Intensity and pressure

1
Intensity and pressure
Introduction to Acoustics

• University of Salford
• Acoustics Audio and Video Group

2
Learning Outcomes

• Define sound intensity, sound pressure and sound
  pressure level
• Explain how power, intensity and pressure are
  related
• Convert pressure and intensity to their decibel
  form and back again.

3
Sound intensity

• The sound intensity of a wave is the power per
  unit area
• The unit area is normal to the direction of
  travel of the wave
• The unit of intensity is watts per metre squared
  Wm-2
• There is a direction associated with intensity

4
Plane wave
5
Spherical wave
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Sound intensity level

• I010-12 Wm-2
• Note
• If I doubles, LI increases by 3 dB
• If I multiplies by 10, LI increases by 10 dB

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Handling intensity levels

• Convert from LI back to I

• Ratio of intensities expressed in dB

8
Pressure

• Atmospheric pressure
• 1 Bar 106 µBar 105 Pascals (Pa)
• Acoustic pressure
• Range 10-5 Pa to 20 Pa

9
Root-mean-square

• Used to average over time
• For a harmonic signal,

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Pressure and intensity

• where
• I is the time-averaged acoustic intensity
• is the mean square acoustic pressure
• ?0 is the density of air
• c is the velocity of sound in air

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Sound pressure level

• p02x10-5 Pa
• Note
• If p doubles, LP increases by 6 dB
• If p multiplies by 10, LP increases by 20 dB

12
Handling SPLs

• Sound pressure level sound intensity level
• Sound pressure from SPL
• Ratio of sound pressures in dB

13
Conclusions

• Intensity I is power divided by area
• Sound intensity level is the decibel form of I
• rms is an average value

• Pressure and intensity
• Sound pressure level is the decibel form of p