Sound
27017
225815398
2008-07-15T15:13:39Z
VASANTH S.N.
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'''{{Refimprove|date=November 2007}}
{{this|audible acoustic waves|Sound (disambiguation)}}
{{Redirect|Soundwave|the Transformer|Soundwave (Transformers)}}
'''Sound''' is vibration transmitted through a solid, liquid, or gas; particularly, sound means those vibrations composed of frequencies capable of being detected by ears.<ref>{{cite book
| authorlink = Houghton Mifflin Company
| title = The American Heritage Dictionary of the English Language, Fourth Edition
| publisher = Houghton Mifflin Company
| date = 2006
| url = http://www.bartleby.com/61/65/S0576500.html }}</ref>
== Perception of sound ==
For humans, hearing is limited to [[frequency|frequencies]] between about 20 [[Hertz|Hz]] and 20,000 Hz (20 [[kHz]]), with the upper limit generally decreasing 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 [[sense]]s, 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 [[frog]]s, [[bird]]s, [[marine mammals|marine]] and terrestrial [[mammal]]s, have also developed special [[organ (anatomy)|organ]]s to produce sound. In some species, these have evolved to produce [[bird vocalization|song]] and (in humans) [[speech communication|speech]]. Furthermore, [[human]]s 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]]: [[gas]]es, [[liquid]]s, [[solid]]s, and [[plasma (physics)|plasma]]s. 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 wave]]s, also called [[compression]] waves. Through solids, however, it can be transmitted as both [[longitudinal wave|longitudinal]] and [[transverse wave]]s. Longitudinal sound waves are waves of alternating [[pressure]] deviations from the [[equilibrium]] pressure, causing local regions of [[physical compression|compression]] and [[rarefaction]], while [[transverse wave]]s in solids, are waves of alternating [[shear stress]].
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 (spatial)|vector]], or wavelength and direction are combined as a [[wave vector]]).
[[Transverse wave]]s, also known as [[Shear stress|shear]] waves, have an additional property of [[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.
Whenever the [[pitch]] of the soundwave is affected by some kind of change, the distance between the sound wave maxima also changes, resulting in a change of [[frequency]]. When the loudness of a soundwave changes, so does the amount of compression in airwave that is travelling through it, which in turn can be defined as [[amplitude]].
===Speed of sound===
{{main|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 20°C (68°F) air at sea level, the speed of sound is approximately 343 m/s (767.3 mph). In fresh water, also at 20°C, the speed of sound is approximately 1482 m/s (3,315.1 mph). In steel the speed of sound is about 5960 m/s (13,332.1 mph).<ref>[http://library.thinkquest.org/19537/Physics4.html The Soundry: The Physics of Sound]</ref> The speed of sound is also slightly sensitive (a second-order 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==
{{main|Sound pressure}}
{{Sound measurements}}
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 in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm <math>-\sqrt{2}</math> Pa) and (1 atm <math>+\sqrt{2}</math> 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 [[deaf]]ening 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''<sub>p</sub> is defined as
:<math>
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}
</math>
:where ''p'' is the [[root-mean-square]] sound pressure and <math>p_\mathrm{ref}</math> 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 [[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===<!-- This section is linked from [[Decibel]] -->
{| class="wikitable"
! Source of sound !! [[root mean square|RMS]] sound pressure !! sound pressure level
|-
!   !! align="center" | [[Pascal_(unit)|Pa]] !! align="center" | [[Decibel|dB]] re 20 µPa
|-
|Theoretical limit for undistorted sound at<br>1 [[atmosphere (unit)|atmosphere]] environmental [[pressure]] || align="right" | 101,325 || align="right" | 191
|-
|1883 [[Krakatoa]] eruption|| align="right" | || align="right" | approx 180 at 100 miles
|-
|[[Stun grenades]]|| align="right" | || align="right" | 170-180
|-
|rocket launch equipment acoustic tests || align="right" | || align="right" | approx. 165
|-
|[[threshold of pain]] || align="right" | 100 || align="right" | 134
|-
|hearing damage during short-term effect || align="right" | 20 || align="right" | approx. 120
|-
|[[jet engine]], 100 m distant || align="right" | 6–200 || align="right" | 110–140
|-
|[[jackhammer]], 1 m distant / [[discotheque]] || align="right" | 2 || align="right" | approx. 100
|-
|[[hearing damage]] from long-term exposure || align="right" | 0.6 || align="right" | approx. 85
|-
|traffic noise on major road, 10 m distant || align="right" | 0.2–0.6 || align="right" | 80–90
|-
|moving [[automobile]], 10 m distant || align="right" | 0.02–0.2 || align="right" | 60–80
|-
|TV set – typical home level, 1 m distant || align="right" | 0.02 || align="right" | approx. 60
|-
|normal talking, 1 m distant || align="right" | 0.002–0.02 || align="right" | 40–60
|-
|very calm room || align="right" | 0.0002–0.0006 || align="right" | 20–30
|-
|quiet rustling leaves, calm human breathing || align="right" | 0.00006 || align="right" | 10
|-
|[[auditory threshold]] at 2 kHz – undamaged human ears || align="right" | 0.00002 || align="right" | 0
|}
== Equipment for dealing with sound ==
Equipment for generating or using sound includes [[musical instrument]]s, [[hearing aid]]s, [[sonar]] systems and [[sound reproduction]] and broadcasting equipment. Many of these use electro-acoustic transducers such as [[microphone]]s and [[loudspeaker]]s.
== References ==
<references/>
==Sound measurement==
<div class="references-small" style="-moz-column-count:2; column-count:2;">
*[[Decibel]], [[sone]], [[Mel scale|mel]], [[phon]], [[hertz]]
*[[Sound pressure level]]
*[[Particle velocity]], [[acoustic velocity]]
*[[Particle displacement]], [[particle amplitude]], [[particle acceleration]]
*[[Sound power]], [[acoustic power]], [[sound power level]]
*[[Sound energy flux]]
*[[Sound intensity]], [[acoustic intensity]], [[sound intensity level]]
*[[Acoustic impedance]], [[sound impedance]], [[characteristic impedance]]
*[[Speed of sound]], [[amplitude]]
*See also [[Template:Sound measurements]]
</div>
== See also ==
[[Pitch]]
[[Acoustics]] |
[[Auditory imagery]] |
[[Audio bit depth]] |
[[Audio signal processing]] |
[[Beat (acoustics)|Beat]]s |
[[Cycle (music)|Cycles]] |
[[Diffraction]] |
[[Doppler effect]] |
[[Echo (phenomenon)|Echo]] |
[[Music]] |
[[Note]] |
[[Phonons]] |
[[Physics of music]] |
[[Pitch (music)|Pitch]] |
[[Psychoacoustics]] |
[[Resonance]] |
[[Rijke tube]] |
[[Reflection (physics)|Reflection]] |
[[Reverberation]] |
[[Sonic weaponry]] |
[[Sound localization]] |
[[Soundproofing]] |
[[Timbre]] |
== External links ==
{{wikiquote}}
{{wikibooks}}
*[http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soucon.html HyperPhysics: Sound and Hearing]
*[http://podcomplex.com/guide/physics.html Introduction to the Physics of Sound]
*[http://www.phys.unsw.edu.au/~jw/hearing.html Hearing curves and on-line hearing test]
*[http://www.audiodesignline.com/howto/audioprocessing/193303241 Audio for the 21st Century]
*[http://www.sengpielaudio.com/calculator-soundlevel.htm Conversion of sound units and levels]
*[http://www.acoustics.salford.ac.uk/schools/index.htm Sounds Amazing a learning resource for sound and waves]
*[http://www.sengpielaudio.com/Calculations03.htm Sound calculations]
*[http://www.audiocheck.net Audio Check: a free collection of audio tests and test tones playable on-line]
[[Category:Sound ]]
[[Category:Acoustics]]
[[Category:Hearing]]
[[Category:Waves]]
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