Course:PHYS341/2018/Calendar/Lecture 33

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Phys341 Lecture 32: Summary and web references

2018.04.03

Textbook Ch.23.1-2 (Recording), 25.1-4 (Perception and stereo sound), 19.11 (Human Voice)

Aural Perception

  1. Live music exists in:
    • Frequency
    • Time
    • Space
    • We hear it with two ears.
  2. Distinguishing left from right (two-ear “Duplex Theory”):
    • Interaural Time Difference (ITD ~ 1 ms) – below 1.5 kHz
      • Low frequency sound diffracts – far ear may hear louder than the nearer ear.
      • Frequency dependent due to diffraction and sound speed differences in air and head.
    • Interaural Intensity Difference (IID) – above 1.5 kHz
      • Less diffraction at higher frequencies – far ear in acoustic “shadow”.
      • Frequency and angle dependent.
    • Limitations
      • Any given ITD defines a cone around line l – not a single direction and no front/back up/down discrimination.
  3. Pinna
    • The geometry of the pinna suppress parts of the acoustic spectrum depending on the direction of the source.
    • If the spectrum is broadband and smooth, then any gap (“notch”) in the spectrum gives a directional clue.
    • Yields up/down and/or forward/backward information.
    • Works with only one ear!
    • Does not work for single-frequency sources, e.g. my bicycle bell.
  4. Analogue Sound Recording
  5. Groove in vinyl record (can be stereo)
  6. Magnetic field on tape

Human Voice

  1. Vocal folds
    • Vibrate like lips in a brass embouchure
    • Frequency controlled by tension in folds
    • Inserts periodic puffs of air into vocal tract: pulse train
    • Little feedback from vocal tract
    • Can “play” any note within range
  2. Disengage vocal folds
    • Rush of air produces white noise – all frequencies
    • No harmonics
    • Formants only
    • Whispering voice
  3. Vocal tract length:
    • ~14 cm for adult females
    • ~17 cm for adult males
  4. Simple tubes of this length would have modes at:
    • (i) 86/0.14 = 614 Hz; 86/0.17 = 506 Hz
    • (ii) 258/0.14 = 1840 Hz; 258/0.17 = 1520 Hz
    • (iii) 430/0.14 = 3070 Hz; 430/0.17 = 2530 Hz
    • Adult female formants:
      • ~250-850, ~850-3000, ~3500-4000 Hz
    • Adult male formants:
      • ~200-700, ~700-2500, ~3000-3500 Hz
  5. Formants have the same effect on the human voice as the soundboard on a string instrument.
    • The input from the vocal folds (strings) is modified as is it radiated, giving the speaker/singer/instrument its distinctive character.
  6. Formants can be manipulated electronically
    • Farinelli clip: https://youtu.be/GIPQtelKN28
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