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Perception of Complex Sounds- Temporal Cues.docx

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McMaster University
Paul Faure

December 3 , 2013 Psych 3A03: Audition Perception of Complex Sounds: Temporal Cues Precedence or Haas Effect - Experiments have shown that the first wavefront arriving at the ears dominates in establishin location of sound source, and that we suppress information coming from later arriving wavefronts (echoes or reflections) - This phenomenon is called law of first wavefront or precedence (Haas) Effect Interaural Time Difference (ITD) versus Interaural Phase Difference (IPD) - Interaural time difference (ITD) does not vary with frequency - This is because the speed of sound (c) is independent of frequency - Interaural phase difference (IPD) does vary with frequency Phase Locking of ANFs - Neurons are phase locking Azimuthal Sound Localization Performance - Subjective judgement: accuracy for broadband noise - Individual frequencies: percent localization errors at 1000Hz due to phase locking - At high frequencies using interaural level differenes - At low frequencies using interaural time differences - Listeners make errors localzing pure tones near 1-3kHz - Listeners make almost no error Interaural Level Difference (ILD) - ILD cues processed in lateral superior olive ILD Cues Increase with Sound Frequency - Plot to right shows interaural level differences calculated for a source in the horizontal plane - The source is at an azimuth q of 10 , 45 , or 90-o- relative to straight ahead - Calculations assume that the ears are at opposite poles of a rigid sphere - Fluctuations due to the head and pinnae - Geometric spreading (inverse square law) does not create appreciable ILD cues - Size of object (e.g. head) relative to wavelength determines magnitude of ILD and sound shadow - Diffraction and reflection primary physical causes of ILD cues - ILD is most prominent at high frequencies ILD Cues and Lateral Superior Olive - LSO: binaural inputs sensitive to interaural level differences (ILD) - Ipsilateral = excitatory (E) - Contralateral = inhibitory (I) - E-I units; most have high CFs Elevational Sound Localization Performance - Listeners make errors localizing pure tones near 2kHz - Listeners make almost no errors when localizing broadband noise signals - Better in azimuthal than elevation - Why is there a difference between broadband and pure tones? Human Sound Localization Performance - Minimum audible angle (MAA) discrimination task - Blindfolded Ss tested to see if they can discriminate sounds broadcast from 2 loudspeakers (1m from S) - Angular separation of loudspeakers varied as a function of azimuth and sound frequency - Listeners had larger MAA when the speakers were located off to the side than when located directly in front (i.e. poor discrimination at side) - More errors in mid-frequency region - Better in line with the visual field’ Duplex Theory of Sound Localization 1. Low frequency tones (<1500Hz) localized by interaural time (phase) difference cues  Very small interaural intensity difference for low-frequency tones  i.e. very little attenuation due to geometric spreading or inverse square law  Phase locking by ANFs occurs for low frequency tones (<4kGz)  Resolution limited by ambiguity: maximum ITD ≈ 670μs corresponding to a whole cycle at 1500Hz (ca. the upper limit for binaural phase sensitivity) 2. High (and close low) frequency tones localized by interaural level difference cues  Sound shadow cast by head greater at high frequencies (20dB at 6kHz) than at low frequencies (3dB at 500Hz)  i.e. head acts as low pass filter  For sounds very close to a listener (<1.5m) the inverse square law gives ILD cues at all frequencies and these differences vary with azimuth independently of any head-shaddow effect  Beyond 1.5m the difference in level between the ears due to this factor is less than 1dB Azimuthal Localization of Complex Sounds - Complex sounds contain both low and high frequencies - Dominant azimuthal localization information is the ITD of the low frequency signal components in the complex wave (e.g. waveform envelope) - No phase ambiguities for complex signals The Cone of Confusion - Both ITD and ILD cues are important for sound localization - Cone of confusion: for listeners that do no move their heads, there are a number of locations in space that provide the same ITD and ILD cues - E.g. mid-sagittal plane (pinnae plays a
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