This article is about compression waves. For other meanings, see sound (disambiguation)

Sound is defined as 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. 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, dB(SPL), or dBSPL. 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.

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 dB SPL, called the threshold of hearing. While there is technically no upper limit, sounds begin to do damage to ears at 85 dB SPL and sounds above approximately 130 dB SPL (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.

See also

• Cycles
• Pitch (music)
• Timbre
• Sound reproduction
• Sound proofing
• Audio signal processing
• Demonstrating sound waves on an oscilloscope
• Voyager Golden Record
• Radiation of sound
• Physics of music
• Rijke tube
• Ultrasound
• Infrasound

• For information on using or contributing sound files to Wikipedia, see Wikipedia:Sound

Sound measurement

• Decibel
• Sone
• Sound pressure
• Sound pressure level
• Sound energy flux
• Sound power
• Sound intensity
• Sound intensity level

External links

• HyperPhysics: Sound and Hearing
• Audio calculations and online acoustics conversion engine