Review of Terminology
- Time signal or waveform
- The description of a sound in the time domain as fluctuations in
some physical property like pressure over time. Often, because the
pressure fluctuations have been transduced by a microphone or other
measurement instrument, we have converted pressure fluctuations to
voltage fluctuations over time.
- Fourier transform or spectrum
- The description of a sound in the frequency-domain as the
amplitude or extent of fluctuation occurring at different frequencies.
- Line Spectrum
- The kind of spectrum that is found for sounds that are purely periodic,
that is, for sounds that repeat the same pattern infinitely. Each line
in a line spectrum is a harmonic of the fundamental period of the
waveform and represents a sinusoid at a particular frequency and
amplitude. Line spectra are the ideal case of harmonic spectra.
- Harmonic Spectrum
- Similar to a line spectrum except that sounds giving rise to
harmonic spectra are not purely periodic, but only
approximately so. Such sounds produce an harmonic spectrum in which
the lines have some discernible width. As sounds deviate increasingly
from true periodicity, their spectra deviate increasingly from line
spectra to approach a continuous spectrum. For example, any sound
that has finite duration is not strictly periodic. Many natural
sounds, like the human voice, are quasi-periodic in that the sound
deviates in a variety of ways from one period to the next.
- Continuous Spectrum
- A spectrum exhibiting non-zero amplitude for one or more broad
regions of the continuous frequency spectrum. This is the kind of
spectrum that is found for aperiodic sounds, that is, sounds that do
not repeat any pattern at all. The ``ideal'' aperiodic sound is an
impulse, that is, a sound consisting of a single instantaneous
pressure spike. The impulse is a sound which has equal amplitude at
- Period or T0
- The duration of a single complete cycle of a periodic
waveform. We sometimes notate the period of a signal as T0.
- Fundamental Frequency or F0
- The fundamental frequency is 1.0/T0, that is, the inverse of the
period. Normally, we express F0 in units of cycles per second or
Hz. This can be slightly confusing since we often express T0 in units
of msec. You must remember to multiply T0 by 1000.0 if it is expressed
in msec to arrive at F0 expressed in Hz. For complex sounds, F0 will
normally be the frequency of the first, or lowest frequency harmonic.
- A line (or near-line) in the spectrum of a periodic (or near-periodic)
signal that can occur at any integer multiple of the fundamental
frequency. In a harmonic spectrum, the harmonics are spaced F0-Hz apart.
- The perceptual correlate of frequency. Normally, the pitch of a
complex sound is a function of its F0. Equal steps in pitch are
roughly equal to logarithmic steps in frequency.
- The perceptual correlate of amplitude. Equal steps in loudness
are roughly equal to logarithmic steps in amplitude.
- Decibel (dB)
- A logarithmic scale of amplitude which is roughly associated with
our perception of loudness. Zero Decibels is near the threshold for
hearing and each Decibel increment in amplitude is roughly one Just
Noticeable Difference in loudness. The formula for computing decibels
Decibels = 20.0 * log(Amplitude/Reference)
is generally something like the smallest perceptible amplitude
- Hertz (Hz)
- Frequency expressed in cycles per second.
- Mel Scale
- A logarithmic scale of frequency based on human pitch
perception. Equal intervals in Mel units correspond to equal pitch
- Bark Scale
- A logarithmic scale of frequency based on human frequency
resolution. Sounds which are separated by more than about one Bark
unit are generally resolvable as separate sounds and do not interact
with each other at a sensory level.
Last Modified: February 22, 1996