Topics for today:

  • Harmonics and complicated periodic vibrations
    • Periodic waveforms have harmonic spectra: signal energy is discrete at frequencies that are integer multiples of the fundamental frequency, known as harmonics, overtones, or harmonic partials.
    • The distribution of energy among the harmonics gives rise to audible changes in the perceived sound quality, or timbre. For example, different sung vowels at the same pitch are recognizable as “ahh,” “ohhh,” “eeee”, and so forth.
  • Audible examples of various effects; experiment to measure the speed of sound, etc.
  • Wave interference
    • When waves arrive from different sources (or from a reflection), the wave pressures may be in phase or out of phase: the pressures may add or subtract from each other.
    • Interference can cause the sound to be louder or softer at different places in a room.
    • Interference can affect an audio recording depending upon the microphone location.

  • Sound level and the decibel

    • Common sound pressures vary from tens of micropascals (very quiet) up to a few pascals (very loud).
    • It can be inconvenient to deal with this wide range of numbers (0.00002 to 2).
    • Customarily, acousticians convert to decibels (dB), so that the wide numerical range of pressure gets converted into a range from zero to 100.
    • The decibel for pressure is defined:   dB = 10 log10((pressure)2/(20 micropascals)2) .
    • This can be adjusted mathematically to be  dB = 20 log10((pressure)/(20 micropascals)) .
    • 20 micropascals is the “threshold of hearing” at frequencies for which the ear is most sensitive (2-3 kHz).  This is zero dB sound pressure level (SPL).
    • 2 pascals is very loud, almost painful, and corresponds to 100 dB SPL.
  • Topics for the next lecture:
    • Reading assignment:  Chapters 9, 10, and 12.
    • Start anatomy and physiology of the ear.