10.5 Binaural hearing and localization
8 min read•august 19, 2024
allows us to pinpoint sound sources in space using both ears. It relies on subtle differences in timing, level, and phase between our ears to determine a sound's direction and distance. This ability is crucial for navigating our acoustic environment.
help us determine a sound's position in 3D space. Interaural time and level differences are key for horizontal localization, while spectral cues from our outer ears aid vertical localization. Understanding these cues is essential for creating realistic spatial audio experiences.
Binaural hearing basics
- Binaural hearing involves the use of both ears to localize sound sources in space
- Enables humans and animals to determine the direction and distance of sound sources
- Plays a crucial role in spatial awareness and navigating complex acoustic environments
Differences between ears
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- Sound waves reach the two ears at slightly different times and levels due to the physical separation of the ears
- These differences provide cues for the brain to determine the location of the sound source
- Anatomical differences between the left and right ear (ear canal, pinna shape) can also contribute to binaural cues
Interaural time differences
- Interaural time differences (ITDs) refer to the difference in arrival time of a sound wave at the two ears
- ITDs are the primary cue for localizing low-frequency sounds (below ~1.5 kHz)
- The brain processes ITDs to determine the (horizontal angle) of the sound source
Interaural level differences
- Interaural level differences (ILDs) are the differences in sound pressure level between the two ears
- ILDs are caused by the acoustic shadow cast by the head, which attenuates high-frequency sounds (above ~1.5 kHz)
- The brain uses ILDs to localize high-frequency sounds in the horizontal plane
Interaural phase differences
- Interaural phase differences (IPDs) occur when the phase of a sound wave differs between the two ears
- IPDs are most effective for localizing sounds with wavelengths comparable to the size of the head
- The brain processes IPDs in conjunction with ITDs to improve localization accuracy
Sound localization cues
Localization in horizontal plane
- Localization in the horizontal plane (azimuth) is primarily based on ITDs and ILDs
- The combination of these cues allows the brain to determine the left-right position of a sound source
- The resolution of horizontal localization is best for sources directly in front or behind the listener
Localization in vertical plane
- Localization in the vertical plane () relies on spectral cues provided by the outer ear (pinna)
- The pinna's complex shape causes frequency-dependent reflections and resonances that vary with the elevation of the sound source
- The brain learns to associate these spectral patterns with specific elevations
Head-related transfer functions
- Head-related transfer functions (HRTFs) describe how the head, torso, and outer ears alter the frequency and time characteristics of sound waves
- HRTFs are unique to each individual and depend on the size and shape of their head and ears
- HRTFs can be measured or simulated to create realistic 3D audio experiences (virtual reality, gaming)
Cone of confusion
- The refers to a region in space where ITDs and ILDs are ambiguous, leading to localization errors
- It occurs when a sound source is equidistant from both ears (e.g., directly in front, behind, or above the listener)
- Resolving front-back confusions often requires head movements to introduce dynamic binaural cues
Binaural recording techniques
Dummy head recording
- involves using a mannequin head with microphones placed in the ear canals
- The mannequin head simulates the acoustic properties of a human head, capturing binaural cues
- Recordings made with a dummy head can create a realistic 3D audio experience when played back over headphones
In-ear microphones
- are small microphones placed inside the ear canals of a human or mannequin head
- They capture the sound pressure at the eardrum, including all the binaural cues introduced by the head and outer ear
- In-ear recordings provide a highly realistic and individualized binaural audio experience
Binaural synthesis
- involves creating binaural audio from mono or stereo recordings using HRTFs
- The original audio is convolved with HRTFs to simulate the spatial cues that would be present in a natural listening environment
- Binaural synthesis allows for the creation of immersive 3D audio without the need for specialized recording techniques
Limitations of binaural recording
- Binaural recordings are most effective when played back over headphones, as loudspeakers can introduce cross-talk between channels
- Individual differences in HRTFs can lead to variations in the perceived spatial quality of binaural recordings
- Head movements during playback can disrupt the binaural illusion, as the spatial cues remain fixed relative to the head
Spatial hearing and architecture
Room acoustics impact on localization
- Room acoustics can significantly influence the ability to localize sound sources in space
- Reflections from walls, ceiling, and floor can interfere with direct sound, affecting binaural cues
- The time and early reflection pattern of a room can enhance or degrade localization accuracy
Reverberation effects on localization
- Reverberation can make it more difficult to localize sound sources, especially in highly reverberant spaces (churches, concert halls)
- The direction and timing of early reflections can provide additional cues for localization
- Excessive reverberation can mask binaural cues and lead to a diffuse, enveloping sound field
Precedence effect in rooms
- The (Haas effect) refers to the dominance of the first-arriving sound in determining localization
- In rooms, the direct sound from a source is followed by early reflections and reverberation
- The brain gives more weight to the localization cues provided by the direct sound and early reflections, suppressing the effect of later reflections
Designing spaces for optimal localization
- Architectural design can be optimized to enhance sound localization and spatial awareness
- Controlling the reverberation time and early reflection pattern can improve localization accuracy
- The use of sound-absorbing materials and diffusers can help reduce the negative effects of excessive reverberation on localization
Binaural technology applications
Virtual reality audio
- Binaural audio is a key component of immersive virtual reality experiences
- Head-tracked binaural rendering allows for dynamic, real-time updating of spatial cues based on the user's head movements
- Binaural audio enhances the sense of presence and realism in virtual environments (gaming, simulations, virtual concerts)
Gaming and immersive audio
- Binaural audio is increasingly used in gaming to create more realistic and engaging sound experiences
- Game engines can simulate the acoustic properties of virtual environments, providing dynamic binaural cues based on the player's actions
- Immersive audio in gaming can improve situational awareness, spatial orientation, and overall gameplay experience
Telepresence and remote collaboration
- Binaural audio can enhance telepresence and remote collaboration by providing a sense of spatial presence
- Capturing and reproducing binaural cues can create the illusion of being in the same physical space as remote participants
- Binaural audio can improve communication and understanding in virtual meetings, conferences, and remote training sessions
Assistive listening devices
- Binaural hearing aids and assistive listening devices can help individuals with hearing impairments better localize sounds
- These devices can preserve and enhance binaural cues, improving spatial awareness and speech understanding in noisy environments
- Binaural noise reduction algorithms can selectively attenuate background noise while preserving the spatial cues of the desired signal
Psychoacoustics of spatial hearing
Minimum audible angle
- The (MAA) is the smallest angular separation between two sound sources that can be reliably discriminated
- MAA varies with the frequency of the sound and the location of the sources relative to the listener
- The human auditory system is most sensitive to changes in the horizontal plane, with MAAs as small as 1-2 degrees for sources near the midline
Localization blur
- refers to the inherent uncertainty in the perceived location of a sound source
- It is influenced by factors such as the frequency content of the sound, the presence of background noise, and the listener's familiarity with the sound
- Localization blur is typically larger for high-frequency sounds and in the vertical plane compared to the horizontal plane
Localization in noise
- The presence of background noise can degrade the ability to localize sound sources
- Noise can mask binaural cues, particularly ITDs and ILDs, making it more difficult to determine the direction of a sound
- The effect of noise on localization depends on the signal-to-noise ratio, the spectral characteristics of the noise, and the listener's age and hearing status
Localization for hearing impaired
- Hearing impairments can significantly affect the ability to localize sounds in space
- Individuals with hearing loss may have reduced sensitivity to binaural cues, particularly ITDs and ILDs
- Asymmetric hearing loss can lead to an imbalance in binaural cues, causing localization errors and difficulty understanding speech in noisy environments
Binaural hearing disorders
Unilateral hearing loss
- (UHL) refers to hearing impairment in one ear, while the other ear has normal hearing
- UHL can cause difficulties in sound localization, as binaural cues are disrupted
- Individuals with UHL may struggle to understand speech in noisy environments and have reduced spatial awareness
Central auditory processing disorder
- (CAPD) is a condition where the brain has difficulty processing auditory information, despite normal hearing sensitivity
- CAPD can affect the ability to localize sounds, as the brain may not effectively integrate binaural cues
- Individuals with CAPD may have trouble understanding speech in noisy environments and following complex auditory instructions
Auditory neglect and extinction
- is a condition where an individual with brain damage (often due to a stroke) fails to respond to sounds on the side opposite the brain lesion
- occurs when an individual can detect sounds on either side alone but fails to respond to sounds on one side when presented with stimuli on both sides simultaneously
- These conditions can severely impact sound localization and spatial awareness
Evaluation and treatment approaches
- Evaluation of binaural hearing disorders involves a combination of audiological tests, spatial hearing assessments, and neuropsychological evaluations
- Treatment approaches may include the use of hearing aids, assistive listening devices, and auditory training programs
- Auditory training can help individuals with binaural hearing disorders better utilize the available cues for sound localization and speech understanding in challenging acoustic environments
Key Terms to Review (21)
Auditory extinction: Auditory extinction is the phenomenon where a sound is perceived as diminished or entirely absent when presented simultaneously with other competing sounds. This effect is closely tied to how the brain processes auditory information and can influence spatial awareness and sound localization in complex acoustic environments.
Auditory neglect: Auditory neglect is a phenomenon where individuals fail to notice or respond to sounds coming from one side of their auditory field, often due to brain injury or damage affecting spatial attention. This condition highlights how our perception of sound is not just about hearing, but also about how our brain processes and prioritizes auditory information, which can significantly impact sound localization and awareness in a binaural context.
Azimuth: Azimuth is the angle measured in the horizontal plane from a reference direction, typically north, to a point of interest, allowing for precise localization of sound sources. This concept is crucial in understanding how we perceive the direction of sounds and plays a significant role in binaural hearing, which helps us determine the location of sounds in our environment.
Binaural hearing: Binaural hearing refers to the ability of humans to perceive sound using both ears, which helps in determining the direction and distance of sounds in our environment. This dual ear input enhances our auditory experience, allowing us to localize sound sources more accurately and distinguish between different pitches and frequencies more effectively, significantly impacting our overall hearing capabilities.
Binaural synthesis: Binaural synthesis is a technique used to recreate 3D sound experiences by simulating how human ears perceive sound from different locations. This process utilizes two microphones placed in a way that mimics the spacing of human ears, capturing sound with all its directional cues. By processing these recordings, binaural synthesis allows listeners to experience a more immersive auditory environment through headphones, enhancing spatial perception and localization of sounds.
Central auditory processing disorder: Central auditory processing disorder (CAPD) is a condition that affects how the brain processes auditory information, leading to difficulties in understanding and interpreting sounds, particularly in complex listening environments. Individuals with CAPD may struggle with tasks like following conversations or localizing sounds, which can significantly impact communication and learning. This disorder highlights the importance of binaural hearing and localization, as it involves the integration of auditory signals from both ears for effective sound perception and spatial awareness.
Cone of confusion: The cone of confusion refers to a region in three-dimensional space where sound sources are perceived to be located at the same angle, making it difficult for listeners to accurately identify the source of the sound. This phenomenon arises due to the way that binaural hearing processes sound waves arriving at each ear, leading to ambiguities in localization, particularly for sounds coming from directly in front or behind a listener.
Dummy head recording: Dummy head recording is a technique used to capture audio that simulates human binaural hearing by utilizing a mannequin head equipped with microphones in the ears. This method replicates the way sounds interact with the human head and ears, allowing for a realistic spatial audio experience. It plays a crucial role in understanding how we perceive direction and distance of sounds in our environment.
Elevation: Elevation refers to the vertical positioning of a sound source in relation to a listener's ears, playing a crucial role in how sounds are perceived in three-dimensional space. Understanding elevation helps in determining where sounds originate, aiding in spatial awareness and auditory localization, which are essential for effective binaural hearing.
Head-related transfer function: The head-related transfer function (HRTF) is a mathematical representation that describes how sound waves from a specific point in space interact with the human head and ears, affecting the sound's frequency and phase. This function is crucial for binaural hearing as it helps the brain determine the location of sounds in three-dimensional space by analyzing differences in the sound signals received at each ear. HRTFs are influenced by factors such as the shape of the outer ear, the position of the listener's head, and environmental acoustics.
In-ear microphones: In-ear microphones are small microphones that fit inside the ear canal, designed for capturing audio in close proximity to the sound source. These microphones are often used in applications like live performances, broadcasting, and recording because they can provide high-quality sound while reducing background noise. Their placement allows for better spatial awareness, making them particularly effective for binaural hearing and localization.
Interaural level difference: Interaural level difference (ILD) refers to the difference in the sound pressure level reaching each ear due to the positioning of the sound source relative to the listener's head. This acoustic phenomenon is essential for binaural hearing, allowing individuals to localize sounds in their environment by analyzing the intensity of sound waves that arrive at each ear. The brain interprets these differences to discern directionality, contributing significantly to spatial awareness and the ability to determine where a sound originates.
Interaural phase difference: Interaural phase difference refers to the difference in the phase of a sound wave that reaches each ear, which is crucial for localizing sound sources. This difference allows the auditory system to determine the direction from which a sound is coming, as sounds arriving at one ear slightly earlier than the other create a detectable phase shift. This phenomenon is a key element in binaural hearing, which enhances our ability to pinpoint the location of sounds in our environment.
Interaural time difference: Interaural time difference refers to the difference in the time it takes for a sound to reach each ear, which is a crucial cue for locating the direction of sounds in the environment. This phenomenon occurs because sounds coming from one side of a person will reach the closer ear slightly earlier than the farther ear, helping the brain to determine the spatial origin of the sound. The brain uses this timing information, along with other auditory cues, to create a sense of where sounds are coming from, which is essential for effective communication and environmental awareness.
Localization blur: Localization blur refers to the reduced accuracy in determining the position of a sound source due to various environmental factors and the limitations of human auditory perception. It occurs when the auditory system struggles to identify the exact location of a sound, often influenced by factors such as distance, reflections, and interference from other sounds. This concept highlights the challenges in spatial hearing and how our ability to pinpoint sounds can be affected in complex auditory environments.
Localization in noise: Localization in noise refers to the ability to determine the origin of a sound source amidst background noise. This process relies on auditory cues and the brain's interpretation of these signals, enabling listeners to discern where sounds are coming from even when competing sounds are present. Effective localization is critical for navigating environments and understanding speech, particularly in complex acoustic situations.
Minimum audible angle: The minimum audible angle is the smallest angular separation between two sound sources that can be perceived as distinct by a listener. This ability to localize sound is crucial for binaural hearing, as it helps individuals determine the direction and distance of sounds in their environment, allowing for better spatial awareness and orientation.
Precedence effect: The precedence effect is a perceptual phenomenon in auditory processing where the brain prioritizes the first sound it hears in a sequence, allowing for better localization of sound sources. This effect occurs because our auditory system uses timing and intensity differences between sounds arriving at each ear to determine their origin, helping us to focus on specific sounds in complex acoustic environments. It plays a crucial role in how we perceive and localize sounds, especially in environments with multiple sound sources.
Reverberation: Reverberation is the persistence of sound in a particular space after the original sound is produced, resulting from multiple reflections off surfaces like walls, ceilings, and floors. It plays a crucial role in shaping the acoustical characteristics of environments, affecting how music and speech are perceived, and can enhance or muddle the clarity of sound depending on its duration and intensity.
Sound localization cues: Sound localization cues are auditory signals that help an organism determine the direction and distance of a sound source in its environment. These cues primarily arise from the differences in time and intensity at which sound reaches each ear, allowing for accurate spatial awareness of sounds. By processing these cues, the brain can discern where a sound is coming from, which is vital for survival and interaction within one's surroundings.
Unilateral hearing loss: Unilateral hearing loss refers to a condition where an individual has hearing impairment in one ear while the other ear has normal hearing. This can significantly impact how sound is perceived and localized, as binaural hearing relies on input from both ears to determine the direction and distance of sounds. With unilateral hearing loss, the brain may struggle to accurately interpret spatial cues, leading to challenges in understanding speech and detecting sounds in the environment.