Simple Models of the Auditory System

1
Simple Models of the Auditory System
Introduction to Acoustics

• University of Salford
• Acoustics, Audio Video Group

2
Learning Outcomes

• Explain how frequency weightings model the
  performance of the ear
• Do calculations involving frequency weightings
• Discuss simple models of spatial hearing

3
Initial questions

• How can we make a device to measure loudness
  level?
• Why do we have two ears (instead of one)?

4
Basic sound level meter

• Handheld
• Powered by battery
• Microphone
• Meter scale in dB
• Can it measure loudness level?

5
Reprise equal loudness contours
6
Modelling loudness

• Contours are all different shapes
• Too complicated for a handheld (analogue) device!
• Pick one (representative) curve, invert it and
  simplify it

7
A-weighting curve
8
A-weighting table
9
A-weighting example calculation

• A sound level meter is used to measure
  single-frequency sounds.
• If the unweighted sound pressure level at 500 Hz
  is 70 dB, what is the A-weighted sound pressure
  level?
• If the A-weighted sound pressure level at 4000 Hz
  is 60 dB, what is the unweighted sound pressure
  level?

10
Other weighting curves
11
Binaural hearing
Binaural localisation how do we locate the
position of a sound source?
12
Interaural time difference
Upper frequency limit reached when ?t half a
period of the sound wave
13
Interaural intensity difference
Low frequency limit reached when wavelength 3d
14
Questions

• At an incident angle of 90, what is the maximum
  frequency for useful ITD?
• What is the minimum frequency for useful IID?

15
Conclusions

• Loudness perception is (crudely) modelled by
  A-weighting sound pressure levels
• Interaural time difference localises at low
  frequencies
• Interaural intensity difference localises at high
  frequencies