LE 12 Localization of Sound

Localization of Sound

  • Challenges in Auditory Localization

    • The auditory system struggles with sound localization due to the inability to preserve spatial information as seen in vision.

    • In vision, light reflects off objects in straight lines preserving object relationships on the retina; in contrast, sounds combine into a single waveform received by the ear.

    • Each sound source must be separated by the auditory system to differentiate between them (e.g., identifying a person speaking or a car horn).

  • Sound Positioning Concepts

    • Sounds can be thought about along two dimensions:

    • Azimuth: Position along the horizon.

    • Elevation: Vertical position of sound.

    • Each sound source is mapped onto an imaginary sphere surrounding the head.

  • Behavioral Data on Sound Localization

    • Participants estimate sound localization in a test arena with different sound positions.

    • Results show:

    • Greater accuracy for sounds coming from the front (blue dots closer to red squares).

    • Reduced accuracy for sounds from the side or behind.

    • Individuals can orient towards sounds for better resolution.

Cues for Sound Localization

  • Main Cues

    • There are three main cues utilized for sound localization:

    1. Interaural Time Differences (ITD): The timing difference between sounds arriving at the two ears.

    2. Interaural Level Differences (ILD): The intensity or loudness difference between sounds reaching each ear.

    3. Head-Related Transfer Function (HRTF): The specific way sound modifies as it interacts with the head and ears.

  • Binaural Cues

    • ITD and ILD require two ears:

    • Interaural Time Difference: Depending on the sound's location, waves will reach one ear before the other.

    • The maximum ITD occurs for sounds located 90 degrees to the side, approximately 0.6 milliseconds for an average headwidth.

    • Interaural Level Difference: Sounds are affected by the head’s size and density; reflections and absorption alter sounds, resulting in varying levels of intensity entering each ear based on the sound's position.

  • Monaural Cues

    • HRTF provides information relevant to each ear, independent from the other.

    • Everyone’s ears have different HRTFs due to anatomical differences, which allow the brain to determine elevation.

Head-Related Transfer Function (HRTF)

  • Definition of HRTF

    • The HRTF is how sound changes in amplitude and frequency as it interacts with a person’s head and ears.

    • The HRTF varies between individuals and across different sound frequencies, leading to perceptual differences in sound quality.

  • Mechanics of HRTF

    • For instance, cupping hands behind ears alters incoming sounds by changing reflections and amplifications.

    • Manipulating the shape of the ear affects the frequency spectrum of sounds, leading to variations based on sound position.

Sound Localization Mechanisms

  • Interaural Time Differences (ITD)

    • ITDs inform on sound location via timing differences:

    • Absolute time differences are small but significant enough (e.g., from 0 to approximately 0.6 milliseconds).

    • As conditions change (left vs right), the ITD shifts systematically.

  • Interaural Level Differences (ILD)

    • Sounds are perceived to be louder from the side they are coming due to reflections & sound absorption by the head.

    • Higher frequency sounds are better at creating ILDs; low frequency sounds usually do not provide useful localization information.

  • The Cone of Confusion

    • Occurs when ITD and ILD cues yield similar values for different spatial locations, leading to difficulty in sound source localization.

    • Resolved by extracting timing information through individual frequency channels (phase locking).

The Role of the Nervous System

  • Neural Processing of Sound Localization

    • Auditory information is processed first at the brainstem, notably in the cochlear nucleus, where auditory nerve cell bodies reside.

    • The signal then projects to the superior olive, where ITDs and ILDs are compared via circuits, including coincidence detection mechanisms called Jeffress coincidence detectors.

  • Jeffress Coincidence Detector

    • A proposed mechanism for processing ITDs; sensitive neurons located in neural arrays respond to phase-locked action potentials from sounds reaching each ear.

Auditory Scene Analysis

  • Auditory Scene and Analysis

    • Defined as the array of sound sources in the environment and their subsequent analysis into separate perceptions.

    • The problem of auditory scene analysis involves distinguishing between overlapping sounds (e.g. conversations at a party).

    • This relates to the cocktail party problem where one must detect and allocate focus on specific sounds amidst a cacophony.

  • Cues for Auditory Scene Analysis

    • Sounds can be grouped based on:

    1. Proximity in space (sounds from similar locations).

    2. Proximity in time (sounds occurring in rapid succession likely from the same source).

    3. Similarity in perceptual qualities (pitch, timbre).

  • Examples in Music and Environment

    • Auditory perception, such as in music, utilizes similarities in pitch and proximity to create melodies perceived as separate streams despite being played on the same instrument.

    • Composers like Bach utilized principles of auditory scene analysis to create distinct musical lines.

Integration of Auditory and Visual Information

  • Ventriloquism Effect

    • Highlights how visual cues can dominate auditory ones, leading to the perception that a sound originates from a seen object rather than its true source.

    • In experiments, auditory and visual signals can influence the perception of events (e.g., perceiving one vs. two flashes based on auditory cues).

  • Environmental Context

    • The auditory system's integration with visual information aids in creating a coherent representation of complex auditory scenes.

Conclusion and Exam Preparation

  • Key Takeaways

    • Localization of sound involves complex auditory processing using binaural cues (ITD, ILD) and monaural cues (HRTF).

    • Understanding auditory scene analysis is crucial for effective communication in complex auditory environments.

  • Upcoming Review Session

    • An exam will be held in a week; a review session is scheduled for the Tuesday before the exam. Students encouraged to attend for clarification on study material.