Binaural Hearing
Or now hear this!
Upcoming Talk: Isabelle PeretzMusical & Non-musical BrainsNov. 22 @ 12 noon + LunchRm 2068B South Building
TLA 6: 2 Two Ear Hearing
• Purpose of TEH – Spatial hearing and understanding
• Activity: – Walk rapidly down a hallway while plugging one ear– Halfway through hallway, switch to plugging the other
ear• Switch order of plugging the two ears and repeat
• Write-up– Does having a plugged ear change how you walk
down a hall? How did changing the plugged ear affect your motion?
Hearing Binaurally(Yost chapter 12)
• Binaural = two ear hearing– Combination of information to determine spatial
position• Azimuth
– Not distance– Not vertical position
– Stationary localization• Different cues available with motion
• Interaural cues for binaural hearing– Interaural Loudness Difference (ILD)
• Interaural Intensity Difference (IID)– Interaural Timing Difference (ITD)– Interaural Phase Difference (IPD)
Interaural Timing Differences (ITD)
• Onset of auditory stimulation– Does not vary across frequency
• Salient with lower frequencies (< 1500 Hz)
– Maximum delay of < 1 ms• Dependent on head-size• Angle of stimulation
• Critical for short events– Clicks, bursts
• Less important for enduring events– Noise, speech
Interaural Phase Differences (IPD)
• Relative phase of stimulus across ears– Critical region is < 800 Hz
• No IPD at 833, 1666 Hz
– Noticeable differences of phase • Minimum displacement 0.2 ms
• Enduring sound events– Noise, speech
• Change in phase triggers change in localization
– Basis of the Precedence Effect
Interaural Loudness Differences (ILD)
• Relative intensity across ears – Critical region
• > 2 kHz• Ecological constraints 800 Hz
– Up to 20 dB SPL attenuation (over 8 kHz)• Sensitive to 1 dB SPL difference• Total masking 8 – 10 dB SPL
– Similar to natural head shadow
• Oldest theory of directional hearing (1870’s)• Ambulance direction
– Open window determines positions for high frequency siren
Duality Theory of Directional Hearing
• Frequency region determines salient cues– Lower frequencies 40 – 1500 Hz IPD, ITD– Higher frequencies 4 – 20 kHz ILD
• Worst localization performance 1500-4000 Hz
• Harnessing Stationary cues– Difficult noises
• Diffuse noise, enduring• Sinewave burst
– Easiest to localize• Broadband click
– Incorporates multiple cues
Minimum Audible Angle (MAA)
• How good is hearing?– Stationary: accuracy separating two
sound sources (Mills, 1958)• Play sound, move left/right play again• Chance performance = 50 %, threshold =
75%
– Results• Azimuth dependence: best at center 0˚,
logarithmic decline to 75˚• Frequency dependent: best 40 – 4000 Hz
– Approx. 3˚ separation (vision 1’)
• Minimum audible movement angle– Velocity – dependent
• Approx. 1˚ separation
Localization with HAs
• Factors affecting localization– Bilateral vs. Unilateral
• 2 ear vs. 1 ear– Symmetric hearing loss?
• All sounds located at hearing ear– If symmetrical bilateral improvement
• Speech in noise release from masking
– BTE vs. ITC/CIC• BTE microphone outside ear canal
– Directional microphones
• ITE/CIC spectral filtering from pinnae
– Better HA performance with ITC/CIC
Localization with Cochlear Implant
• Test unilateral, bilateral cochlear implant users– ITD, IPD cues– ILD cues
• HYPOTHESES?
• 3x precision with bilateral implants– Large individual differences
• Duration using bilateral implants• Speech ability
Head-related Transfer Functions (HRTFs)
• HRTF: calculation of the sum of spatial parameters– Distance between the ears– Pinna filtering
• Spectral shape of resonance harmonics
– Head attenuation• Nose directionality• Body absorption• Hair on the head
• Calculation of HRTF for simulated reality– Convolve microphone input– Dummy-head recordings– Binaural recordings
• Which is best? • Front-back confusions
Binaural Masking• Vary position of noise & energetic masker• Monaural
– No difference of spatial position and noise– Similar amount of energetic masking in all positions
• Diotic– No difference of spatial position of noise– Similar amount of energetic masking
• Dichotic – Noise to one ear, masker to other– Release from masking
• Better detection of signal
Hearing the Silent World
• Localization– Study of sound sources
• Sound producing objects relative to listener
• Are sound sources the basis of hearing? – Visual world
• Light producing objects– Sun, lamps
• Light reflecting surfaces– Tables, faces, trees
– Can we detect sound obscuring/reflecting surfaces?
Hearing the Silent World• Sound obstructing surfaces
– Diffuse sound field set behind sound attenuating surfaces
• Are listeners sensitive to position of surfaces?
• Test behavioral judgment– Is the aperture large enough to allow
passage?• Ego-centric judgment facilitates
accuracy– Aperture size affects intensity,
spectra• Randomize intensities, sine wave
signals– Listeners can detect position of
sound obstructing surfaces
Elevation
• Height relative to listener– How can this be determined?
• Interaural cues?– Timing difference between the ears
• Mid-Saggital plane
– Loudness difference between the ears• Absorption by head & pinna
– Front-back confusions
• Pinna cues– Forward, downward facing– Partially resolve front-back errors
Distance
• How far away is a sound source? – Interaural cues?
• Azimuth does not indicate relative distance
– Pinna cues?• Slight-downward facing
– More distant cues higher in the perceptual plane
• Salient cues for distance– Intensity
• Attenuation over distance– Frequency dependent
• Unreliable indicator– Reverberation
• Increase in number and lag of echoes
– DEMO
Improving Accuracy
• How do listeners judge distance? – Metrics of perception
• Absolute distance: objective scale• Egocentric distance: metric in body relations
• Test– Judge baby rattle distance egocentric scale
• 1 vs. 2 degrees of freedom – Arm vs. Arm + body lean
– Highly accurate judging 1 or 2 degrees• Better accuracy than found with absolute distance
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