Spectral centroid

1. Spectral centroid • 6 harmonics: f0 = 100Hz • E.g. 1: Amplitudes: 6; 5.75; 4; 3.2; 2; 1 • [(100*6)+(200*5.75)+(300*4)+(400*3.2)+(500*2)+(600*1)] / 21.95 • = 265.6Hz • E.g. 2: Amplitudes 1; 2; 6; 5.75; 4; 3.2 • [(100*1)+(200*2)+(300*6)+(400*5.75)+(500*4)+(600*3.2)] / 21.95 • = 301.86Hz

2. Masking • A sound may become inaudible due to the presence of one or more other sounds • Explained in terms of an increase in the hearing threshold of the weaker sound • Formal definition: • “The process (or amount) by which the threshold of audibility for one sound is raised by the presence of another (masking) sound” • Amount – measured in dB

3. Masking • A sound is most easily masked by another sound that has frequency components close to it • Related to the BM frequency resolution – our ability to separate the components of a complex sound • Masking occurs if the frequency selectivity of the ear is insufficient to separate the signal and the masker

4. Types of masking • Simultaneous masking – signal present at the same time as the masker • Backward masking – signal present before the masker • Forward masking – signal present after the masker • Asa trk 23-25

5. Mechanism of simultaneous masking • Two conceptions: • The masker swamps the neural activity evoked by the signal • The masker suppresses the activity which the signal would evoke if presented alone – two-tone suppression

6. Forward masking • The amount of forward masking is greater the nearer in time to the masker the signal occurs • limited to signals which occur within about 200ms after the cessation of the masker • Influenced by the relation between the frequencies of the signal and masker

7. forward masking • Some explanations: • BM response rings after end of masker - temporal overlap of vibration patterns on the BM – for small delay times between masker and signal • fatigue in the auditory nerve or higher centres – reduces the response to the signal after the masker • The auditory processes underlying forward and backward masking are not well understood

8. Sound Localisation

9. Sound Localisation • Two ears • To determine the direction and distance of a sound source • Locate sounds in the horizontal plane, the vertical plane (elevation) and distance – for each of these we use a number of different cues: • Interaural time difference (ITD) • Interaural level difference (ILD) • Pinna and head cues - head-related transfer function (HRTF), head movement, movement of sound source

10. Locating sounds in the azimuth • Azimuth– locations on an imaginary circle that extends around us in a horizontal plane, measured in angle degrees • Locating a sound source in the azimuth: • Interaural time difference (ITD) • Interaural level difference (ILD)

11. Interaural time difference (ITD) • Time difference between the sound arriving at both ears. • ITD approx. range: 0 for a sound straight ahead to about 690 µs for a sound at 90° azimuth (directly opposite one ear) • Location of sound source for max ITD? • Location of sound source for min ITD?

13. ITD • Medial superior olives – first brain stem region where inputs from both ears converge – contributes to detection of ITD – neurons here respond to timing differences between inputs of both ears

14. Interaural level difference (ILD) • Difference in level (intensity) between a sound arriving at one ear versus the other • Properties: • Sounds are more intense at the ear closer to the source • Largest at 90°, -90° and min. at 0° and 180°

16. ILD • Head blocks high-frequency sounds much more than low-frequency sounds, • low frequency sounds have a wavelength which is long compare with the size of the head – sound bends around the head • ILDs are greatest for high frequency sounds

17. ILD • Neurons sensitive to intensity differences are found in the lateral superior olives

18. Summary ITD / ILD • Frequency dependency only for pure tones – not for complex tones • Sounds with more than one frequency – comparisons across frequency of ITD and ILD – most common ITD / ILD • ITD and ILDs are not sufficient to tell us completely where a sound is coming from.

19. Summary ITD / ILD • Do not indicate if the sound is from the front or back, or higher / lower (elevation) • Head movement, movement of the sound source • Other cues: • Direction-dependent filtering of the head and pinnae • important for judgements of vertical location and front / back discrimination

20. Pinnae and head cues • Spectral changes by head and pinnae used to judge location of a sound. • Spectral changes by the pinnae are limited to frequencies >6kHz - head, torso may modify the spectrum at lower frequencies • The head and pinnae modify the spectra of sounds in a way that depends on where the sound is – form a complex direction-dependent filter

21. Pinnae and head cues • Characterised by measuring the spectrum of the sound source and the spectrum of the sound reaching the eardrum –ratio of these two, expressed in dB, gives Head Related Transfer Function (HRTF) • HRTFs differ across individuals, due to head and pinnae shape and sizes. • Listeners can use these changes in intensity across frequency to learn where a sound comes from. • Visual feedback

22. The Precedence / Haas effect

23. Auditory distance perception • Determine how far away a sound is • Cue: relative intensity of a sound – become less intense with greater distance • Cue: spectral composition of sounds – high frequencies dampen (decrease in energy) more than low frequencies for far away sounds – sound of close vs far away thunder • Cue: relative amounts of direct vs. reverberant energy – a closer sound – more direct energy, also time delay between direct and reflected sound

24. Auditory distance perception • Change in intensity as listener moves toward the sound source • Relies on many cues: • In order to estimate the distance of a sound source the listener can combine absolute intensity, changes in intensity with distance (a moving source), spectral composition, and relative amounts of direct and reflected energy.