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Acoustic and perceptual effects of air sacs

Acoustic and perceptual effects of air sacs. Bart de Boer University of Amsterdam. The air sac. Brown Howler monkey. Humans. Siamang. Orang Utan. Air sac prevalence. All apes have air sacs But humans don’t Why? Speech?. The anatomy of air sacs. “siamang”. “howler monkey” (subhyoid).

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Acoustic and perceptual effects of air sacs

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  1. Acoustic and perceptual effects of air sacs Bart de Boer University of Amsterdam

  2. The air sac Brown Howler monkey Humans Siamang Orang Utan

  3. Air sac prevalence • All apes have air sacs • But humans don’t • Why? • Speech?

  4. The anatomy of air sacs “siamang” “howler monkey” (subhyoid)

  5. The function of air sacs? • Speculation • Acoustics: • Resonators (e. g. Avril, 1963; Schön, 1971) • Impedance matchers (e.g. Fitch & Hauser, 1995) • Suppressing resonances (Haimoff, 1983) • Non-acoustic • Accidental byproduct (Brandes, 1932) • Rebreathing air (Negus, 1949) • Preventing hyperventilation (Hewitt et al., 2002)

  6. Why is this even important? • For the evolution of speech • Correlation air sacs – hyoid bone • Bones fossilize!

  7. Human (http://www.anatomyatlases.org/atlasofanatomy/ plate01/08hyoidbone.shtml) Chimpanzee (Avril 1962) Shape correlation • When an air sac is present (at least in primates) the hyoid has a cup-shaped front (the bulla) Brown howler monkey

  8. Dikika baby, A. afarensis (Alamseged et al. 2006) Neanderthal (Arensburg et al 1989) Fossil evidence • Four fossil hyoid bones are known • Neanderthal (60 KyA) • 2x H. heidelbergensis (530 KyA) • Australopithecus afarensis (3.3 MyA) H. Heidelbergensis (Martínez et al. 2008)

  9. Fossil conclusion • The latest common ancestor of Neanderthals and modern humans did not have air sacs • Australopithecus afarensis did • (+ possibility to find evidence for intermediate ancestors)

  10. Implications for speech? • Did air sacs disappear because of speech? • But we need to be sure about their acoustic effect!

  11. An acoustic model • Important elements: • The neck • The cavity • The wall • Radiation • Can be analyzedas an electricalcircuit

  12. [a] [ə] [y] The acoustic effect • A new peak at ~the resonance of the air sac • Original formants get shifted up and closer together

  13. [y] 5 10 0 10 Rel. Power -5 10 100 500 1000 1500 2000 2500 3000 3500 4000 Frequency (Hz) Perception issues • We could measure distances in formant space • But which formants do we take? • Does the air sac resonance take over the role of F1? • And how about F2 – F2’? F2 F1 Fs2 F3 F4 Fs3 Fs1

  14. Perception experiment • Expose subjects to stimuli, and let them classify • [a]/[ə], [a]/[y], [ə]/[y], [a]/[ə]/[y] • (Two or three - unforced choice) • With and without air sac • Iteratively find noise level at which performance halfway between perfect and chance

  15. Example trial

  16. Perception experimental results * *

  17. Conclusion • Apes have air sacs, humans don’t • Neanderthals don’t • Australopithecines do • Lost in evolution • Why? • Change spectrum of speech • Can hear the difference less well • Lost because of speech • Hypothesis: • Neanderthals could speak, Australopithecines not • (proto-) speech is at least 500 000 years old

  18. Analysis

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