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Sound

Sound is a disturbance ofmechanical energy that propagates throughmatter as a longitudinal wave. Sound is characterized by the properties ofsound waves, which are frequency, wavelength, period, amplitude, andspeed.

Humans perceive sound by the sense of hearing. By sound, wecommonly mean the vibrations that travel through air and can be heard byhumans. However, scientists and engineers use a wider definition of soundthat includes low and high frequency vibrations in air that cannot be heard byhumans, and vibrations that travel through all forms of matter, gases, liquidsand solids.

The matter that supports the sound is called the medium. Soundpropagates as waves of alternating pressure, causing local regions ofcompression and rarefaction. Particles in the medium are displaced by the

wave and oscillate. The scientific study of sound is called acoustics.Noise is often used to refer to an unwanted sound. In science and

engineering, noise is an undesirable component that obscures a wantedsignal.

Perception of sound

A schematic representation of hearing. (Blue: sound waves. Red:eardrum. Yellow: cochlea. Green: auditory receptor cells. Purple: frequencyspectrum of hearing response. Orange: nerve impulse)

Sound is perceived through the sense ofhearing. Humans and manyanimals use their ears to hear sound, but loud sounds and low-frequencysounds can be perceived by other parts of the body through the sense oftouch as vibrations. Sounds are used in several ways, notably for

communication through speech and music. They can also be used to acquireinformation about properties of the surrounding environment such as spatialcharacteristics and presence of other animals or objects. For example, batsuse echolocation, ships and submarines use sonar and humans candetermine spatial information by the way in which they perceive sounds.

Humans can generally hear sounds with frequencies between 20 Hzand 20 kHz (the audio range) although this range varies significantly withage, occupational hearing damage, and gender; the majority of people can no

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longer hear 20,000 Hz by the time they are teenagers, and progressively losethe ability to hear higher frequencies as they get older. Most human speechcommunication takes place between 200 and 8,000 Hz and the human ear ismost sensitive to frequencies around 1000-3,500 Hz. Sound above thehearing range is known as ultrasound, and that below the hearing range asinfrasound.

The amplitude of a sound wave is specified in terms of its pressure.The human ear can detect sounds with a very wide range of amplitudes andso a logarithmic decibel amplitude scale is used. The quietest sounds thathumans can hear have an amplitude of approximately 20 Pa (micropascals)or a sound pressure level (SPL) of 0 dB re 20 Pa (often incorrectlyabbreviated as 0 dB SPL). Prolonged exposure to a sound pressure levelexceeding 85 dB can permanently damage the ear, resulting in tinnitus andhearing impairment. Sound levels in excess of 130 dB are more than thehuman ear can safely withstand and can result in serious pain and permanentdamage. At very high amplitudes, sound waves exhibit nonlinear effects,including shock.

Speed of sound

The speed at which sound travels depends on the medium throughwhich the waves are passing, and is often quoted as a fundamental propertyof the material. In general, the speed of sound is proportional to the squareroot of the ratio of the elastic modulus (stiffness) of the medium and itsdensity. Those physical properties and the speed of sound change withambient conditions. For example, the speed of sound in air and other gasesdepends on temperature. In air, the speed of sound is approximately 344m/s, in water 1500 m/s and in a bar of steel 5000 m/s. The speed of sound is

also slightly sensitive (to second order) to the sound amplitude, resulting innonlinear propagation effects, such as the weak production of harmonics andthe mixing of tones (see parametric array).

Sound pressure

Sound pressure is the pressure deviation from the local ambientpressure caused by a sound wave. Sound pressure can be measured using amicrophone in air and a hydrophone in water. The SI unit for sound pressureis the pascal (symbol: Pa). The instantaneous sound pressure is the deviationfrom the local ambient pressure caused by a sound wave at a given locationand given instant in time. The effective sound pressure is the root mean

square of the instantaneous sound pressure averaged over a given interval oftime. In a soundwave, the complementary variable to sound pressure is theacoustic particle velocity. For small amplitudes, sound pressure and particlevelocity are linearly related and their ratio is the acoustic impedance. Theacoustic impedance depends on both the characteristics of the wave and themedium. The local instantaneous sound intensity is the product of the soundpressure and the acoustic particle velocity and is, therefore, a vector quantityin time.

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Sound pressure level

As the human ear can detect sounds with a very wide range ofamplitudes, sound pressure is often measured as a level on a logarithmicdecibel scale.

The sound pressure level (SPL) or Lp is defined as

where p is the root-mean-square sound pressure and p0 is a reference soundpressure. (When using sound pressure levels, it may be important to quotethe reference sound pressure used.) Commonly used reference soundpressures, defined in the standard ANSI S1.1-1994, are 20 Pa in air and 1Pa in water.

Since the human ear does not have a flat spectral response, soundpressure levels are often frequency weighted so that the measured level willmatch perceived levels more closely. The International ElectrotechnicalCommission (IEC) has defined several weighting schemes. A-weightingattempts to match the response of the human ear to noise and A-weightedsound pressure levels are labeled dBA. C-weighting is used to measure peak

levels.

Examples of sound pressure and sound pressure levels

Source of sound sound pressuresound pressurelevel

pascal dB re 20 Pathreshold of pain 100 134hearing damage during short-termeffect

20 approx. 120

jet, 100 m distant 6 - 200 110 - 140

jack hammer, 1 m distant / discotheque 2 approx. 100hearing damage during long-termeffect

0.6 approx. 90

major road, 10 m distant 0.2 - 0.6 80 - 90passenger car, 10 m distant 0.02 - 0.2 60 - 80

TV set at home level, 1 m distant 0.02 ca. 60normal talking, 1 m distant 0.002 - 0.02 40 - 60very calm room 0.0002 - 0.0006 20 - 30

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leaves noise, calm breathing 0.00006 10auditory threshold at 2 kHz 0.00002 0

Sound Waves

Sound is a series of compression waves that moves through air orother materials. These sound waves are created by the vibration of someobject, like a radio loudspeaker. The waves are detected when they cause adetector to vibrate. Your eardrum vibrates from sound waves to allow you tosense them. Sound has the standard characteristics of any waveform.

Sound is waveform in matter

Sound is a waveform that travels through matter. Although it iscommonly associated in air, sound will readily travel through many materialssuch as water and steel. Some insulating materials absorb much of the soundwaves, preventing the waves from penetrating the material.

Does not travel in vacuum

Because sound is the vibration of matter, it does not travel through avacuum or in outer space. When you see movies or TV shows about battles inouter space, you should only be able to see an explosion but not hear it. Thesounds are added for dramatic effect.

Some atoms in space

Note that in outer space, there are actually some widely-spaced atomsand molecules floating around. But since they are so far apart, regular wavemotion would not be great enough to detect.

Sound waves different than light waves

Also note that light and radio waves are electromagnetic waves. Theyare completely different than sound, which is vibration of matter.

Electromagnetic waves are related to electrical and magnetic fields andreadily travel through space.

Sound is a compression wave

The back-and-forth vibration of an object creates the compressionwaves of sound. The motions of a loudspeaker cone, drumhead and guitarstring are good examples of vibration that cause compression waves. This is

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different than the up and down or transverse motion of a water wave. (SeeGeneral Wave Motion for more information.)

Transverse Wave (water wave)

Compression Wave (sound)

The illustration above shows a comparison of a transverse wave suchas a water wave and the compression wave sound wave.

Characteristics of sound

A sound wave has characteristics just like any other type of wave,including amplitude, velocity, wavelength and frequency.

Amplitude

The amplitude of a sound wave is the same thing as its loudness. Sincesound is a compression wave, its loudness or amplitude would correspond tohow much the wave is compressed. It is sometimes called pressureamplitude.

Decibel

A common measurement of loudness is the decibel (dB). It is really1/10 of a bel, which was named after the inventor of the telephone,Alexander Graham Bell. It is a complex unit that varies as the ratio of thelogarithms of loudness.

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Decrease in loudness

A sound wave will spread out after it leaves its source, decreasing itsamplitude or loudness. The amplitude decreases as the square of thedistance from the source. Also, if there is some absorption in the material,the loudness of the sound will decrease as it moves through the substance.

Speed or velocity of sound

The speed or velocity of sound in air is approximately 344meters/second, 1130 feet/sec. or 770 miles per hour at room temperature of20oC (70oF). The speed varies with the temperature of air, such that soundtravels slower at higher altitudes or on cold days.

Note: The difference between speed and velocity is that velocity usuallyincludes direction the of travel. We'll interchange them here, but in somecases the distinction is important.

A jet plane traveling at the speed of sound would be moving at about680 mph at sea level. At very high altitudes, the speed required would bemuch lower.

Wavelength

Wavelength is the distance from one crest to another of a wave. Sincesound is a compression wave, the wavelength is the distance betweenmaximum compressions.

Frequency

The frequency of sound is the rate at which the waves pass a givenpoint. It is also the rate at which a guitar string or a loud speaker vibrates.Frequency is also called the pitch of a sound. It is called the note in musicalsounds.

Relationship

The relationship between velocity, wavelength and frequency is:

velocity = wavelength x frequency

Since the velocity of sound is approximately the same for all wavelengths,frequency is often used to better describe the effects of the differentwavelengths.

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Pitch

The pitch or note of a sound that we experience is determined by itswavelength or its frequency. The shorter the wavelength, the higher thefrequency becomes, and the higher the pitch that we hear.

Creating and detecting sounds

Creating and detecting sounds are similar effects, but opposite. Theydemonstrate the duality of nature.

Creating sound

Whenever an object in air vibrates, it causes compression waves in theair. These waves move away from the object as sound. There are many formsof the vibration, some not so obvious.

The back and forth movement of a loudspeaker cone, guitar string ordrum head result in compression waves of sound. When you speak, yourvocal cords also vibrate, creating sound.

Blowing across a bottle top can also create sound. In this case, the airinside the bottle goes in a circular motion, resulting in sound waves beingformed. Wind blowing through trees can also create sound this indirect way.

Sound can also be created by vibrating an object in a liquid such aswater or in a solid such as iron. A train rolling on a steel railroad track willcreate a sound wave that travels through the tracks. They will then vibrate,creating sound in air that you can hear, while the train may be a great

distance away.

Detecting sound

When a sound wave strikes an object, it can cause the object tovibrate. This leads to the method to detect sound, which requires changingthat vibration into some other type of signal--usually electrical.

The main way you detect or sense sounds is through your ears. Thesound waves vibrate your ear drum, which goes to the inner ear and ischanged to nerve signals you can sense.

You can also feel sounds. Stand in front of a stereo or hi-fi loudspeakeron at full volume, and you can feel some of the vibrations from the music.

There are mechanical devices that detect sounds, such as themicrophone. The sound vibrates a membrane, which creates an electricsignal that is amplified and recorded.

Doppler effect

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The Doppler effect, named after Christian Doppler, is the change infrequency and wavelength of a wave that is perceived by an observer movingrelative to the source of the waves. For waves, such as sound waves, thatpropagate in a wave medium, the velocity of the observer and of the sourceare reckoned relative to the medium in which the waves are transmitted. Thetotal Doppler effect may therefore result from either motion of the source or

motion of the observer. Each of these effects is analyzed separately. Forwaves which do not require a medium, such as light or gravity in specialrelativity only the relative difference in velocity between the observer andthe source needs to be considered.

Development

Doppler first proposed to the effect 1842 in the monograph ber dasfarbige Licht der Doppelsterne und einige andere Gestirne des Himmels -Versuch einer das Bradleysche Theorem als integrirenden Theil in sichschliessenden allgemeineren Theorie (On the coloured light of the binaryrefracted stars and other celestial bodies - Attempt of a more general theoryincluding Bradley's theorem as an integral part) [1]. The hypothesis was testedfor sound waves by the Dutch scientist Christoph Hendrik Diederik BuysBallot in 1845. He confirmed that the sound's pitch was higher as the soundsource approached him, and lower as the sound source receded from him.Hippolyte Fizeau discovered independently the same phenomenon onelectromagnetic waves in 1848 (in France, the effect is sometimes called"effet Doppler-Fizeau"). It is often overlooked that in Doppler's publications(and also Einstein's in his discussion of the Doppler effect) he explicitlyacknowledges that his formulas are only approximate since he made several

mathematical approximations in his derivation. Doppler's derivation isrepeated more or less verbatim in most modern textbooks but often withoutthe warning that the formulas are only valid in some (experimentally oftenseen) limits.

General

For waves that travel through a medium (sound, ultrasound, etc...) therelationship between observed frequency f' and emitted frequency f is givenby:

where is the speed of waves in the medium (in air at T degrees Celsius, thisis 332 + 0.59T m/s) is the velocity of the source (the thing emitting thesound)

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For waves that travel at the speed of light, such as radio waves, therelationship between observed frequency f' and emitted frequency f is givenby:

Change infrequency

Observedfrequency

where

is the transmitted frequencyis the velocity of the transmitter relative to the receiver in

meters/second: positive when moving towards one another, negativewhen moving away

is the speed of light in a vacuum m/sis the wavelength of the light

Because the detected frequency increases for objects moving towardthe observer, the object's velocity must be subtracted when motion is movingtoward the observer. (This is because the source's velocity is in thedenominator.) Conversely, detected frequency decreases when the objectmoves away, and so the object's velocity is added when the motion is away.As mentioned previously this equation is only a first approximation to anexact formula, but works reasonably well in the case considered by Doppler,i.e. when the source and/or receiver moves slowly and the signal or wavemoves rapidly, as well as simultaneously also being the case that the sourceand receiver are far apart from each other. When any of theseapproximations that Doppler made are violated the formulas discussed here

are no longer valid except as a very rough approximation to the exactformulas.

Analysis

It is important to realize that the frequency of the sounds that thesource emits does not actually change. To understand what happens,consider the following analogy. Someone throws one ball every second in aman's direction. Assume that balls travel with constant velocity. If thethrower is stationary, the man will receive one ball every second. However, ifthe thrower is moving towards the man, he will receive balls more frequentlybecause the balls will be less spaced out. The converse is true if the thrower

is moving away from the man. So it is actually the wavelength which isaffected; as a consequence, the perceived frequency is also affected.

If the moving source is emitting waves through a medium with anactual frequency f0, then an observer stationary relative to the mediumdetects waves with a frequency f given by:

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which can be written as:

where v is the speed of the waves in the medium and vs, r is the speed of thesource with respect to the medium (positive if moving away from theobserver, negative if moving towards the observer), radial to the observer.

With a relatively slow moving source, vs, r is small in comparison to vand the equation approximates to:

A similar analysis for a moving observer and a stationary source yieldsthe observed frequency (the observer's velocity being represented as vo):

where the same convention applies : vo is positive if the observer is movingaway from the source, and negative if the observer is moving towards thesource.

These can be generalized into a single equation with both the source

and receiver moving. However the limitations mentioned above still apply.When the more complicated exact equation is derived without using anyapproximations (just assuming that everything: source, receiver, and wave orsignal are moving linearly) several interesting and perhaps surprising resultsare found. For example, as Lord Rayleigh noted in his classic book on sound2by properly moving it is possible to hear a symphony being playedbackwards. This is the so-called "time reversal effect" of the Doppler effect.Other interesting cases are that the Doppler effect is time dependent ingeneral (thus we need to know not only the source and receivers' velocitiesbut also their positions at a given time) and also in some circumstances it ispossible to receive two signals or waves from a source (or no signal at all). Inaddition there are more possibilities than just the receiver approaching the

signal and the receiver receding from the signal.

All these additional complications are for the classical i.e.nonrelativistic Doppler effect. However all these results also hold for therelativistic Doppler effect as well.

The first attempt to extend Doppler's analysis to light waves was soonmade by Fizeau. In fact, light waves do not require a medium to propagate

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Redshift of spectral lines in the optical spectrum of a supercluster ofdistant galaxies (right), as compared to that of the Sun (left).

The Doppler effect for electromagnetic waves such as light, is of greatuse in astronomy, and results in either a so-called redshift or blueshift. It hasbeen used to measure the speed at which stars and galaxies are approachingtoward or receding from us, that is, the radial velocity. This is used to detectif a single star is, in fact, a close binary and even to measure the speed of

rotation of stars and galaxies.

The use of the Doppler effect for light in astronomy depends on thefact that the spectra of stars are not continuous. They show absorption linesat well defined frequencies that are correlated with the energies required toexcite electrons in various elements from one level to another. The Dopplereffect is recognizable in the fact that the absorption lines are not always atthe frequencies that are obtained from the spectrum of a stationary lightsource. Since blue light has a higher frequency than red light, the spectrallines from an approaching astronomical light source show a blueshift andthose of receding sources show a redshift.

Among the nearby stars, the largest radial velocities with respect tothe Sun are +308 km/s (BD-154041, also known as LHS 52, 81.7 light-yearsaway) and -260 km/s (Woolley 9722, also known as Wolf 1106 and LHS 64,78.2 light-years away). Positive radial velocity means the star is recedingfrom the Sun, negative that it is approaching.

Temperature measurement

http://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Astronomyhttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Blueshifthttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Velocity#Polar_coordinateshttp://en.wikipedia.org/wiki/Binary_starhttp://en.wikipedia.org/wiki/Astronomyhttp://en.wikipedia.org/wiki/Electromagnetic_spectroscopyhttp://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/List_of_nearest_starshttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/w/index.php?title=BD-15%C2%B04041&action=edithttp://en.wikipedia.org/w/index.php?title=Woolley_9722&action=edithttp://en.wikipedia.org/wiki/Image:Redshift.pnghttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Astronomyhttp://en.wikipedia.org/wiki/Redshifthttp://en.wikipedia.org/wiki/Blueshifthttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Galaxyhttp://en.wikipedia.org/wiki/Velocity#Polar_coordinateshttp://en.wikipedia.org/wiki/Binary_starhttp://en.wikipedia.org/wiki/Astronomyhttp://en.wikipedia.org/wiki/Electromagnetic_spectroscopyhttp://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/List_of_nearest_starshttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/w/index.php?title=BD-15%C2%B04041&action=edithttp://en.wikipedia.org/w/index.php?title=Woolley_9722&action=edit

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Another use of the Doppler effect which is found mostly in astronomy,is the estimation of the temperature of a gas which is emitting a spectral line.Due to the thermal motion of the gas, each emitter can be slightly red or blueshifted, and the net effect is a broadening of the line. This line shape is calleda Doppler profile and the width of the line is proportional to the square root ofthe temperature of the gas, allowing the Doppler-broadened line to be used

to measure the temperature of the emitting gas.

Radar

The Doppler effect is also used in some forms ofradar to measure thevelocity of detected objects. A radar beam is fired at a moving target - a car,for example, as radar is often used by police to detect speeding motorists -as it recedes from the radar source. Each successive wave has to travelfurther to reach the car, before being reflected and re-detected near thesource. As each wave has to move further, the gap between each waveincreases, increasing the wavelength. In some situations, the radar beam isfired at the moving car as it approaches, in which case each successive wavetravels a lesser distance, decreasing the wavelength. In either situation,calculations from the Doppler effect accurately determine the car's velocity.

The Proximity fuze which was developed during World War II also relieson Doppler radar.

Medical imaging and blood flow measurement

An echocardiogram can, within certain limits, produce accurateassessment of the direction of blood flow and the velocity of blood andcardiac tissue at any arbitrary point using the doppler effect. One of the

limitations is that the ultrasound beam should be as parallel to the blood flowas possible. Velocity measurements allow assessment of cardiac valve areasand function, any abnormal communications between the left and right sideof the heart, any leaking of blood through the valves (valvular regurgitation),and calculation of the cardiac output. Contrast-enhanced ultrasound usinggas-filled microbubble contrast media can be used to improve velocity orother flow-related medical measurements.

Although "Doppler" has become synonymous with "velocitymeasurement" in medical imaging, in many cases it is not the frequency shift(Doppler shift) of the received signal that is measured, but the phase shift(when the received signal arrives).

Velocity measurements of blood flow are also used in other fields ofmedical ultrasonography, such as obstetric ultrasonography and neurology.Velocity measurement of blood flow in arteries and veins based on dopplereffect is an effective tool for diagnosis of vascular problems like stenosis [2].

Flow measurement

http://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Doppler_broadeninghttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Policehttp://en.wikipedia.org/wiki/Proximity_fuzehttp://en.wikipedia.org/wiki/World_War_IIhttp://en.wikipedia.org/wiki/Echocardiogramhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Cardiac_outputhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Medical_ultrasonographyhttp://en.wikipedia.org/wiki/Obstetric_ultrasonographyhttp://en.wikipedia.org/wiki/Neurologyhttp://en.wikipedia.org/wiki/Doppler_Effect#_note-1http://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Doppler_broadeninghttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Policehttp://en.wikipedia.org/wiki/Proximity_fuzehttp://en.wikipedia.org/wiki/World_War_IIhttp://en.wikipedia.org/wiki/Echocardiogramhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Cardiac_outputhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Medical_ultrasonographyhttp://en.wikipedia.org/wiki/Obstetric_ultrasonographyhttp://en.wikipedia.org/wiki/Neurologyhttp://en.wikipedia.org/wiki/Doppler_Effect#_note-1

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Instruments such as the laser Doppler velocimeter (LDV), and AcousticDoppler Velocimeter (ADV) have been developed to measure velocities in afluid flow. The LDV and ADV emit a light or acoustic beam, and measure thedoppler shift in wavelengths of reflections from particles moving with theflow. This technique allows non-intrusive flow measurements, at highprecision and high frequency.

http://en.wikipedia.org/wiki/Laser_Doppler_velocimetryhttp://en.wikipedia.org/wiki/Acousticshttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Laser_Doppler_velocimetryhttp://en.wikipedia.org/wiki/Acousticshttp://en.wikipedia.org/wiki/Velocity