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.

Frequency (Hz) Power Noise spectrum Signal

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

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.

Frequency (Hz) Power Internal filter Noise spectrum Signal

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

Frequency Filter weighting (dB) -40 7

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

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

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

Vertical angles 0º 180º 90º 270º

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.

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

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.

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

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

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

Vertical angles 0º 180º 90º 270º

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.

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

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.

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

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

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).

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

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.