Psychoacoustics 4 – Spatial Hearing

1. Alexis Baskind Psychoacoustics 4 Spatial Hearing Alexis Baskind, https://alexisbaskind.net

2. Alexis Baskind Psychoacoustics 4 – Spatial Hearing Course series Fundamentals of acoustics for sound engineers and music producers Level undergraduate (Bachelor) Language English Revision January 2020 To cite this course Alexis Baskind, Psychoacoustics 4 – Spatial Hearing, course material, license: Creative Commons BY-NC-SA. Full interactive version of this course with sound and video material, as well as more courses and material on https://alexisbaskind.net/teaching. Except where otherwise noted, content of this course material is licensed under a Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International License. Psychoacoustics 3 - Spatial Hearing

3. Alexis Baskind Outline 1. Introduction 2. Interaural level and time differences 3. Cone of confusion 4. Estimating distance in a dry environment 5. Cocktail-Party effect 6. Summing Localization 7. Precedence effect Psychoacoustics 3 - Spatial Hearing

4. Alexis Baskind Introduction • Sound localization, i.e. our ability to estimate the position of objects in 3 dimensions based on sound only, is one of the fundamental attributes of human perception, as it complements vision (which is only frontal) in spatial perception of the environment • It consists in 3 aspects: 1. Lateralization (left-right localization) 2. Perception of height 3. Perception of distance • It’s a quite complex mechanism that relies on objective cues (time and level differences between both ears, filtering because of reflections…) as well as other sources of information (previous knowledge, interaction with vision and movements, etc…) • This course will only focus on the direct sound (i.e. without considering neither the reverberation nor the reflections) Perception of direction Psychoacoustics 3 - Spatial Hearing

6. Alexis Baskind Interaural Level and Time Differences • Depending on the sound wave incidence, the sound differs between both ears; • The head creates an acoustic shadow • At the ear which is the furthest away from the source, the sound is: – softer and filtered (especially at high frequencies) – delayed • The ear relies on those interaural time and level differences in order to localize the source Psychoacoustics 3 - Spatial Hearing

7. Alexis Baskind Interaural Level Differences • Interaural level differences (“ILD”) depend on the sound incidence: • If the source is in the so- called median plane (on the front, the rear or on the top, for example), it is at the same distance to both ears the level difference is zero • If the source is 90° on one side, then the shadowing is maximum, and thus the level difference is also maximal Psychoacoustics 3 - Spatial Hearing

8. Alexis Baskind Interaural Level Differences • Interaural level differences (“ILD”) depend on frequency: • The head is an obstacle only at frequencies, for which the wavelength is smaller as the dimensions of the head (i.e. above 1000-1500 Hz) • At low frequencies, diffraction of the sound around the head dominates, and level differences decrease with frequency. Psychoacoustics 3 - Spatial Hearing

9. Alexis Baskind Interaural Level Differences Example of Interaural level differences as a function of frequency and angle Note: those values vary from one individual to another ! Psychoacoustics 3 - Spatial Hearing

10. Alexis Baskind Interaural Time Differences • Interaural time differences (“ITD”) depend on the sound incidence: Closest ear (shorter path) Furthermost ear (longer path) • If the source is in the so- called median plane (on the front, the rear or on the top, for example), it is at the same distance to both ears the time difference is zero • If the source is 90° on one side, then the distance to the furthermost ear is maximum, and so the time difference is also maximal Psychoacoustics 3 - Spatial Hearing

11. Alexis Baskind Interaural Time Differences • Interaural time differences (“ITD”) depend on frequency: Closest ear (shorter path) Furthermost ear (longer path) • The maximum time difference is specific to each person, but is on average around 0.6 ms • At low frequencies, those differences are small pertaining to the period: the localisation blur increases with decreasing frequency • At high frequencies, the period is very small possible ambiguity of the time delay Increasing localization blur Conclusion: time-based sound localization reaches its maximum of precision in the medium frequencies Psychoacoustics 3 - Spatial Hearing

12. Alexis Baskind Interaural Time Differences Interaural time differences as a function of angle Note: those values also vary from one individual to another ! Psychoacoustics 3 - Spatial Hearing

14. Alexis Baskind Cone of Confusion • Interaural level and time differences, which carry the most important information to localize sources, are ambiguous • For given interaural level and time differences, there is an infinity of possible incidences Image from Stefan Weinzierl, „Handbuch der Audiotechnik“ Typical example: Front/back confusions Psychoacoustics 3 - Spatial Hearing

15. Alexis Baskind Blauert Bands • Special case: sound incidence in the median plane  Level and time differences are zero  Without any other source of information, the source is localized not thanks to its actual direction, but only based on its frequency content („Blauert bands“) Experiment by J. Blauert (1969/70): • The sound source is in the median plane (either in the front, above or behind) • The signal consists in third-octave noise The Localization does not depend on the incidence, only on the frequency band (Image: Wikipedia)Frequency in kHz Probabilityofestimated directionofincidencein% Blauert bands front frontback backabove Psychoacoustics 3 - Spatial Hearing

16. Alexis Baskind Cone of Confusion How to solve this ambiguity? 1. Thanks to vision: if one sees the sound source or at least ist rough direction, the ambiguity is solved 2. Thanks to previous-knowledge: if one already has a good idea about the possible position of the source 3. Thanks to head movements even small: the modification of the interaural time differences is enough  For binaural synthesis (3D-audio for headphones) this means, that head movements must be measured in real time in order to adapt the synthesis accordingly  Head-Tracking 4. Thanks to the effect of distance on level and spectrum (see below) Psychoacoustics 3 - Spatial Hearing

17. Alexis Baskind Cone of Confusion How to solve this ambiguity? 5. Thanks to the effect of reflections on schoulders and ear conch (comb filters) Image in D.R. Begault, 3-D Sound For Virtual Reality And Multimedia Psychoacoustics 3 - Spatial Hearing

19. Alexis Baskind Estimating the distance of the source In a dry environment (i.e. without reverberation), estimating the distance relies on two kinds of information: 1. The level of the sound at the ears: if a sound is further, it will be softer (distance law). => But this means knowing how loud is the source ! For example, a loud whispered voice will be considered as close, or a very brassy, but soft, trumpet, will be considered as far, even if the sound level is the same If we don’t know the source, and if there is no reverberation, we cannot know how far is the sound source Psychoacoustics 3 - Spatial Hearing

20. Alexis Baskind Estimating the distance of the source In a dry environment (i.e. without reverberation), estimating the distance relies on two kinds of information: 2. The spectrum of the sound: – If the source is close and directive, low frequencies are boosted – If the source is far, high frequencies will decline because of air absorption (low frequencies travel further than high frequencies) But this requires as well to know the spectrum of the sound source. If we don’t know the sound, we cannot know how its spectrum is modified with the distance Psychoacoustics 3 - Spatial Hearing

22. Alexis Baskind Cocktail-Party Effect • The cocktail-party effect (also called selective attention) is our ability to focus our attention on a given sound source when two or more sources are playing simultaneously • It relies on two different mechanisms: 1. Spectral and time cues: the hearing system will try to separate sources that have different spectra, and are not simultaneous (homorhythmic for music). But this is harder if sources are at the same position Psychoacoustics 3 - Spatial Hearing

23. Alexis Baskind Cocktail-Party Effect • The cocktail-party effect (also called selective attention) is our ability to focus our attention on a given sound source when two or more sources are playing simultaneously • It relies on two different mechanisms: 2. Spatial cues: if two sounds present different level and time differences between both ears, they will be spatially separated by the audition and therefore perceived as two distinct sound sources: => That’s why a stereo mix sounds in general clearer than a mono mix, and a surround mix sounds in general clearer than a stereo mix Psychoacoustics 3 - Spatial Hearing

25. Alexis Baskind Summing Localization • With two-channel stereophony, the listener becomes 4 information, 2 for each source • Hearing is tricked because of this non natural situation and assumes, that there is only one source between both loudspeakers => Phantom source • This is called Summing localization Phantom source Sound image Psychoacoustics 3 - Spatial Hearing

26. Alexis Baskind Summing Localization • Summing localization is the basis of stereophony • It occurs when two or more spatially distinct sources radiate identical or at least coherent signals with a time difference smaller than 1.5 ms • In this case there is one perceived sound event, which localization depends on the time and level differences at the ears between the sound sources: – If the time and level differences are zero, the phantom source is perceived exactly at the midpoint between both loudspeakers – If the time and level differences are not zero, the phantom source shifts towards the loudspeaker for which the signal is the loudest and/or the earliest Psychoacoustics 3 - Spatial Hearing

27. Alexis Baskind Summing Localization Localization of the phantom source based on level differences (left figure) and time differences (right figure) (Figure: J. Blauert, “Spatial Hearing”. Dashed plot: speech, head free to move; solid plot: impulse signals, head immobilized) Psychoacoustics 3 - Spatial Hearing

28. Alexis Baskind Time difference of the loudspeaker signals Leveldifferenceoftheloudspeakersignals Phantom source in the middle (rightlouder)(leftlouder) (right earlier) (left earlier) Phantom source at the right Loudspeaker Time and level differences can be combined with each others Mixed Stereophony, near-coincident pairs (ex: ORTF) Summing Localization Phantom source at the left Loudspeaker Psychoacoustics 3 - Spatial Hearing

29. Alexis Baskind Summing Localization Sweet-Spot 60° 60° 60° • The previous information is only valid if the listener is at the Sweet-Spot (i.e. the listener and the loudspeakers have to form an equilateral triangle) • If it‘s not the case, extra time and level differences are introduced and the phantom source moves in the direction of the closest loudspeaker Psychoacoustics 3 - Spatial Hearing Sound image

31. Alexis Baskind Precedence effect • If the same (or almost the same) signals reach the listener from different directions with a delay between them, the resulting perceived event differs as a function of the delay: 1. If the time delay is less than 1.5 ms, the summing localization applies 2. If the time delay is greater than 1.5 ms and the delayed sound is much quieter than the first, the auditory system only perceives the direction of the sound signal arriving first. The second sound is not distinctly perceived (=precedence effect, or Haas effect), but the sound is perceived louder, farther and/or wider. Psychoacoustics 3 - Spatial Hearing

32. Alexis Baskind Precedence effect • If the same (or almost the same) signals reach the listener from different directions with a delay between them, the resulting perceived event differs as a function of the delay: 3. If the time delay is greater than 1.5 ms and the delayed sound is loud enough, the auditory system perceives both sound signals distinctly. The second sound is then called echo Psychoacoustics 3 - Spatial Hearing

33. Alexis Baskind Precedence effect • The level threshold between the Haas effect and a distinct perception of an echo is called echo threshold, and depends on the time delay and on the sound itself : • The greater the time delay, the lower the echo threshold • The echo threshold is lower for sounds with short, sharp transients Psychoacoustics 3 - Spatial Hearing

34. Alexis Baskind Precedence effect The precedence effect is used everywhere in music production: 1. Sound reinforcement: delayed loudspeakers are set at the back of the room. The listener does not perceive them distinctly, still perceiving the sound coming from the stage, but louder. Psychoacoustics 3 - Spatial Hearing

35. Alexis Baskind Precedence effect The precedence effect is used everywhere in music production: 2. Studio-Mixing: – to make a mono recording wider, short delays (approx. 10ms) can be applied to it, which are not perceived as echoes. The delays must be different on the left and right side to decrease the correlation (see previous part) – If the time delay is a bit longer (20-50ms), more envelopment is generated Psychoacoustics 3 - Spatial Hearing

36. Alexis Baskind Precedence effect The precedence effect is used everywhere in music production: 2. Studio-Mixing : – These are the basics of the perception of a reverberation: Direct sound (more width/envelopment) elate intense reflections cause echoes late reverberationFirst reflections Psychoacoustics 3 - Spatial Hearing

37. Alexis Baskind To go further • J. Blauert, Spatial Hearing, The Psychophysics of Human Sound Localization, MIT Press • F. Rumsey, Spatial Audio, Focal Press • Eberhard Sengpiel‘s webseite, among others – http://www.sengpielaudio.com/InterchannelLevelDiffe rencesAndInterchannelTimeDifferences2.pdf – http://www.sengpielaudio.com/calculator- localisationcurves.htm Psychoacoustics 3 - Spatial Hearing

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