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  1. Acoustics
  2. AKG Acoustics
  3. Audio feedback
  4. Audio level compression
  5. Audio quality measurement
  6. Audio-Technica
  7. Balanced audio connector
  8. Beyerdynamic
  9. Blumlein Pair
  10. Capacitor
  11. Carbon microphone
  12. Clipping
  13. Contact microphone
  14. Crosstalk measurement
  15. DB
  16. Decibel
  17. Directional microphone
  18. Dynamic range
  19. Earthworks
  20. Electret microphone
  21. Electrical impedance
  22. Electro-Voice
  23. Equal-loudness contour
  24. Frequency response
  25. Georg Neumann
  26. Harmonic distortion
  27. Headroom
  28. ITU-R 468 noise weighting
  29. Jecklin Disk
  30. Laser microphone
  31. Lavalier microphone
  32. Loudspeaker
  33. M-Audio
  34. Microphone
  35. Microphone array
  36. Microphone practice
  37. Microphone stand
  38. Microphonics
  39. Nevaton
  40. Noise
  41. Noise health effects
  42. Nominal impedance
  43. NOS stereo technique
  44. ORTF stereo technique
  45. Parabolic microphone
  46. Peak signal-to-noise ratio
  47. Phantom power
  48. Pop filter
  49. Positive feedback
  50. Rode
  51. Ribbon microphone
  52. Schoeps
  53. Sennheiser
  54. Shock mount
  55. Shure
  56. Shure SM58
  57. Signal-to-noise ratio
  58. Soundfield microphone
  59. Sound level meter
  60. Sound pressure
  61. Sound pressure level
  62. Total harmonic distortion
  63. U 47
  64. Wireless microphone
  65. XLR connector



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Sound pressure

From Wikipedia, the free encyclopedia

(Redirected from Sound pressure level)

Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.

The sound pressure deviation p is

p = \frac{F}{A}


F = force,
A = area.

The entire pressure ptotal is

p_\mathrm{total} = p_0 + p \,


p0 = local ambient pressure,
p = sound pressure deviation.

Sound pressure level

Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms pressure of a particular sound relative to a reference sound source. It is usually measured in decibels (dB (SPL), dBSPL, or dBSPL).

L_p=10 \log_{10}\left(\frac{p^2_{rms}}{p^2_0}\right) =20 \log_{10}\left(\frac{p_{rms}}{p_0}\right)\mbox{ dB}

where p0 is the reference sound pressure and prms is the rms sound pressure being measured.

The commonly used reference sound pressure in air is p0 = 20 Pa (rms). In underwater acoustics, the reference sound pressure is p0 = 1 Pa (rms).

It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure. See also Weber-Fechner law.

Measuring sound pressure levels

When making measurements in air (and other gases), SPL is almost always expressed in decibels compared to a reference sound pressure of 20 Pa, which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). Thus, most measurements of audio equipment will be made relative to this level. However, in other media, such as underwater, a reference level of 1 Pa is more often used.[1] These references are defined in ANSI S1.1-1994.[2] In general, it is necessary to know the reference level when comparing measurements of SPL. The unit dB (SPL) is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself.

The human ear is a sound pressure sensitive detector. It does not have a flat spectral response, sound pressure levels are often frequency weighted so that the measured level will match perceived sound level. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise, while C-weighting is used to measure peak sound levels.[3] If the actual, as opposed to weighted, SPL is desired, many instruments allow a "flat" or unweighted measurement to be made. See also Weighting filter.

When measuring the sound created by an object, it is important to measure the distance from the object as well, since the SPL decreases in distance from a point source with 1/r (and not with 1/r2, like sound intensity). It often varies in direction from the source, as well, so many measurements may be necessary, depending on the situation. An obvious example of a source that varies in level in different directions is a bullhorn.

Sound pressure p in N/m2 or Pa is

p = Zv = \frac{J}{v} = \sqrt{JZ}\,


Z is acoustic impedance, sound impedance, or characteristic impedance, in Pas/m
v is particle velocity in m/s
J is acoustic intensity or sound intensity, in W/m2

Sound pressure p is connected to particle displacement (or particle amplitude) ξ, in m, by

\xi = \frac{v}{2 \pi f} = \frac{v}{\omega} = \frac{p}{Z \omega} = \frac{p}{ 2 \pi f Z}.

Sound pressure p is

p = \rho c \omega \xi = Z \omega \xi = { 2 \pi f \xi Z} = \frac{a Z}{\omega} = c \sqrt{\rho E} = \sqrt{\frac{P_{ac} Z}{A}},

normally in units of N/m2 = Pa.


The distance law for the sound pressure p is inverse-proportional to the distance r of a punctual sound source.

p \propto \frac{1}{r} (proportional)
\frac{p_1} {p_2} = \frac{r_2}{r_1}
p_1 = p_{2} \cdot r_{2} \cdot \frac{1}{r_1}

The assumption of 1/r with the square is here wrong. That is only correct for sound intensity.

Note: The often used term "intensity of sound pressure" is not correct. Use "magnitude", "strength", "amplitude", or "level" instead. "Sound intensity" is sound power per unit area, while "pressure" is a measure of force per unit area. Intensity is not equivalent to pressure.

Examples of sound pressure and sound pressure levels

See also

  • Decibel, especially the Acoustics section
  • Acoustics
  • Sone
  • Weber-Fechner law
  • Sound power level


  1. ^ Underwater Acoustics Federation of American Scientists
  2. ^ Glossary of Noise Terms Sound pressure level definition
  3. ^ Glossary of Terms Cirrus Research plc.
  4. ^ Decibel level chart.
  • Beranek, Leo L, "Acoustics" (1993) Acoustical Society of America. ISBN 0-88318-494-X

External links

  • Conversion of sound pressure to sound pressure level and vice versa
  • The level of sound is dB
  • Table of Sound Levels - Corresponding Sound Pressure and Sound Intensity
  • SPL of many different sounds - txt
  • Ohm's law as acoustic equivalent - calculations
  • Definition of sound pressure level
  • A table of SPL values
  • Relationships of acoustic quantities associated with a plane progressive acoustic sound wave - pdf
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