Lecture 3 Notes: Spatial Hearing

Spatial Hearing: Lecture 3

Sven Mattys

Background Readings: Plack - Chapter 9

Why Locate Sound Sources?

1. Information: Sound location is crucial information in itself, aiding survival and environmental awareness.

2. Orientation: The location of a sound source can immediately orient visual attention towards it, facilitating quicker responses to potential threats or points of interest.

3. Separation: Sound localization assists in segregating different auditory streams, allowing us to focus on individual sounds even in complex acoustic environments.

Lecture Objectives

1. Horizontal Plane Localization: To understand the cues listeners utilize to localize sounds in the horizontal plane.

   • Inter-aural Timing Differences (ITD): differences in arrival time of sound between the ears.

   • Inter-aural Level Differences (ILD): differences in intensity of sound between the ears.

2. Front vs. Back Discrimination: Understanding mechanisms that help listeners distinguish whether a sound comes from the front or the back, addressing potential ambiguities.

3. Elevation Discrimination: Exploring how listeners perceive sounds coming from above and below, which involves different cues compared to horizontal localization.

4. Precedence Effect: Comprehending the principle of the Precedence Effect and its significant role in localization within enclosed spaces, managing echoes, and reinforcing directional perception.

Coordinates of Spatial Hearing

• Azimuth: The angle representing the sound source’s horizontal position, measured relative to the median plane (midline of the head), indicating left or right.

• Elevation: The angle indicating the sound source's vertical position, measured relative to the horizontal plane, describing location above or below.

Minimum Audible Angle (MAA)

• MAA is the smallest angular change that can be detected by a listener between two sound sources; a measure of spatial hearing acuity.

• MAA is typically $1°$ for tones presented directly in front at low frequencies, showcasing high sensitivity in this condition.

• MAA increases around $1500$ Hz but then improves at higher frequencies, though not as acutely as at low frequencies, indicating frequency-dependent spatial acuity.

• MAA is generally larger for sounds originating from non-frontal locations, demonstrating that spatial acuity varies with the sound source's position.

Cues for Sound Localization

• Interaural Timing Difference (ITD):

  • When a sound originates from the left, it reaches the left ear slightly before the right ear. This temporal disparity aids in determining the sound's horizontal position.

• Interaural Level Difference (ILD):

  • When a sound comes from the left, the head acts as an obstacle, creating a 'head shadow' that attenuates the sound reaching the right ear. This intensity difference helps localize the sound source.

Interaural Timing Difference (ITD)

• When a sound comes from one side at $+90°$ azimuth, the ITD is about $0.65$ milliseconds ($650$ μs).

• Smallest distinguishable ITD: a mere $10$ μs corresponds to approximately $1°$ deviation from straight ahead, illustrating remarkable sensitivity to timing differences.

Interaural Level Difference (ILD)

• When a sound comes from $+90°$ azimuth, the ILD measures around $20$ dB at $6000$ Hz but diminishes to nearly $0$ dB at $200$ Hz, showing frequency dependence due to head shadow effects.

• Smallest distinguishable ILD: around $1$ dB across various frequencies, indicating a high sensitivity to level differences.

Frequency Range of ITDs and ILDs

• ILDs are effectively used at higher frequencies where the head shadow is more pronounced.

ITD Ambiguity

• ITDs are unambiguous at low frequencies such as $385$ Hz (Period = $1/385$ s = $2600$ μs), providing clear localization cues.

• ITDs become ambiguous around $770$ Hz (Period = $1/770$ s = $1300$ μs), leading to potential localization errors.

• ITDs can be misleading above $770$ Hz as the phase differences caused by ITDs may wrap around, confusing the auditory system (e.g., $1155$ Hz, Period = $1/1155$ s = $866$ μs).

• ITDs are less likely to be ambiguous for modulated tones because the auditory system can track changing patterns and envelope information, effectively extending the usable range of ITDs.

Interim Summary

• Pure Tones:

  • ITDs are utilized at low frequencies ($<750$ Hz).

  • ILDs become more effective at higher frequencies ($>1500$ Hz).

• Frequency-Modulated (Complex) Tones:

  • ITDs offer more reliable cues across a broader frequency spectrum.

• ITDs are generally the primary cue for localizing most natural sounds due to their robustness and reliability at lower frequencies.

Neural Mechanisms for ITD Calculation

• Superior Olive: plays a crucial role in analyzing sound source location, acting early in the ascending auditory pathway because it requires very precise timing analysis.

Measuring ITDs: Delay Lines and Coincidence Detectors

• Delay lines (axons) transmit auditory signals from each ear, preserving the temporal information critical for ITD processing.

• Neural coincidence detectors fire when signals from both ears arrive simultaneously, allowing precise measurement of ITDs.

• Barn owls utilize these delay lines and coincidence detectors extensively for accurate localization in the azimuth.

• Terrestrial mammals, including humans, likely employ a similar neural mechanism, although the exact implementation may vary.