Sound, in the broad sense, is the oscillatory motion of the particles of an elastic medium propagating as waves in gaseous, liquid or solid media— the same as elastic waves; in the narrow sense it is a phenomenon perceived subjectively by the organ of hearing of humans and animals. A human being hears sound in the frequency range from 16 Hz to 20 kHz. Inaudible sound with a frequency below 16 Hz is called infrasound, that above 20 kHz ultrasound, and the highest-frequency elastic waves, in the range from 109 to 1012—1013 Hz, hypersound.
An important characteristic of sound is its spectrum, obtained by resolving the sound into simple harmonic oscillations (the so-called frequency analysis of sound). The fundamental frequency then determines the pitch of the sound as it is heard, and the set of harmonic components determines its timbre. The spectrum of the sound of speech contains formants — stable groups of frequency components corresponding to particular phonetic elements. The energy characteristic of sound oscillations is the sound intensity, which depends on the amplitude of the sound pressure and also on the properties of the medium itself and on the waveform. The subjective characteristic of sound connected with its intensity is loudness, which depends on frequency. The human ear is most sensitive in the frequency region of 1—5 kHz.
A source of sound can be any phenomenon that produces a local change of pressure or of mechanical stress. Sources of sound in the form of oscillating solid bodies are widespread (the cones of loudspeakers and the diaphragms of telephones, the strings and soundboards of musical instruments, for example); in the ultrasonic frequency range these are plates and rods of piezoelectric or magnetostrictive materials. Electroacoustic transducers form an extensive class of sound sources.
Receivers of sound include, in particular, the hearing organ of humans and animals. In technology, sound is received mainly by electroacoustic transducers: microphones in air, hydrophones in water and geophones in the Earth's crust.
The propagation of sound waves is characterised first of all by the speed of sound. In a number of cases dispersion of the speed of sound is observed, i.e. a dependence of its propagation speed on frequency. As a sound wave propagates, the sound gradually decays, i.e. its intensity and amplitude decrease, which is caused to a considerable extent by the absorption of sound, bound up with the irreversible conversion of sound energy into other forms (mainly into heat). In the propagation of waves of large amplitude (see Nonlinear acoustics) the sinusoidal form of the wave is gradually distorted and approaches the form of a shock wave.
An important characteristic of sound is its spectrum, obtained by resolving the sound into simple harmonic oscillations (the so-called frequency analysis of sound). The fundamental frequency then determines the pitch of the sound as it is heard, and the set of harmonic components determines its timbre. The spectrum of the sound of speech contains formants — stable groups of frequency components corresponding to particular phonetic elements. The energy characteristic of sound oscillations is the sound intensity, which depends on the amplitude of the sound pressure and also on the properties of the medium itself and on the waveform. The subjective characteristic of sound connected with its intensity is loudness, which depends on frequency. The human ear is most sensitive in the frequency region of 1—5 kHz.
A source of sound can be any phenomenon that produces a local change of pressure or of mechanical stress. Sources of sound in the form of oscillating solid bodies are widespread (the cones of loudspeakers and the diaphragms of telephones, the strings and soundboards of musical instruments, for example); in the ultrasonic frequency range these are plates and rods of piezoelectric or magnetostrictive materials. Electroacoustic transducers form an extensive class of sound sources.
Receivers of sound include, in particular, the hearing organ of humans and animals. In technology, sound is received mainly by electroacoustic transducers: microphones in air, hydrophones in water and geophones in the Earth's crust.
The propagation of sound waves is characterised first of all by the speed of sound. In a number of cases dispersion of the speed of sound is observed, i.e. a dependence of its propagation speed on frequency. As a sound wave propagates, the sound gradually decays, i.e. its intensity and amplitude decrease, which is caused to a considerable extent by the absorption of sound, bound up with the irreversible conversion of sound energy into other forms (mainly into heat). In the propagation of waves of large amplitude (see Nonlinear acoustics) the sinusoidal form of the wave is gradually distorted and approaches the form of a shock wave.