Chapter 7- Sound Localization Flashcards
Which does more processing to form a map of space, the visual or auditory system?
Auditory system takes in 360 degrees input and also has to analyze frequency information
Visual system only has to combine 2 inputs.
What is diotic? What is dichotic?
Diotic= same signal to both ears
Dichotic= different signals across ears
What assumptions are made with normal hearing?
Left/right ears are copies:
- Same frequency response
- Same inner-ear filtering
- Same loudness growth, etc.
Why are 2 ears helpful?
- Sound localization
- Understanding speech in noisy environments
How can you calculate the interaural phase difference?
ITD= IPD/f
What is the minimum audible angle (MAA)?
Smallest detectable change in azimuthal positions
Best in front, worse on the side
What is the duplex theory of sound localization?
ILDs
- Used mostly >1.5 kHz
- Heads produce negligible ILDs <1.5 kHz
ITDs
- Best at low frequencies <1.5 kHz
Why are ITDs not good at frequencies above 1.5 kHz?
- Rate limitations of AN firing
- Human head interferes with the traveling wave
- Head width approximately 760 us (760 us*1500 Hz= 1 cycle)
- Start getting slip cycles
What are slip cycles?
ITDs greater than half a period are lateralized to the wrong side.
The first stimulus is directly one period out of phase; listener cannot tell the sound has moved.
Two tones that have a certain period
- If you have 2 tones, and they’re in phase, there’s ) ITD
What is centrality?
Neurons that are firing near the midline seem to matter more than those near the edge
What is straightness?
Binaural system checks across frequency to obtain information about where the sound is
- ITDs don’t change much with frequency
- Can disambiguate slip cycle issues
- For complex tones, you can probably use high frequency information as long as you have auditory channels that provide your system that information
What is adaptation?
Natural acoustic environments contain many transients
- Information is concentrated near transients
Adaptation removes redundant information
- Allows for efficient neural encoding
How is vertical sound localization achieved with a pinna?
Pinna acts like an acoustic antenna
- Cavities amplify some frequencies
Low frequencies (<4 kHz)
- Collect energy
- Not directionally dependent
High frequencies (4-16 kHz) - Path differences occur
What are HRTFs?
Head-Related Transfer Functions (HRTFs)
- Composed of ITDs, ILDs, and spectral cues
What happens when you remove characteristic filtering from HRTFs?
Sound can become internalized and harder to localize
What is the monaural head shadow?
Better SNR in one ear (~6 dB)
Accessible to NH, HI, and CI users
What is binaural summation/redundancy?
2 ears louder than 1 (~1 dB)
What is binaural unmasking?
Added advantage from neural processing and auditory stream segregation
- NH listeners good at this
What are the caveats to the duplex theory?
ITDs
• Can be processed >1500 Hz if there are slow envelope modulations, like if you have a SAM or transposed tone
• Can be processed >1500 Hz if stimulus has energy across several auditory filters (NBN)
• Band must have the same ITD (i.e., must have same straightness across frequencies)
ILDs
• Can be processed <1500 Hz if you artificially apply them, as would occur for headphones or if the sound source were closed to the head (inverse square law would produce ILDs if close enough)
How does the human head interfere with the traveling wave?
Stimulus < 1500 Hz
o Wavelength > maximum time delay between ears
o Auditory system calculates phase difference to localize sound
Stimulus > 1500 Hz
o Wavelength < maximum time delay
o Head shadow effect is produced
o Auditory system uses ILDs to localize
Stimulus is in region 1000-1500 Hz
o Wavelength is similar to time delay between ears, which reduces phase differences
What is the precedence effect?
Precedence effect explains how a direct sound and its reflection interact to affect the listener’s perception of where that sound is located.
∂12= 0 ms
• 2 sources slightly lower than 1 source data
• Due to summing localization
∂12 ≤ 8 ms
• Source 1 (lead) easy to distinguish ITD (localization dominance)
• Source 2 (lag) suppressed
∂12 ≥ 8 ms
• Source 1 (lead) lower than source 2 (lag)
• Due to temporal-order confusion
What is the effect of BW on BMLDs?
Performance increases with wideband noise
Performance decreases with narrowband noise
How does hearing loss affect vertical plane sound localization?
Otte et al. 2013
• As subjects age, they lose their high frequencies and their pinnae get bigger
• If pinna get bigger, spectral peaks/notches get shifted downwards
• Maybe ears grow bigger to compensate for age related hearing loss
• Measured HRTFs
• OHI subjects were better at lower-frequency discrimination, but the pinna size does not fully compensate for high-frequency age-related hearing loss
