SPATIAL HEARING l Ability to locate the direction

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

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

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

Vertical angles 90º 0º 180º 270º

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

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

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

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

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

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

Distance perception l Loudness changes with distance l Precedence effect Multiple reflections from surfaces Auditory system processes the first wavefront and suppresses location information from laterarriving wavefronts

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

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

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

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

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

STANDARD 1000 Hz 1. 0 d. B: 0 phon curve COMPARISON 100 Hz 250 Hz 2. 10 d. B: 10 phon curve 500 Hz 3. 20 d. B: 20 phon curve 2000 Hz 4. 30 d. B: 30 phon curve 4000 Hz … 120 d. B: 120 phon curve 8000 Hz 10000 Hz

Equal loudness contours: Fletcher and Munson, 1933

Characteristics of loudness perception: Effect of frequency l Equal loudness contours not parallel to each other l The loudness of low intensity sounds is highly dependent on the frequency of the sound l 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 l Loudness increases with intensity l NOT A LINEAR RELATION

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

Scales of pitch l Musical scale: Octave – Semitones - Cents l Non-musical scale: Mel l For pure tones, perceived pitch corresponds to frequency l 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 l Example, noise (continuous spectrum) with spectral peaks at 750, 1000, 1250, 1500, 1750 and 2000 Hz l No fundamental frequency or periodicity in this case l Reported pitch: 250 Hz (corresponds to spacing between the spectral peaks)

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