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

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

Horizontal angles 0º : Front 0º : Front 180º : Back 180º : Back 90º : Right 90º : Right 270º : Left 270º : Left0º180º 90º270º

Vertical angles 0º 180º90º270º

Localization in the horizontal plane Based on inter-aural time and inter-aural intensity differences Based on inter-aural time and inter-aural intensity differences Duplex theory of localization For low frequencies: Inter- aural time differences For low frequencies: Inter- aural time differences For high frequencies: Inter- aural intensity/level 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. For sounds of any frequency, ITD and ILD highest at 90 degrees azimuth. ILD high for high frequencies (head shadow effect) ILD high for high frequencies (head shadow effect) ITD approximately same across frequencies, but most useful at low frequencies ITD approximately same across frequencies, but most useful at low frequencies

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

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

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

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

Distance perception Loudness changes with distance Loudness changes with distance Precedence effect 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. Experiments under headphones more controlled. Sounds are perceived ‘inside’ the head 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. 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. If more than 2 ms ITD, sound heard in both ears.

Loudness Subjective attribute of intensity Subjective attribute of intensity Measuring loudness: Loudness matching task Standard or reference tone Standard or reference tone Comparison tone Comparison tone Subject’s task: To match the loudness of the comparison tone to that of the standard tone. Subject’s task: To match the loudness of the comparison tone to that of the standard tone. Do they sound equally loud? If not, adjust the level of the comparison sound till they sound equally loud. Do they sound equally loud? If not, adjust the level of the comparison sound till they sound equally loud.

1000 Hz 250 Hz 500 Hz 2000 Hz 4000 Hz STANDARD COMPARISON 1000 Hz

Units of loudness Loudness expressed in units of phons or sones Phon: Loudness level Sone: Loudness

Equal loudness contours (Fletcher and Munson, 1933) Standard tone: 1000 Hz tone presented at various intensities Standard tone: 1000 Hz tone presented at various intensities Frequency of the comparison tone varied Frequency of the comparison tone varied Each curve represents a different intensity of the standard Each curve represents a different intensity of the standard Every point or frequency on a given curve has the same loudness level Every point or frequency on a given curve has the same loudness level

1000 Hz 1.0 dB: 0 phon curve 2.10 dB: 10 phon curve 3.20 dB: 20 phon curve 4.30 dB: 30 phon curve …120 dB: 120 phon curve STANDARD 250 Hz 500 Hz 2000 Hz 4000 Hz 8000 Hz Hz 100 Hz COMPARISON

Equal loudness contours: Fletcher and Munson, 1933

Characteristics of loudness perception: Effect of frequency Equal loudness contours not parallel to each other Equal loudness contours not parallel to each other The loudness of low intensity sounds is highly dependent on the frequency of the sound The loudness of low intensity sounds is highly dependent on the frequency of the sound At higher intensities, loudness does not depend as much on frequency At higher intensities, loudness does not depend as much on frequency For sounds of equal intensity, loudness is not necessarily equal.

Characteristics of loudness perception: Effect of intensity Loudness increases with intensity Loudness increases with intensity NOT A LINEAR RELATION NOT A LINEAR RELATION

Pitch Subjective attribute of frequency Subjective attribute of frequency Sounds above 1000 Hz need to be at least 10 ms long for ‘pitch’ to be perceived Sounds above 1000 Hz need to be at least 10 ms long for ‘pitch’ to be perceived Pitch is a much more complex phenomenon than loudness Pitch is a much more complex phenomenon than loudness No direct correspondence between pitch and the actual frequencies present in the stimulus No direct correspondence between pitch and the actual frequencies present in the stimulus

Scales of pitch Musical scale: Octave – Semitones - Cents Musical scale: Octave – Semitones - Cents Non-musical scale: Mel Non-musical scale: Mel For pure tones, perceived pitch corresponds to frequency For pure tones, perceived pitch corresponds to frequency For complex sounds, the pitch may not correspond to an actual frequency present in the sound For complex sounds, the pitch may not correspond to an actual frequency present in the sound

For periodic complex sounds: ‘Missing fundamental’: Highest fundamental frequency for which the rest of the components could be harmonically related Reported pitch: 100 Hz Pitch perceived based on the envelope of the complex sound

For non-periodic complex sounds Example, noise (continuous spectrum) with spectral peaks at 750, 1000, 1250, 1500, 1750 and 2000 Hz Example, noise (continuous spectrum) with spectral peaks at 750, 1000, 1250, 1500, 1750 and 2000 Hz No fundamental frequency or periodicity in this case No fundamental frequency or periodicity in this case Reported pitch: 250 Hz (corresponds to spacing between the spectral peaks) Reported pitch: 250 Hz (corresponds to spacing between the spectral peaks)

In other cases Here, pitch corresponds neither to periodicity, nor to the frequency spacing of the tones. Here, pitch corresponds neither to periodicity, nor to the frequency spacing of the tones. If based on missing fundamental: Should be 25 Hz. If based on missing fundamental: Should be 25 Hz. If based on frequency spacing: Should be 100 Hz. If based on frequency spacing: Should be 100 Hz. Reported pitch: 104 Hz Reported pitch: 104 Hz