Sound

I

Introduction

Sound, physical phenomenon that stimulates the sense of hearing. In human beings, hearing takes place whenever vibrations of frequencies between about 15 and 20,000 hertz reach the inner ear. The hertz (Hz) is a unit of frequency equalling one cycle per second. Such vibrations reach the inner ear when they are transmitted through air, and the term sound is sometimes restricted to such airborne vibrational waves. Modern physicists, however, usually extend the term to include similar vibrations in liquid or solid media. Sounds of frequencies higher than about 20,000 hertz are called ultrasonic. See Ultrasonics.

This article deals with the major outlines of this field of physics. For the architectural science of designing rooms and buildings with desirable properties of sound propagation and reception, see Acoustics. For the nature of the physiological process of hearing sounds, and the anatomy of the human and animal hearing mechanism, see Ear. For the general properties of the generation and propagation of vibrational waves, including sound waves, see Wave Motion. See also Oscillation.

In general, waves can be propagated transversely or longitudinally. In both cases, only the energy of wave motion is propagated through the medium; no portion of the medium itself actually moves very far. As a simple example, a rope may be tied securely to a post at one end, and the other end pulled almost taut and then shaken once. A wave will travel down the rope to the post, and at that point it will be reflected and returned to the hand. No part of the rope actually moves longitudinally towards the post, but each successive portion of the rope moves transversely. This type of wave motion is called a transverse wave. Similarly, if a rock is thrown into a pool of water, a series of transverse waves moves out from the point of impact. A cork floating near the point of impact will bob up and down, that is, move transversely with respect to the direction of wave motion, but will show little if any outward, or longitudinal, motion. A sound wave, on the other hand, is a longitudinal wave. As the energy of wave motion is propagated outward from the centre of disturbance, the individual air molecules that carry the sound move back and forth, parallel to the direction of wave motion. Thus, a sound wave is a series of alternate compressions and rarefactions of the air. Each individual molecule passes the energy on to neighbouring molecules but, after the sound wave has passed, each molecule remains in about the same location.

II

Physical Characteristics

Any simple sound, such as a musical note, may be completely described by specifying three perceptual characteristics: pitch, loudness (or intensity), and quality (or timbre). These characteristics correspond exactly to three physical characteristics: frequency, amplitude, and harmonic constitution, or waveform, respectively. Noise is a complex sound, a mixture of many different frequencies or notes not harmonically related.

A

Frequency

Sounds can be produced at a desired frequency by different methods. For example, a sound of 440 hertz can be created by actuating a loudspeaker with an oscillator tuned to this frequency (see Sound Recording and Reproduction). An air blast can be interrupted by a toothed wheel with 44 teeth, rotating at 10 revolutions per second; this method is used in operating an ordinary siren. The sound of the speaker and that of the siren at the same frequency are very different in quality, but will correspond closely in pitch, equivalent to the A above middle C on a piano. The next higher A on the piano, the note one octave above, has a frequency of 880 hertz. Similarly, notes one and two octaves below have frequencies of 220 and 110 hertz, respectively. Thus, by definition, an octave is the interval between any two notes the frequencies of which are in a two-to-one ratio.

A fundamental law of harmony states that two notes an octave apart, when sounded together, produce a euphonious combination. A fifth and a major third produce successively less euphonious combinations. Physically, an interval of a fifth is the relationship between two notes, the frequencies of which bear the arithmetical ratio three to two; in a major third, the ratio is five to four. Fundamentally, then, the law of harmony states that two or more notes sound euphonious when played together if their frequencies are in the ratio of small whole numbers; if the frequencies are not in such ratios, a dissonance is produced. On a fixed-pitch instrument, such as a piano, it is not possible to arrange the notes so that all of these ratios hold exactly, and some compromise is necessary in tuning, in accordance with the meantone system, or tempered scale.

B

Amplitude

The amplitude of a sound wave is the degree of motion of air molecules within the wave, which corresponds to the extent of rarefaction and compression that accompanies the wave. The greater the amplitude of the wave, the harder the molecules strike the ear drum and the louder the sound that is perceived. The amplitude of a sound wave can be expressed in terms of absolute units by measuring the actual distance of displacement of the air molecules, or the pressure differential in the compression and rarefaction, or the energy involved. Ordinary speech, for example, produces sound energy at the rate of about one hundred-thousandth of a watt. All of these measurements are extremely difficult to make, however, and the intensity of sounds is generally expressed by comparing them to a standard sound, measured in decibels (see Sensations of Tone below).