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Encyclopedia > Sounds
A schematic representation of auditory signaling

Sound is a series of mechanical compressions and rarefactions or longitudinal waves that succesively pass one into another and propagate through materials (medium) that are at least a little compressible (solid, liquid or gas but not vacuum). A sound wave is usually represented graphically by a wavy, horizontal line; the upper part of the wave (the crest) indicates a compression and the lower part (the trough) indicates a rarefaction. In sound waves parts of matter (molecules or groups of molecules) move in a direction of the spreading of the disturbance (as opposite to transversal waves). The cause of sound waves is called the source of waves, e.g. a violin string vibrating upon being bowed or plucked.

The characteristics of sound are frequency, wavelength, amplitude and velocity.

The frequency is the number of oscillations of a particular point in the course of soundwaves in a second. One single oscillatory cycle per second corresponds to 1 Hz(1/s). The wavelength is the distance between two successive crests and is the path that a wave travels in the time of one oscillatory cycle. In the case of longitudinal harmonic sound waves we can describe it with the equation

where y(x,t) is the displacement of particles from the stable position (y0) in the direction of spreading of waves, while x is the displacement of the source of waves, c is the speed of waves, ω is the angle speed of the souce of waves and x/c is the time that the wave needs to travel the path x. Time of one oscillatory cycle is denoted by t.

The amplitude is the magnitude of sound pressure change within the wave. It is the maximal displacement of particles of matter that is obtained in compressions, where the particles of matter move towards each other and pressure increases the most and in rarefactions, where the pressure lessens the most. 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 level and measured in decibels, 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 decibels for a more thorough discussion.

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.

Noises are irregular and disordered vibrations, they include all possible frequencies, their picture does not repeat in time. The noise is an aperiodic series of waves.

Sounds that are sine waves with fixed frequency and amplitude are perceived as pure tones. While sound waves are usually visualised as sine waves, 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 overtones or harmonics and any sound can be thought of as being composed of sine waves (see additive synthesis). Waveforms commonly used to approximate harmonic sounds in nature include sawtooth waves, 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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## References

• (2005) Computation Provides a Virtual Recording of Auditory Signaling. (http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0030026) PLoS Biol 3(1): e26.

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 Sound and Vision Magazine - Home Theater Equipment, Audio and Video Equipment, Plasma and LCD HDTVs, HD-DVD vs Blu-Ray (270 words) When a New York City landmark needed an innovative new sound system, Definitive Technology came to the rescue. Sound and Vision's expert reviewers recommend the best gear to buy right now. Let Sound and Vision help you find the best equipment for your home, taste, and budget.
 Sound & Light (440 words) The energy of a sound wave travels away from the source trough a series of molecule collisions parallel to the direction of the wave. A sound wave with a wavelength of 1.24 m is moving at 343 m/s. In this example, the sound produced by the source is always the same; however, John is receiving more oscillations per second because the source is moving toward him.
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