SPA 3011: Introduction to Speech Science






Th, 5/31

Aperiodic sounds

Fry (8)


  1. Periodic versus aperiodic sounds
    1. So far we have been talking about periodic sounds (sounds which repeat exactly over and over again) that only exist as a theoretical construct
    2. Recall that "periodic" sounds in the real world (including speech) are quasi-periodic (they repeat in a noticeably similar way over an over) and so we examine them in the same way as periodic sounds because they aren't very different
    3. There is another class of sounds that is very different from the periodic sounds
    4. Sounds that are not periodic (or quasi-periodic) are called aperiodic
      1. Aperiodic sounds are sounds that do not repeat
      2. There are two ways we can imagine that a sound would not repeat
      3. A transient sound is one that occurs just once, briefly (For example, a popping balloon, dropping a pen, or synthesizing one cycle of a sine wave)
      4. A noise sound is one where the pressure at each moment is random and does not depend on the previous moment (For example, a tire leaking air or static on a tv)
    5. Both transient and noise sounds are distinctive in their appearance on waveform and spectrum graphs
      1. A transient on a waveform is a brief period of positive or negative pressure
      2. A noise on a waveform is a random jumble of points with no apparent pattern
      3. The spectrum of a transient has noise components of all frequencies, so it looks like a horizontal line (can sort of see this by imagining adding together sine waves of all frequencies with the same phase to get one big point of positive pressure and lots of zeros)
      4. The spectrum of noise also covers all frequencies, but different "types" of noise can have some frequencies louder than others
      5. For example, white noise has equal components of all frequencies, so the spectrum of white noise looks like a horizontal line
      6. The spectra of other types of noise are curves over the frequency range (similar to the spectra of resonances and filters, and very different from the comb spectra of simple periodic or harmonic sounds)
      7. Note that the spectra of a transient and of white noise are the same, so it would be nice if we had another representation of sound that covered both the frequency and time domains

  2. Source, filter, and the vocal tract
    1. The concepts of periodic sounds, aperiodic sounds, and filtering (resonance) are the fundamental building blocks of the theory of how speech articulation creates sound
    2. In normal speech, the larynx acts as a source of periodic sound (and, in fact, a periodic sounds that is rich in harmonics)
    3. In whispered speech (or for talkers with a breathy voice quality, or for /h/), the larynx acts as a source of aperiodic sound
    4. Other articulators can also provide sources of aperiodic sound, particularly the tongue, lips, and teeth
    5. Regardless of the source of sound (periodic or aperiodic), the shape of the vocal tract downstream from the source (toward the mouth) filters the frequencies of the source
    6. Different combinations of the sound source with the shape of the vocal tract are used to make the distinctive phonemes of speech
    7. A major component of speech science is the study of the acoustic and articulatory correlates of the distinctive phonemes (and other phonological units) of speech

Revised: 5/30/01