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Sound is a disturbance of mechanical energy that propagates through matter as a wave. Humans perceive sound by the sense of hearing. Sound is commonly the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider definition of sound that includes low and high frequency vibrations in air that cannot be heard, and vibrations that travel through all forms of matter, gases, liquids and solids. The matter that supports the sound is called the medium. Sound propagates as waves of alternating pressure causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate.
Attributes of soundAs a wave, sound is characterized by the properties of waves including frequency, wavelength, period, amplitude and velocity or speed. SpeedThe speed of sound depends on the type, temperature and pressure of the medium through which it propagates. Typical values for the speed of sound are 343 m/s in air and 1500 m/s in water. Frequency and wavelengthThe range of human hearing is from 20 Hz to 20 kHz which corresponds to wavelengths of 17 m to 17 mm in air. The fundamental frequency or pitch of a typical human spoken voice is in the range 150 Hz to 400 Hz. In music, middle C has a frequency of approximately 260 Hz. AmplitudeThe amplitude of a sound wave is specified in terms of its pressure. The human ear can detect sounds of a very wide range of amplitudes and so a logarithmic decibel scale is used. A standard unit of sound pressure amplitude is sound pressure level. Types of soundsNoises are irregular and disordered vibrations including all possible frequencies. Their wave diagram does not repeat in time. 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, as well as how the sound changes over time (see timbre.) For convenience in this article, however, it is best to think of sound waves as sine waves. Perception of soundThe 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 0dBSPL, called the threshold of hearing. Sound is technically at its upper limit at 194.09 dB. Above this level it should be called a shock wave. Sounds begin to do damage to ears at 85 dBSPL (with prolonged exposure - eg 8 hrs per day) 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 characteristics 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. See alsoWikibooks has more about this subject: School science how-toAudio signal processing | Auditory imagery | Beats | Cycles | Doppler Effect | Infrasound | Music | Soundproofing | Sound reproduction | Phonons | Physics of music | Pitch (music) | Radiation of sound | Resonance | Rijke tube | Steam whistle | Timbre | Voyager Golden Record | Ultrasound | Wave | Noise | Reflection | Echo | Reverberation | Decibel | Sound localization | Microphone | Loudspeaker | Sound measurement
References
External linksWikiquote has a collection of quotations related to: Sound
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