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