Binaural Hearing

1. Binaural Hearing Or now hear this! Upcoming Talk: Isabelle Peretz Musical & Non-musical Brains Nov. 22 @ 12 noon + Lunch Rm 2068B South Building

2. 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?

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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?

14. 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

15. 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

16. 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 Distance

17. 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