This chapter explains how sound is produced, how it travels, and how it is described and used. Sound is a form of energy produced by vibrating objects; the vibration disturbs the particles of a medium, which pass the disturbance on as a wave without themselves travelling along. Sound is a longitudinal wave — the particles of the medium move back and forth along the direction the wave travels, creating regions of compression (particles crowded, high pressure) and rarefaction (particles spread out, low pressure). Sound needs a material medium (solid, liquid or gas) and cannot travel through a vacuum, which is why astronauts in space cannot hear each other directly. A sound wave is described by its frequency (number of vibrations per second, in hertz, deciding pitch), wavelength (distance between two consecutive compressions or rarefactions), amplitude (deciding loudness) and time period. Wave speed links these as \(v = f\lambda\). Sound travels fastest in solids and slowest in gases, and faster in warmer air. When sound reflects off a surface we hear an echo if the reflected sound returns after about 0.1 second; multiple reflections cause reverberation. Humans hear roughly 20 Hz to 20,000 Hz; sound above this range is ultrasound and below it is infrasound. Applications include SONAR (using ultrasound echoes to find depth and underwater objects), bats and dolphins navigating by echoes, and ultrasound scans in medicine and industry.
Production and propagation of soundLongitudinal waves: compressions and rarefactionsFrequency, wavelength, amplitude and \(v = f\lambda\)Speed of sound, echo and reverberationAudible range, ultrasound and SONAR
Key terms
Longitudinal wave
A wave in which the particles of the medium vibrate back and forth along the same direction in which the wave travels, as in sound.
Compression and rarefaction
Regions of a sound wave where particles are crowded (high pressure) and where they are spread apart (low pressure), respectively.
Frequency
The number of vibrations (or compressions) produced per second, measured in hertz (Hz); it determines the pitch of a sound.
Amplitude
The maximum displacement of particles from their mean position; it determines the loudness of the sound.
Wave speed relation
The speed of a wave equals its frequency times its wavelength, \(v = f\lambda\).
Echo
A repetition of sound heard after it is reflected from a distant surface, perceived when it returns after about 0.1 second.
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Sound is produced by objects that are:
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