5) Audition II

Cognitive Psychology 1 - Audition II

• Auditory Scene Analysis: The challenge of separating sounds from multiple sources in a complex auditory environment.

Learning Objectives

• Describe the cues used to determine the location of a sound source.
• Understand the basic principles of auditory grouping that help us to make sense of the auditory scene.

Sound Localisation

• Visual vs. Auditory: Visual cues locate objects in a scene within the retinal image, unlike audition where the activation point on the cochlea doesn't directly pinpoint location.

• Binaural Cues: Sound localisation relies on comparison of signals from both ears.

  • Interaural Time Differences (ITDs): The time difference between when a sound reaches each ear. This is crucial for determining azimuth (left-right). If a sound source is directly ahead, there's no ITD. If it's to one side, the nearer ear receives the sound first. ITDs are most useful for low frequencies or abrupt-onset sounds and the range varies depending on the speed of sound (typically 330 m/s), and the distance between ears. The maximum ITD in humans is typically around 600µs (0.6ms).
  • Interaural Level Differences (ILDs): The difference in sound pressure level between the two ears. High-frequency sounds are significantly affected by the head's acoustic shadow, resulting in a decrease in level at the far ear. This effect is minimal for low-frequency sounds.

• Monaural Cues:

  • Elevation: The pinna (outer ear) filters frequencies differently depending on sound source elevation. The relative intensity of different frequencies changes with the sound's source elevation.
  • Distance: Sound intensity diminishes with distance. Reverberation (echoes and reflections) also provide cues about distance, by combining multiple reflections.

Interaural Time Differences (ITDs)

• The relative time a sound arrives at each ear relates to its location in azimuth (left-right).

Interaural Level Differences (ILDs)

• Differences in sound pressure between ears depend on the sound source's location in azimuth. This relies on the acoustic shadow of the head for high-frequency sounds.

Physiology of Binaural Processing

• Processing of ITDs and ILDs begins in the brainstem's superior olivary complex.
  • Lateral Superior Olive (LSO): Sensitive to ILDs.
  • Medial Superior Olive (MSO): Sensitive to ITDs.

Strengths and Weaknesses of Binaural Cues

• Complementary information for azimuth, well-suited for low-frequency sounds but not elevation or distance.
• Cone of Confusion: A set of possible positions in azimuth that yield identical ITDs and ILDs.

Auditory Scene Analysis Strategies

• Spectral Grouping: Grouping sounds based on similar frequencies perceived as likely originating from the same source. Harmonicity plays a role, as does the common frequency changes perceived within a sound source.
• Sequential Grouping: Grouping sounds based on common characteristics over time, like timbre and constant or smoothly changing sounds perceived as continuous even when interrupted.
• Temporal Proximity: Sounds that occur in rapid succession tend to be grouped together.

Summary

• Binaural Cues (ITDs/ILDs): Crucial for azimuth localization, yet they do not provide information about elevation or distance.
• Monaural Cues: Used for elevation (pinna filtering) and distance (intensity/reverberation).
• Auditory Scene Analysis: Processes the mixture of sounds in a scene, segregating sounds that originate from different sources and grouping those from the same source.

Test your knowledge on auditory scene analysis and sound localisation. This quiz covers the principles of how we perceive sound sources in our environment, including binaural cues and interaural time differences. Explore the differences between visual and auditory localisation skills.