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Sound is a mechanical compression and rarefaction or a longitudinal displacement wave that propagates through a medium (solid, liquid or gas). The speed of this propagation depends on the type, temperature and pressure of the medium. Under normal conditions, however, because air is nearly a perfect gas, it does not depend on the air pressure. In dry air at 20 °C (68 °F) the speed of sound is approximately 343 m/s. A real-world estimate is nearly 1 meter per 3 milliseconds.
Sounds can often be thought of as having two components: frequency and amplitude. The frequency is the rate of change within the wave and is measured in hertz (Hz). The amplitude is the magnitude of sound pressure change within the wave. See also particle displacement and particle velocity. While the pressure can be measured in pascals, the amplitude is more often referred to as sound pressure levelSound pressure level (SPL) or sound level L is a logarithmic measure of the energy of a particular noise relative to a reference noise source. It is almost always expressed in decibels compared to a reference source of 20 µPa (micropascal) sound pressure. and measured in decibelThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM)s, or dBSPL, sometimes written as dBspl or dB(SPL). When the measurement is adjusted based on how the human ear perceives loudness based on frequency, it is called dBA or A-weighting. See decibelThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM)s for a more thorough discussion.
Sounds that are sine waves with fixed frequency and amplitude are perceived as pure toneThe word tone is used in several different fields with different meanings. Alternate meanings Note Tone In music, the abstract concept of note refers both to a unit of fixed pitch which has been given a name, and also to the graphic representation of thats. While sound waves are usually visualised as sine waveAn ideal sine wave is a waveform whose graph is identical to the generalized sine function y A''sin omega ''x − alpha ] + C where A is the amplitude, omega is the angular frequency (2π P where P is the wavelength), alpha is the phase shift, and Cs, sound waves can have arbitrary shapes and frequency content, limited only by the apparatus that generates them and the medium through which they travel. In fact, most sound waves consist of multiple overtoneAn overtone is a sinusoidal component of a waveform, of greater frequency than its fundamental frequency. Usually the first overtone is the second harmonic, the second overtone is the third harmonic, etc. Use of the term overtone is generally confined tos or harmonicIn acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integral multiple of the fundamental frequency. For a sine wave, it is an integral multiple of the frequency of the wave. For example, if the freqs and any sound can be thought of as being composed of sine waves (see additive synthesisAdditive synthesis is a technique of audio synthesis which creates musical timbre. Since different instruments' timbre is composed of varying amounts of harmonics that change over time, with respect to a base tone, additive synthesis emulates this behavio). Waveforms commonly used to approximate harmonic sounds in nature include sawtooth waveThe sawtooth wave is a kind of basic waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw. A bandlimited sawtooth wave pictured in the time domain (top) and frequency domain (bottom). The fundamental is at 220 Hz (As, square waves and triangle waves.
While a sound may still be referred to as being of a single frequency (for example, a piano striking the A above middle C is said to be playing a note at 440 Hz), the sound perceived by a listener will be colored by all of the sound wave's frequency components and their relative amplitudes (see timbre.) For convenience in this article, however, it is best to think of sound waves as sine waves.
The frequency range of sound audible to humans is approximately between 20 and 20,000 Hz. This range varies by individual and generally shrinks with age. It is also an uneven curve - sounds near 3,500 Hz are often perceived as louder than a sound with the same amplitude at a much lower or higher frequency. Above and below this range are ultrasound and infrasound, respectively. The amplitude range of sound for humans has a lower limit of 0 dBSPL, called the threshold of hearing. While there is technically no upper limit, sounds begin to do damage to ears at 85 dBSPL and sounds above approximately 130 dBSPL (called the threshold of pain) cause pain. Again, this range varies by individual and changes with age.
The perception of sound is the sense of hearing. In humans and many animals this is accomplished by the ears, but loud sounds and low frequency sounds can be perceived by other parts of the body through the sense of touch. Sounds are used in several ways, most notably for communication through speech or, for example, music. Sound perception can also be used for acquiring information about the surrounding environment in properties such as spatial characterics and presence of other animals or objects. For example, bats use one sort of echolocation, ships and submarines use sonar, and humans can determine spatial information by the way in which they perceive sounds.
The study of sound is called acoustics and is performed by acousticians. A notable subset is psychoacoustics, which combines acoustics and psychology to study how people react to sounds.
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