1. Fletcher’s band-widening experiment (1940)
Present a pure tone in the presence of a broadband noise.
Both signal and noise are presented simultaneously.
Signal is kept constant (in frequency and intensity).
Bandwidth of the noise is increased, keeping spectrum level constant.

2. Frequency (Hz)
Power
Noise spectrum
Signal

3. Concept of the critical bandwidth
Results: At first, the masked threshold of the signal increased. After a certain bandwidth, no more change in the signal threshold.
This bandwidth was termed the critical bandwidth.
Bandwidth of masker
Masked
signal
threshold
Critical bandwidth

4. Explanation of results
Theory: Bandpass ‘internal’ filter centered around signal frequency.
Both signal and the noise passed through this bandpass filter.
Only the bandwidth of the noise that passes through the internal filter has an effect on the signal threshold.
Beyond this point, noise only increases in loudness.

5. Frequency (Hz)
Power
Internal filter
Noise spectrum
Signal

6. Characteristics of internal critical band filter
Shape of the derived filter does not change with frequency
Bandwidth increases with the center frequency of the filter.
Bandwidth increases with increasing signal intensity. 6

7. Frequency
Filter weighting (dB)
-40 7

8. SPATIAL HEARING Ability to locate the direction of a sound.
Localization: In free field
Lateralization: Under headphones 8

9. Sound localization Planes Horizontal/Azimuth: Left to right
Vertical: Up to down
Distance: Near to far 9

10. Horizontal angles 0º : Front 180º : Back 90º : Right 270º : Left 0º10

11. Vertical angles 0º
180º
90º
270º

12. Localization in the horizontal plane
Based on inter-aural time and inter-aural intensity differences
Duplex theory of localization
For low frequencies: Inter-aural time differences
For high frequencies: Inter-aural intensity/level differences.

14. Effect of azimuth and frequency
For sounds of any frequency, ITD and ILD highest at 90 degrees azimuth.
ILD high for high frequencies (head shadow effect)
ITD approximately same across frequencies, but most useful at low frequencies

15. Errors in absolute localization
Depends on the type of stimulus
For pure tones: Most errors in the mid-frequencies
For broadband noise: Very few errors.

16. Localization in the vertical plane
Cone of confusion: All sounds that lie in the mid-sagittal plane have the same inter-aural differences
Inter-aural differences not very useful for sound sources in the cone of confusion
Types of confusions: Front-Back and Back-Front
Useful cues: Head related transfer functions, mainly for high frequencies

17. Errors in vertical localization
Depends on type of stimulus
Poor for low frequency pure tones
Broadband stimuli: Not as good as horizontal localization

18. Horizontal angles 0º : Front 180º : Back 90º : Right 270º : Left 0º

19. Vertical angles 0º
180º
90º
270º

20. Localization in the horizontal plane
Based on inter-aural time and inter-aural intensity differences
Duplex theory of localization
For low frequencies: Inter-aural time differences
For high frequencies: Inter-aural intensity/level differences.

22. Effect of azimuth and frequency
For sounds of any frequency, ITD and ILD highest at 90 degrees azimuth.
ILD high for high frequencies (head shadow effect)
ITD approximately same across frequencies, but most useful at low frequencies

23. Errors in absolute localization
Depends on the type of stimulus
For pure tones: Most errors in the mid-frequencies
For broadband noise: Very few errors.

24. Localization in the vertical plane
Cone of confusion: All sounds that lie in the mid-sagittal plane have the same inter-aural differences
Inter-aural differences not very useful for sound sources in the cone of confusion
Types of confusions: Front-Back and Back-Front
Useful cues: Head related transfer functions, mainly for high frequencies

25. Errors in vertical localization
Depends on type of stimulus
Poor for low frequency pure tones
Broadband stimuli: Not as good as horizontal localization

26. Discrimination
Minimal audible angle (MAA): Smallest detectable angular separation between two loudspeakers.
Smallest MAA when the loudspeaker is directly in front of the listener (when listener’s head is stationary).

27. Distance perception Loudness changes with distance Precedence effect
Multiple reflections from surfaces
Auditory system processes the first wavefront and suppresses location information from later-arriving wavefronts

28. Lateralization Experiments under headphones more controlled.
Sounds are perceived ‘inside’ the head
Fused image: For inter-aural time difference less than 2 ms, sounds arriving at both ears are ‘fused’ into one image.
If more than 2 ms ITD, sound heard in both ears.