Sound is the range of frequencies that can be heard by a living organism. Fundamentally, sound consists of a pattern of vibrations through some propagation medium such as a gas. Sound is most typically experienced by humans via the medium of air, but it also travels through liquids such as water and through some solids. The vibrations associated with sound can sometimes be felt as well as heard, e.g. when a heavy vehicle passes close by.
Sound moves through a medium as a wave, much like ripples on the surface of a pond as viewed from the side. A wave can be measured in terms of both frequency and amplitude. The amplitude, or maximum departure from the average or equilibrium value, can be expressed in terms of either the motion of particles or the variation in pressure as the sound wave travels through a medium. Amplitude corresponds to the loudness of the sound in that a louder sound has a greater amplitude.
A change in pressure from the average or equilibrium value, up to a maximum pressure, down to a minimum pressure, and then back to the equilibrium value takes place over a single cycle, which can then repeat. The number of cycles per second is equal to the frequency value in Hertz (Hz), and frequency corresponds to the pitch of the sound. The normal range of human hearing is about 20 to 20,000 Hz. Cycles, or vibrations, may be periodic, i.e. regular and with perhaps something of a musical quality, or aperiodic, i.e. comprising irregular patterns. Aperiodic sounds can include noises that humans may find unpleasant, such as the sound of a jet engine.
Sound is involved in human communication, e.g. in the perception and production of speech, as well as in non-linguistic utterances (such as cries or laughter). In linguistics, the physical mechanisms of speech are studied in the fields of acoustic, auditory and articulatory phonetics. The study of speech processing in language is a matter for disciplines including theoretical phonology and psycholinguistics.
Speech sounds are divided into two main groups, obstruents and sonorants. Acoustically, the production of obstruents such as [p], [f] and [s] involve an irregular wave, while sonorants such as vowels and nasal consonants are more regular. Obstruents which are voiced (involve vocal cord vibration), such as [v] and [z], are both periodic and aperiodic, in that voicing is periodic and the rest of the articulation is aperiodic.
- Davenport and Hannahs (2010: 56-61).