Binaural hearing and spatialization

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Binaural hearing and spatialization

• Perfecto Herrera
• Psicoacústica

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• La natura ha proveït lésser humà amb una
  llengua, però amb dues orelles, de manera tal que
  pugui escoltar dues vegades més que parlar
• Zenó

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Why 2 ears?

• Locating sounds in space acuity is poorer than
  in vision by up to two orders of magnitude, but
  extends in all directions. Role in alerting and
  orienting?
• Separating sound sources signals from 2
  receivers can be compared and operated on to
  enhance the reception of one sound source over
  another. The same applies to signals in a
  reverberant environment.
• Definitions
• Monaural Sound presented to one ear
• Binaural Sound presented to both ears
• Diotic Sounds identical at the two ears
• Dichotic Sounds different at the two ears
• Azimuth position in the left-right axis
• Elevation position in the up-down axis
• Blauert, J. (1997). Spatial Hearing The
  Psychophysics of Human Hearing. Cambridge, MA
  MIT Press.

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Localisation in 3 dimensions

• Azimuth (left/right)
• (Arab. as-sumut, i.e. as al the sumut, pl of
  samt way)
• Binaural cues Interaural Time Differences and
  Interaural Level Differences
• Lateralization is a simplified localisation
  (placement in the head)
• Median-plane (front, up, back, down)
• Pinna-induced spectral cues
• Head movements
• Distance
• Absolute level, Inverse-square law (intensity
  decreases 6dB each time we double the distance),
  spectral balance, reverberation

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Anatomy of the auditory system
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Interaural Level Difference (ILD)
From David McAlpine
Processed in Lateral Superior Olive
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ILD is greater for higher frequencies
Interaural level differences calculated for a
source in the horizontal plane. The source is at
an azimuth q of 10 (green curve), 45 (red), or
90 (blue) relative to straight ahead. The
calculations assume that the ears are at opposite
poles of a rigid sphere.
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Interaural Time Difference (ITD)
From David McAlpine
Processed in Medial Superior Olive
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Interaural time-difference - ITD
t R
t L
ITD t R - t L
Maximum c 0.6 ms
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Phase Ambiguity
This particular case is not a problem since max
ITD 0.6 ms But for frequencies above 1500 Hz it
IS a problem
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Phase Ambiguity
Both possible times are less than the maximum ITD
of 0.6 ms
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Raleighs Duplex theoryfor pure tones

• Low frequency pure tones (lt1500 Hz) localised by
  interaural time differences
• High frequency pure tones localised by intensity
  differences

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Raleighs Duplex theoryfor pure tones (2)

• 1. Low frequency tones (lt1500 Hz) localised by
  phase differences
• Very small interaural intensity difference for
  low-frequency tones.
• Phase locking present for low frequency tones
  (lt4kHz).

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Raleighs Duplex theoryfor pure tones (2)

• 1. Low frequency tones (lt1500 Hz) localised by
  phase differences
• Phase locking present for low frequency tones
  (lt4kHz).
• Limited by phase ambiguity Maximum ITD 670 µs
  corresponding to a whole cycle at 1500 Hz (the
  upper limit for binaural phase sensitivity)

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Raleighs Duplex theoryfor pure tones (3)

• High (and close low) frequency tones localised by
  intensity differences
• Shadow cast by head greater at high (20 dB at 6
  kHz) than low frequencies (3 dB at 500 Hz) i.e.
  head acts as a lowpass filter.
• For close sounds (lt1.5m) the inverse square law
  gives intensity differences between the ears for
  all frequencies. These differences vary with
  azimuth independently of any head-shadow effect.
  Beyond 1.5m the difference in level between the
  ears due to this factor is less than 1 dB.

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Pinna notch
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Head-Related Transfer Functions
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Precedence (or Haas) effect
Reflections arriving in the 0-40ms range are
linked to the direct sound source Longer delays
are detached from the direct sound source and
considered echoes
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Applications of the precedence effect
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Perception of distance

• More distant sounds are
• Quieter (inverse-square law)
• More muffled (high frequencies dont travel so
  well)
• More reverberant (direct is quieter relative to
  reflected)

For very close sounds, the difference in distance
from the source to the two ears becomes
significant -gt excess IID from inverse-square law.
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Perception of distance