Sources, distance and directivity

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Sources, distance and directivity
Introduction to Acoustics

• University of Salford
• Acoustics Audio and Video Group

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Learning Outcomes

• Define directivity factor and directivity index
• Explain how (and why) intensity varies with
  distance from different sound sources
• Calculate the sound pressure level at a given
  distance from a particular source

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Sound level vs. distance

• Point source
• Spherical wave fronts
• 6dB decrease when distance doubles

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Sound level vs. distance

• Line source
• Cylindrical wave fronts
• 3dB decrease when distance doubles

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Spherical spreading

• Point source gt spherical waves
• Starting from
• and
• we find

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Inverse square law

• Each time the distance from the source is doubled
  the sound pressure level reduces by 6dB
• Each time the distance from the source is halved
  the sound pressure level increases by 6dB
• This is referred to as the inverse square law
• Comes from I proportional to 1/r2

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Source directivity

• Directivity index,
• Directivity factor,
• So, now

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Directivity patterns

• Baffled loudspeaker at 1.5 kHz

• Unbaffled loudspeaker

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Directivity patterns

• Baffled loudspeaker at 17 kHz

• Baffled loudspeaker at 7 kHz

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Point sources on hard surfaces

• Source on one hard surface
• Source at junction of two hard surfaces
• Source in corner

• Q2 and DI3 dB
• Q4 and DI6 dB
• Q8 and DI9 dB

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Conclusions

• Point source
• spherical wave, 6 dB drop per distance doubled
• Non-uniform directivity
• Line source
• cylindrical wave, 3 dB drop per distance doubled