How Can You Localize Sound?

1. How Can You Localize Sound? • Ponder this: • Imagine digging two trenches in the sand beside a lake so that water can flow into them. Now imagine hanging a piece of cloth in the water in each trench. Your job is to determine the number and location and type of every fish, duck, person, boat, etc. simply by examining the motion of the cloth. That’s what your auditory system does! - Al Bregman

2. Localization • All you have is a pair of instruments (basilar membranes) that measure air pressure fluctuations over time

3. Localization • There are several clues you could use:

4. Localization Left Ear Right Ear Compression Waves

5. Localization • There are several clues you could use: • arrival time - sound arrives first at ear closest to source

6. Localization Left Ear Right Ear Compression Waves

7. Localization • There are several clues you could use: • arrival time • phase lag (waves are out of sync) - wave at ear farthest from sound source lags wave at ear nearest to source

8. Localization Left Ear Right Ear Compression Waves

9. Localization • There are several clues you could use: • arrival time • phase lag (waves are out of sync) - wave at ear farthest from sound source lags wave at ear nearest to source • Head shadow

10. Localization • Arrival Time • Phase Lag • Head Shadow InterauralTiming Differences (ITD) InterauralIntensity Difference (IID)

11. Localization • What are some problems or limitations?

12. Localization • Low frequency sounds aren’t attenuated by head shadow because sound bends around the head with little loss of amplitude Sound is the same SPL at both ears Left Ear Right Ear Compression Waves

13. Localization • Low frequency sounds aren’t attenuated by head shadow • Your brain preferentially uses ITD cues for low-frequency sounds

14. Localization • High frequency sounds have ambiguous phase lag because more than one wavelength “fits” between the ears Left Ear Left Ear Right Ear Right Ear Two locations, same phase information!

15. Localization • High frequency sounds have ambiguous phase lag • Your brain preferentially uses IID cues for high-frequency sounds

16. Localization • These cues only provide azimuth (left/right) angle, not altitude (up/down) and not distance Left Ear Right Ear Azimuth

17. Localization Additional cues:

18. Localization Additional cues: Head Related Transfer Function: Pinnae modify the frequency components differently depending on sound location

19. Localization Additional cues: Room Echoes: For each sound, there are 6 “copies” (in a simple rectanguluar room!). Different arrival times of these copies provide cues to location of sound relative to the acoustic space

20. Localization • What would be the “worst case” scenario for localizing a sound?

21. Pitch and Music

22. Pitch • Pitch is the subjective perception of frequency Period - amount of time for one cycle Frequency - number of cycles per second (1/Period) Air Pressure time ->

23. Pitch • Pure Tones - are sounds with only one frequency f = 400 hz f = 800 hz

24. Tone Height • Tone Height is our impression of how high or low a sound is • but there’s something more to our impression of how something sounds than just its tone height…

25. Chroma • Tone Chroma is the subjective impression of what a tone sounds like • Notes that have the same Chroma sound similar 500 Hz 400 hz 800 Hz

26. Chroma • Tones that have the same Chroma are octaves apart

27. Chroma • chroma is best represented as a helix • chroma repeats every octave • tones with the same chroma are above or below each other on a helix

28. Chroma • Tones that are octaves apart have the same chroma • one octave is a doubling in frequency

29. Chroma • frequency is determined (in part) by location of stimulation on the basilar membrane

30. Chroma • frequency is determined (in part) by location of stimulation on the basilar membrane • but that relationship is not linear (it’s logarithmic)

31. Chroma • doublings of frequency map to equal spacing on the basilar membrane