Sensitivity of the Ear

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Loudness Curves

Loudness curves can be produced using the following process.

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Producing loudness curves

  • Test subjects listen to pure tones at various frequencies and at different intensities, using headphones in a soundproof room.
  • For each frequency and intensity, they also listen to a reference tone at 1000 Hz.
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Producing loudness curves 2

  • The listener perceives when each tone is the same loudness as the test tone and a graph is produced of the sound level (measured in decibels) required to produce the same intensity at different frequencies.

Human Perception of Relative Intensity

The human ear does not respond linearly to sound pressure, but approximately in a logarithmic fashion.

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Relative intensity

  • Intensity is measured by comparison with a reference level.
  • Relative intensity (measured in bels) =log10(I1I0)= \log_{10}(\frac{I_1}{I_0})
    • where I0 is 10-12 Wm-2.
  • In practice the decibel (dB) is used, so:
    • Relative intensity =10log10(I1I0)= 10 \log_{10}(\frac{I_1}{I_0}) dB
  • 1 decibel = 10 bels
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Frequency dependence

  • The response of the ear to sound is dependent on the frequency of the sound.
  • The single sound pressure level obtained by simply adding the contribution from all frequencies will not correlate well with the non-linear frequency response of the human ear.
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Frequency dependence 2

  • In the "A-weighting" scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts to give one single sound pressure level value.
  • This value is designated as dB(A).
  • The dB(A) more accurately reflects the frequency response of the human ear.

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