Sound is a form of energy (Crystal 32). It is a series of pressure fluctuations in a medium (Johnson 4). In speech the medium is usually air, although sound can propagate through solid objects and water, for example. Once the air particles become energised by the vocal folds vibration, a series of rarefaction and compression events begin. Compression occurs when particles are shifted closer to each other, which results in increased density within medium. Rarefaction is the opposite, when particles retract so density in medium reduces.
Compression, rarefaction, and other terms related to acoustics are often explained through a simple device – a pendulum. A pendulum, or a swing, is “a weight hung from a fixed point so that it can swing freely” (Oxford Dictionary). Once set in motion it will oscillate between two maximum points and its central, equilibrium, position.
Here is a graphical representation of a pendulum. The point E is the equilibrium, while the points M1 and M2 mark the maximum points on both sides of the pendulum. The swinging motion from E to M1, then back to E and up to M2, can be shown in the coordinate system as a sinusoid. The figure shows such a sinusoid, with a series of maximum and minimum swinging points. The crossing point of the sinusoid and the line show the phase in oscillation when the pendulum reaches its starting point E. Particles do not travel through a medium; instead, they create a propagating pressure fluctuation: “A sound wave is a travelling pressure fluctuation that propagates through any medium that is elastic enough to allow molecules to could together and move apart” (Johnson 3). In other words, while each particle moves back and forth and acts “like the bob of pendulum … the waves of compression move steadily outward” (Ladefoged, Elements, 8). Here is an animation of the air molecules in a sound wave propagation.
Combined, a pendulum and a sinusoid illustrate the properties of sound waves and they help explain the terminology related to the physics of speech. For example, the distance between points E and M1 (or E and M2) is the amplitude. It shows the maximum oscillation points of the particles or, in sound, “the extent of maximum variation in air pressure” (Ladefoged, Elements, 14). A pendulum’s period (or a cycle) is a trajectory from E to M1, M2 and back to E. The number of such periods in a second is frequency, and it is measured in hertz (Hz). A pendulum with one oscillation per second has 1 Hz (equation 1). A sound of 100 Hz has an identifiable part that repeats once in a tenth part of a second.
1 Hz = 1/s
The energy of a sound wave depends on the force that created it. The bigger the energy in making the sound wave, the bigger pressure level in the medium it creates. The energy of a sound wave is related to its amplitude: a very strong wave will have big amplitude, and vice versa. The sound pressure, or its intensity, is measured in dB (decibels).
The human ear is very sensitive to pressure variations, estimated at 1013 units of intensity (Crystal 36). For easier reference, the logarithmic scale is used. Thus, units of 1013 are scaled to 130 dB (36).
A simple sinusoid below is an abstraction of a simple periodic sine wave. For its description, three items are needed: amplitude, frequency and phase  (Johnson 7). From the picture we see that the frequency of the sound is 1 per unit of time, while the amplitude reaches its peaks at 2 and -2 on the vertical scale. Unlike simple periodic waves, complex periodic waves “are composed of at least two sine waves” (8). One such complex wave has a pressure oscillation (an amplitude) that is the result of the pressure oscillations of at least two waves (Ladefoged, Elements 37), and, of course, the phases of the waves involved. Every complex wave can be seen as composed of several simple waves, and the merit of such model is that “any complex waveform can be decomposed into a set of sine waves having particular frequencies, amplitudes and phase relations)” (Johnson 11). The process of “breaking complex wave down into its sinusoidal components” (Clark 203) is well-known in physics and is called the Fourier analysis, named after the scientist who “developed its mathematical basis” (203) in XIX century.
The second group of waves is aperiodic waves. They are characterised by the lack of repetitive pattern. Two types of waves are grouped under the term aperiodic: white noise and transients. White noise contains a completely random waveform, while waveform in transients does not repeat; in speech, an example for white noise is a fricative such as [s] (Johnson 12). Aperiodic sounds can also be subjected to Fourier analysis.
Sometimes pressure fluctuations in form of sound that hit an object cause the object to vibrate. The vibrations occur if the acting frequency is within the “effective frequency range” or resonator bandwidth (Ladefoged, Elements 68). Such induction of vibrations by another vibrating object is called resonance. Every object has a specific range of frequencies that it can respond to, and those frequencies correspond to the dominant frequencies of the sound the object can create – or as Ladefoged explains it: “… [T]he resonance curve of a body has the same shape as its spectrum” (65). In speech, the speech organs have the function of resonators: they filter (enhance and dampen) properties of waves, recognised as the speech sounds.
 Phase is “the timing of the waveform relative to same reference point” (Johnson 8).
This post is based on a draft for one of the introductory chapters in my paper.
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