Notes

• Auditory events can be perceived in all directions from observer.
• Auditory events can be localized internally or externally at various distances.
• Audition also supports motion perception
  • Change in direction.
  • Doppler shift (A source coming towards you is perceived as having an higher pitch than a source getting away from you).

Cocktail Party Effect

• In environments with many sound sources it is easier to process auditory streams if they are separated spatially.
• Spatial sound techniques can help in sound discrimination, detection and speech comprehension in busy immersive environments.

Spatial Auditory Cues

Two basic types of head-centric direction cues

• Binaural cues
• Spectral cues (they are the cues that change in frequency depending on where the sound is coming from).

Binaural Directional Cues

• When a source is located eccentrically it is closer to one ear than the other
  • The sound arrives later and weaker to one ear, Interaural Time Difference (ITD)
  • The head "shadow" also weakens the sound that arrives from the opposite ear, Interaural Intensity Difference (IID)
• Binaural cues are robust but ambiguous.

Interaural Time Differences (ITD)

• ITD increase with directional deviation from the median plane. It is about 600μs for a source located directly to one side.
• Humans are sensitive to as little as 10μs as ITD. Sensitivity decreases with ITD.
• For a given ITD, phase difference is a linear function of frequency.
• For pure tones, phase based ITD is ambiguous (imagine listening to a sinusoidal wave with ITD between the two ear, it is difficult to "match the peaks" because on one ear they could either arrive early or late but it's difficult to tell which of the two).
• At low to moderate frequencies phase difference can be detected. At high frequencies we can use ITD in signal envelope.
• ITD cues appear to be integrated over a window of 100-200ms (binaural sluggishness).

Interaural Intensity Differences (IID)

• With lateral sources head shadow reduces intensity at opposite ear.
• Effect of head shadow is most pronounced for high frequencies.
• IID cues are most effective above about 2000Hz.
• IID of less than 1dB are detectable. At 4000Hz a source located at 90 degrees gives about 30dB IID.

Ambiguity and Lateralization

• These binaural cues are ambiguous. The same ITD/IID can arise from sources anywhere along a "cone of confusion".
• Spectral cues and changes in ITD/IID with observer/object motion can help disambiguate.
• When directional cues are used in headphone systems, sounds are lateralized left versus right but seem to emanate from inside the head (not localized).
• Also for near sources (less than 1m) there is significant IID due to differences in distance to each ear even at lower frequencies.
• Intersection of these "near field" IID curves with cones of confusion constrains them to toroids of confusion.

Spectral Cues

• Pinnae or outer ears and head shadow each ear and create frequency dependent attenuation of sounds that depend on direction of source.
• Pinnae are relatively small, spectral cues are effective predominately at higher frequency (i.e., above 6000 Hz).
• Direction estimation requires separation of spectrum of sound source from spectral shaping by the pinnae.
• Shape of the pinnae shows large individual differences which is reflected in differences in spectral cues.