sound is, however, propagated through air. Air comprises some 400 billion billion molecules per cubic inch at ordinary pressures. Air molecules tend to maintain a constant pressure between themselves and thus a constant distance from each other. This distance acts in an elastic fashion. If something pushes the molecules of air closer together than usual, they resist and try to move back to the original position. Likewise, if pulled apart, they resist and try to return to the original position.
A simple analogy is to think of the molecules of air connected to each other by springs. If one molecule moves towards the next, the spring be-tween them is compressed and will try to push them apart. On the other hand, if the molecules are pulled apart, the spring is stretched and will try to pull them back to their original position.
If we take our tuning fork and molecule of air from Figure 7.1 and add several other molecules of air, we can see how vibration is propagated. The tuning fork and molecules of air are connected by springs; thus the move-ment of one molecule of air causes a corresponding movemove-ment on the next one, and so on. Figure 7.3 shows this. Note that the wave motion travels
Figure 7.3 The cyclical propagation of waves
Time
away from the source of energy (the tuning fork); however, the individual molecules of air do not move far from their original position.
In our model, the wave would continue for ever. In real life, of course, the tuning fork loses energy and gradually comes to rest. In addition, as the wave spreads out in a circular fashion from the tuning fork, the energy is spread over an increasingly large area, and the wave gradually dissipates.