10.2.2

# Sensitivity of the Ear

Test yourself

## Loudness Curves

Loudness curves can be produced using the following process.

### 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.

### 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.

### Relative intensity

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

### 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.

### 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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