Spatial Hearing and Sound Localization

Introduction to Spatial Hearing

• Spatial hearing refers to how we localize sounds in space and is crucial for everyday auditory perception.

Key Definitions

• Binaural: Refers to using both ears (bi- for two).

• Monaural: Refers to using one ear (mono- for one).

• Unilateral Loss: Hearing loss affecting only one ear, resulting in difficulty localized sounds.

Importance of Two Ears in Sound Localization

• Sound localization requires input from both ears, as they help the brain determine the location of sounds by analyzing:

  • Timing Differences: When a sound arrives at one ear before the other.

  • Level Differences: The difference in loudness between the ears due to the head creating a sound shadow.

Introduction to Key Concepts

• Sound Localization: The ability to identify the position of a sound source in space.

• Lateralization: The perception of sound movement between ears, typically experienced through headphones.

• Azimuth: Refers to the angle of a sound source on the horizontal plane, measured in degrees (with 0 degrees directly in front).

Sound Localization Mechanisms

1. Interaural Timing Differences (ITDs)

   • The time delay experienced when sound reaches one ear before the other.

   • Most effective for low to mid-range frequencies.

   • A critical cue for the brain to determine sound direction.

2. Interaural Level Differences (ILDs)

   • The difference in sound intensity as it reaches each ear, primarily impactful at high frequencies.

   • Created by sound shadows cast by the head.

Binaural Hearing Effects

• Binaural Summation: At superthreshold levels (louder than the softest detectable sound), binaural hearing can yield a sound boost of up to 6 dB when the same sound reaches both ears.

• Threshold Levels: When sounds are at the threshold of hearing, the binaural benefit drops to approximately 3 dB or less, especially if hearing varies between ears.

Sound Localization in the Azimuth Plane

• In azimuth, ITDs and ILDs help the brain triangulate the position of sounds.

• ITD Calculation: Time sound takes to travel to the nearest ear compared to the further ear.

• ILD Calculation: The ear that is further from the sound source receives a less intense sound due to the head blocking some sound energy.

Understanding Frequency Impact

• Higher frequencies produce greater ILDs because they are more likely to be affected by the sound shadow.

• Lower frequencies tend to wrap around the head, resulting in more reliance on ITDs.

• Essential frequency cutoff: The average human head begins to block frequencies around 1944 Hz.

Combining ITDs and ILDs

• Both ITDs and ILDs generally occur simultaneously in real-world sound environments.

• Unlike isolated pure tones, complex sounds consist of multiple frequencies, enabling the use of both localization cues.

Perception of Distance

• Distance perception is determined by:

  • The loudness of sounds.

  • Reflections and reverberations.

Challenges with Pure Tones

• Identifying the precise location of pure tones (single frequency sounds) can be difficult, particularly around the mid-frequency range of 1000-2000 Hz, where both ITDs and ILDs provide minimal information.

Conclusion

• Understanding spatial hearing and sound localization is essential for auditory rehabilitation and enhancing communication in environments with background noise.

• Continued exploration of auditory pathways and the effects of hearing impairments will be covered in forthcoming lectures.