11.4 Spatial hearing and sound localization

Sound localization is crucial for our spatial awareness. Binaural cues, like interaural time and level differences, help us pinpoint sound sources. The duplex theory explains how our brains use these cues across different frequency ranges for accurate localization.

Spectral cues from our outer ear shape add another layer to sound localization. These unique patterns, captured by Head-Related Transfer Functions, help resolve ambiguities like front-back confusion. However, localization has limitations, including minimum audible angles and distance perception challenges.

Binaural Cues and Sound Localization

Binaural cues for localization

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• Interaural Time Difference (ITD)
  • Time difference of sound arrival between ears enables localization
  • Effective for low-frequency sounds (below 1500 Hz) due to wavelength size
  • Calculated as $ITD = \frac{r\theta}{c}$, r = head radius, θ = azimuth angle, c = speed of sound
• Interaural Level Difference (ILD)
  • Intensity difference between ears aids in sound source identification
  • Effective for high-frequency sounds (above 1500 Hz) due to head shadow effect
  • Increases with frequency and source angle
• Duplex theory
  • Combines ITD and ILD for accurate localization across frequency ranges
  • Low frequencies rely on ITD, high frequencies on ILD
• Neural processing
  • Auditory cortex integrates binaural cues for spatial perception
  • Specialized neurons detect interaural differences (superior olivary complex)

Cone of confusion implications

• Cone of confusion
  • Surface where ITD and ILD remain constant creates localization ambiguity
  • Conical surface extends from each ear, symmetrical around interaural axis
• Ambiguities in localization
  • Front-back confusion occurs when sound sources on cone have identical binaural cues
  • Up-down confusion arises from similar ITD and ILD values
• Resolution methods
  • Head movements break symmetry and provide dynamic localization cues
  • Spectral cues from pinna offer additional spatial information
• Implications
  • Challenges in virtual audio systems require advanced HRTF modeling
  • Importance of dynamic cues in natural listening environments for accurate perception

Spectral Cues and Localization Limitations

Spectral cues from outer ear

• Pinna's role in sound localization
  • Direction-dependent filter modifies spectral content of incoming sounds
  • Complex folds create unique spectral patterns for different source locations
• Head-Related Transfer Function (HRTF)
  • Acoustic transfer function from source to ear canal captures individual anatomy
  • Includes effects of head, torso, and pinna on sound propagation
• Elevation cues
  • Pinna notches create frequency-specific attenuation (5-10 kHz range)
  • Spectral peaks from resonances in ear canal and concha enhance vertical localization
• Monaural vs. binaural cues
  • Spectral cues primarily monaural, useful for single-ear localization
  • Complement binaural cues for comprehensive 3D sound localization

Limitations of sound localization

• Minimum Audible Angle (MAA)
  • Smallest detectable change in sound source position varies with stimulus
  • Typically 1-2° for frontal sources, larger for lateral positions
• Distance perception limitations
  • Accuracy decreases with increasing distance beyond a few meters
  • Reliance on intensity and reverberation cues for distance estimation
• Factors affecting localization accuracy
  • Source characteristics: frequency content, duration, onset/offset
  • Environmental factors: reverberation, background noise, multiple sources
  • Individual differences: ear shape, hearing sensitivity, experience
• Localization in complex environments
  • Cocktail party effect demonstrates selective attention in noisy settings
  • Precedence effect in reverberant spaces suppresses echoes for clear localization
• Technological challenges
  • Virtual and augmented reality audio requires precise HRTF reproduction
  • Hearing aid design for spatial awareness must balance amplification and localization