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Learn more about "Sound"

Sound

- ''This page is about audible acoustic waves. For other uses see Sound (disambiguation)
Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a threshold of hearing|level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.

Perception of sound

For humans, hearing is normally limited to frequencies between about 12 Hertz|Hz and 20,000 Hz (20 kHz)http://books.google.com/books?id=RUDTFBbb7jAC&pg=PA248, although these limits are not definite. The upper limit generally decreases with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. As a signal perceived by one of the major senses, sound is used by many species for defence mechanism (biology)|detecting danger, navigation, predation, and communication. Earth's atmosphere, hydrosphere|water, and virtually any physical phenomenon, such as fire, rain, wind, ocean surface wave|surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine mammals|marine and terrestrial mammals, have also developed special organ (anatomy)|organs to produce sound. In some species, these have evolved to produce bird vocalization|song and speech communication|speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.

Physics of sound

The mechanical vibrations that can be interpreted as sound are able to travel through all State of matter|forms of matter: gases, liquids, solids, and plasma (physics)|plasmas. The matter that supports the sound is called the Transmission medium|medium. Sound cannot travel through vacuum.

Longitudinal and transverse waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called Physical compression|compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the Mechanical equilibrium|equilibrium pressure, causing local regions of physical compression|compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation. Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra physical compression|compression (in case of longitudinal waves) or lateral displacement Strain (materials science)|strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium.

Sound wave properties and characteristics

Sound waves are characterized by the generic Wave#Physical description of a wave|properties of waves, which are frequency, wavelength, Periodicity|period, amplitude, Intensity (physics)|intensity, speed of sound|speed, and Direction (geometry, geography)|direction (sometimes speed and direction are combined as a velocity Vector (geometric)|vector, or wavelength and direction are combined as a wave vector). Transverse waves, also known as Shear stress|shear waves, have an additional property of Polarization (waves)|polarization. Sound characteristics can depend on the type of sound waves (longitudinal versus transverse) as well as on the physical properties of the transmission medium.

Speed of sound

The speed of sound depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In air at the sea level, the speed of sound is approximately using the formula "v = (331 + 0.6T) m/s". In fresh water, also at 20 °C, the speed of sound is approximately . In steel, the speed of sound is about .The Soundry: The Physics of Sound The speed of sound is also slightly sensitive (a second-order Anharmonicity|anharmonic effect) to the sound amplitude, which means that there are nonlinear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).

Acoustics and noise

The scientific study of the propagation, absorption, and reflection of sound waves is called acoustics. Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.

Sound pressure level

Sound pressure is defined as the difference between the average local pressure of the medium outside of the sound wave in which it is traveling through (at a given point and a given time) and the pressure found within the sound wave itself within that same medium. A square of this difference (i.e. a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of such average is taken to obtain a root mean square (RMS) value. For example, 1 Pascal (unit)|Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm

-\sqrt{2}

Pa) and (1 atm

+\sqrt{2}

Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny (relative to atmospheric) variation in air pressure at an audio frequency will be perceived as quite a deafening sound, and can cause hearing damage, according to the table below. As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The '''sound pressure level''' (SPL) or ''L''_p is defined as
-

L_\mathrm{p}=10\, \log_{10}\left(\frac{{p}^2}{{p_\mathrm{ref}}^2}\right) =20\, \log_{10}\left(\frac{p}{p_\mathrm{ref}}\right)\mbox{ dB}

- where ''p'' is the root-mean-square sound pressure and

p_\mathrm{ref}

is a reference sound pressure. Commonly used reference sound pressures, defined in the standard American National Standards Institute|ANSI S1.1-1994, are 20 micropascal|µPa in air and 1 micropascal|µPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level. Since the human ear does not have a flat Responsivity|spectral response, sound pressures are often frequency weighted so that the measured level will match perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.

Examples of sound pressure and sound pressure levels

Equipment for dealing with sound

Equipment for generating or using sound includes musical instruments, hearing aids, sonar systems and sound reproduction and broadcasting equipment. Many of these use electro-acoustic transducers such as microphones and loudspeakers.