Chapter 7- Sound Localization

Question 1

Which does more processing to form a map of space, the visual or auditory system?

Answer 1

Auditory system takes in 360 degrees input and also has to analyze frequency information

Visual system only has to combine 2 inputs.

Question 2

What is diotic? What is dichotic?

Answer 2

Diotic= same signal to both ears

Dichotic= different signals across ears

Question 3

What assumptions are made with normal hearing?

Answer 3

Left/right ears are copies:

• Same frequency response
• Same inner-ear filtering
• Same loudness growth, etc.

Question 4

Why are 2 ears helpful?

Answer 4

• Sound localization

- Understanding speech in noisy environments

Question 5

How can you calculate the interaural phase difference?

Answer 5

ITD= IPD/f

Question 6

What is the minimum audible angle (MAA)?

Answer 6

Smallest detectable change in azimuthal positions

Best in front, worse on the side

Question 7

What is the duplex theory of sound localization?

Answer 7

ILDs

• Used mostly >1.5 kHz
• Heads produce negligible ILDs <1.5 kHz

ITDs
- Best at low frequencies <1.5 kHz

Question 8

Why are ITDs not good at frequencies above 1.5 kHz?

Answer 8

• 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

Question 9

What are slip cycles?

Answer 9

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

Question 10

What is centrality?

Answer 10

Neurons that are firing near the midline seem to matter more than those near the edge

Question 11

What is straightness?

Answer 11

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

Question 12

What is adaptation?

Answer 12

Natural acoustic environments contain many transients

• Information is concentrated near transients

Adaptation removes redundant information
- Allows for efficient neural encoding

Question 13

How is vertical sound localization achieved with a pinna?

Answer 13

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

Question 14

What are HRTFs?

Answer 14

Head-Related Transfer Functions (HRTFs)

- Composed of ITDs, ILDs, and spectral cues

Question 15

What happens when you remove characteristic filtering from HRTFs?

Answer 15

Sound can become internalized and harder to localize

Question 16

What is the monaural head shadow?

Answer 16

Better SNR in one ear (~6 dB)

Accessible to NH, HI, and CI users

Question 17

What is binaural summation/redundancy?

Answer 17

2 ears louder than 1 (~1 dB)

Question 18

What is binaural unmasking?

Answer 18

Added advantage from neural processing and auditory stream segregation

• NH listeners good at this

Question 19

What are the caveats to the duplex theory?

Answer 19

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)

Question 20

How does the human head interfere with the traveling wave?

Answer 20

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

Question 21

What is the precedence effect?

Answer 21

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

Question 22

What is the effect of BW on BMLDs?

Answer 22

Performance increases with wideband noise

Performance decreases with narrowband noise

Question 23

How does hearing loss affect vertical plane sound localization?

Answer 23

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