The Spectrograph & Spectral
A sound spectrograph is
a laboratory instrument that displays
a graphical representation of the strengths
of the various component frequencies
of a sound as time passes. Voice researchers
use the spectrograph as a tool for analyzing
vocal output. In current research, it
is used for identifying the strength
and frequencies of formants, and for
real-time biofeedback in voice training
There are two
main kinds of voice analysis
performed by the spectrograph, wideband (with
a bandwidth of 300-500 Hz)
and narrowband (with
a bandwidth of 45-50 Hz).
Wideband (a.k.a. 'broadband') spectrograms, when used for normal speech with
a fundamental frequency of around 100-200 Hz, will pick up energy from several
harmonics at once and add them together, as seen below. The Fo (fundamental
frequency) can be determined from the graphic by counting the number of individual
vertical lines per unit time. Also, the frequencies and relative strengths
of the first two formants (F1 and F2) are visible as
dark, rather blurry concentrations of energy.
The wide bandwidth
in this type of analysis allows
for excellent time resolution;
you can see the energy peaks
from each individual vibration
of the vocal folds in the graph.
However, the wide bandwidth
also means that individual
harmonics cannot be singled
out by this kind of spectrogram.
This means that for the wideband
spectrograph, frequency components
within a 300-500 Hz bandwidth
are not easily distinguished.
The narrowband spectrogram has different strengths; it is able to pick out
each individual harmonic, unlike the wideband spectrogram, but its time resolution
is not good enough to isolate each individual cycle of vibration, and the formant
structure of the sound is not rendered as clearly as with a wideband analysis.
Note the dark horizontal stripes, representing each harmonic, in the graphic
below. Also note that the large clusters of formant energy which we saw in
the wideband spectrogram are not present:
spectrograph is often used
in research, it can also serve
as a helpful tool in the voice
clinic to provide feedback
as part of a voice therapy
or training program.