Page 117 - The Master Handbook Of Acoustics

92 CHAPTER FIVE

in other spectrograms. The following discussion owes much to the
clear presentation of Flanagan. 1

Sound Sources for Speech
The artificial larynx is based on the fact that there are really two more
or less independent functions in the generation of speech sounds: the
sound source and the vocal system. In general, it is a series flow as pic-
tured in Fig. 5-3A, in which the raw sound is produced by a source
and subsequently shaped in the vocal tract. To be more exact, there are
really three different sources of sound to be shaped by the vocal tract
as indicated in Fig. 5-3B. First, there is the one we naturally think of—
the sounds emitted by the vocal cords. These are formed into the
voiced sounds. They are produced by air from the lungs flowing past
the slit between the vocal cords (the glottis), which causes the cords to
vibrate. The air stream, broken into pulses of air, produces a sound
that can almost be called periodic, that is, repetitive in the sense that
one cycle follows another.
The second source of sound is that made by forming a constriction
at some point in the vocal tract with the teeth, tongue, or lips and forc-
ing air through it under high enough pressure to produce significant
turbulence. Turbulent air creates noise. This noise is shaped by the
vocal tract to form the fricative sounds of speech such as the conso-
nants f, s, v, and z. Try making these sounds, and you will see that
high-velocity air is very much involved.
The third source of sound is produced by the complete stoppage of
the breath, usually toward the front, a building up of the pressure, and
then the sudden release of the breath. Try speaking the consonants k,
p, and t, and you will sense the force of such plosive sounds. They are
usually followed by a burst of fricative or turbulent sound. These three
types of sounds—voiced, fricative, and plosive—are the raw sources
that are shaped into the words we casually speak without giving a
thought to the wonder of their formation.

Vocal Tract Molding of Speech
The vocal tract can be considered as an acoustically resonant system.
This tract, from the lips to the vocal cords, is about 6.7 in (17 cm) long.
Its cross-sectional area is determined by the placement of the lips, jaw,
tongue, and velum (a sort of trapdoor that can open or close off the