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Encyclopedia > Sound intensity
Sound measurements
Sound pressure p
Sound pressure level (SPL)
Particle velocity v
Particle velocity level (SVL)
   (Sound velocity level)
Particle displacement ξ
Sound intensity I
Sound intensity level (SIL)
Sound power Pac
Sound power level (SWL)
Sound energy density E
Sound energy flux q
Acoustic impedance Z
Speed of sound c

The sound intensity, I, (acoustic intensity) is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location. For instantaneous acoustic pressure pinst(t) and particle velocity v(t) the average acoustic intensity during time T is given by Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. ... It has been suggested that this article or section be merged into Sound pressure. ... Particle velocity is the velocity v of a particle (real or imagined) in a medium as it transmits a wave. ... The particle velocity level or the sound velocity level tells the ratio of a sound incidence in comparison to a reference level of 0 dB. It shows the ratio of the particle velocity v1 and the particle velocity v0. ... Particle displacement or particle amplitude (represented in mathematics by the lower-case Greek letter ξ) is a measurement of distance (in metres) of the movement of a particle in a medium as it transmits a wave. ... Sound intensity level or acoustic intensity level is a logarithmic measure of the sound intensity in comparison to the reference level of 0 dB (decibels). ... Sound power or acoustic power Pac is a measure of sonic energy E per time t unit. ... Sound power level or acoustic power level is a logarithmic measure of the sound power in comparison to a specified reference level. ... The sound energy density or sound density (symbol E or w) is an adequate measure to describe the sound field at a given point as a sound energy value. ... The sound energy flux is the average rate of flow of sound energy for one period through any specified area. ... The acoustic impedance Z (or sound impedance) is a frequency f dependent parameter and is very useful, for example, for describing the behaviour of musical wind instruments. ... Sound is a vibration that travels through an elastic medium as a wave. ... Sound power or acoustic power Pac is a measure of sonic energy E per time t unit. ... In physics, intensity is a measure of the time-averaged energy flux. ... Sound pressure p (or acoustic pressure) is the measurement in pascals of the Root Mean Square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. ... Particle velocity is the velocity v of a particle (real or imagined) in a medium as it transmits a wave. ...

I = frac{1}{T} int_{0}^{T}p_{inst}(t) cdot v(t),dt

Notice that both v(t) and I are vectors, which means that both have a direction as well as a magnitude. The direction of the intensity is the average direction in which the energy is flowing. The SI units of intensity are W/m2 (watts per square metre). A vector going from A to B. In physics and in vector calculus, a spatial vector, or simply vector, is a concept characterized by a magnitude and a direction. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...


For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:

I_r = frac{P_{ac}}{A} = frac{P_{ac}}{4 pi r^2} ,

Here Pac (upper case) is the sound power and A the surface area of a sphere of radius r. Thus the sound intensity decreases with 1/r2 the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r-distance law.

I sim {p^2} sim dfrac{1}{r^2} ,

where p (lower case) is the RMS sound pressure (acoustic pressure). RMS may refer to any of the following: Railway Mail Service, US mail transportation service until the mid-20th century Royal Mail Ship or Steamer, ship prefix for vessels that carry mail under contract to the British Royal Mail Royal Melbourne Show Royal Meteorological Society Royal Society of Miniature Painters...


Hence

p sim dfrac{1}{r} ,

The sound intensity I in W/m2 of a plane progressive wave is:

I = frac{p^2}{Z} = Z cdot v^2 = &# 0;2 cdot omega^2 cdot Z = frac{a^2 cdot Z}{omega^2} = E cdot c = frac{P_{ac}}{A}

where:

Symbol Units Meaning
p pascals RMS sound pressure
f hertz frequency
ξ m, metres particle displacement
c m/s speed of sound
v m/s particle velocity
ω = 2πf radians/s angular frequency
ρ kg/m3 density of air
Z = c · ρ N·s/m³ characteristic acoustic impedance
a m/s² particle acceleration
I W/m² sound intensity
E W·s/m³ sound energy density
Pac W, watts sound power or acoustic power
A m² area

Sound intensity level, LI, is the magnitude of sound intensity, expressed in logarithmic units (decibels). The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. ... MHZ redirects here. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... The metre (American English:meter) is a measure of length. ... Particle displacement or particle amplitude (represented in mathematics by the lower-case Greek letter ξ) is a measurement of distance (in metres) of the movement of a particle in a medium as it transmits a wave. ... The metre (American English:meter) is a measure of length. ... Look up second in Wiktionary, the free dictionary. ... Sound is a vibration that travels through an elastic medium as a wave. ... Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. ... Particle velocity is the velocity v of a particle (real or imagined) in a medium as it transmits a wave. ... In mathematics and physics, the radian is a unit of angle measure. ... Look up second in Wiktionary, the free dictionary. ... It has been suggested that this article or section be merged into Angular velocity. ... The U.S. National Prototype Kilogram, which currently serves as the primary standard for measuring mass in the U.S. It was assigned to the United States in 1889 and is periodically recertified and traceable to the primary international standard, The Kilogram, held at the Bureau International des Poids et... The metre (American English:meter) is a measure of length. ... The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earths atmosphere, and is a useful value in aeronautics. ... The newton (symbol: N) is the SI unit of force. ... Look up second in Wiktionary, the free dictionary. ... The metre (American English:meter) is a measure of length. ... The acoustic impedance Z (or sound impedance) is a frequency f dependent parameter and is very useful, for example, for describing the behaviour of musical wind instruments. ... The metre (American English:meter) is a measure of length. ... Look up second in Wiktionary, the free dictionary. ... In a compressible sound transmission medium - mainly air - air particles get an accelerated motion: the particle acceleration or sound acceleration with the symbol a in metre/second². In acoustics or physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... Look up second in Wiktionary, the free dictionary. ... The metre (American English:meter) is a measure of length. ... The sound energy density or sound density (symbol E or w) is an adequate measure to describe the sound field at a given point as a sound energy value. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... Sound power or acoustic power Pac is a measure of sonic energy E per time t unit. ... Sound power or acoustic power is a measure of sonic energy E per time t unit. ... The metre (American English:meter) is a measure of length. ... Area is a physical quantity expressing the size of a part of a surface. ... Sound intensity level or acoustic intensity level is a logarithmic measure of the sound intensity in comparison to the reference level of 0 dB (decibels). ... The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power) relative to a specified or implied reference level. ...

L_I=10 log_{10} frac {|I|}{I_o} (dB-SIL),

where Io is the reference intensity, 10-12 W/m2



Note 1^ : The term "intensity" is used exclusively for the measurement of sound in watts per unit area.
To describe the strength of sound in terms other than strict intensity, one can use "magnitude" "strength", "amplitude", or "level" instead. In physics, intensity is a measure of the time-averaged energy flux. ... The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs. ... Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation, that is, magnitude of the maximum disturbance in the medium during one wave cycle. ... Look up level in Wiktionary, the free dictionary. ...


Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure which is related to sound intensity. In stereo the level differences have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be valuable in music recording if it could. Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. ...


See also

In physics, intensity is a measure of the time-averaged energy flux. ...

External links


  Results from FactBites:
 
Loudness (894 words)
Sound loudness is a subjective term describing the strength of the ear's perception of a sound.
This is one of the motivations for using the decibel scale to measure sound intensity.
A widely used "rule of thumb" for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud.
Sound Intensity (493 words)
To assess sound loudness, as distinct from an objective intensity measurement, the sensitivity of the ear must be factored in.
Since audible sound consists of pressure waves, one of the ways to quantify the sound is to state the amount of pressure variation relative to atmospheric pressure caused by the sound.
Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:
  More results at FactBites »


 

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