Page 115 - The Master Handbook Of Acoustics

90 CHAPTER FIVE

Sometimes people lose their voices. Perhaps the vocal cords are para-
lyzed, or the larynx was removed surgically. For these people, the
Western Electric Company offers a prosthetic device, which when
held against the throat, produces pulses of sound that simulate the
sounds produced by the natural vocal cords as they interrupt the air
stream. This battery-operated device even has a pitch control for con-
trolling “voice” pitch. Then the tongue, lips, teeth, nasal passages, and
throat perform their normal function of molding the pulsed noise into
words. Even if the overall effect has a somewhat duck-like quality, it
enables the user to speak by shaping the noise appropriately.

Sound Spectrograph
An understanding of speech sounds is necessary to understand how
the sounds are produced. Speech is highly variable and transient in
nature, comprising energy chasing up and down the three-dimen-
sional scales of frequency, sound level, and time. It takes the sound
spectrograph to show all three on the same flat surface such as the
pages of this book. Examples of several commonly experienced
sounds revealed by the spectrograph are shown in Fig. 5-1. In these
spectrographs, time progresses horizontally to the right, frequency
increases from the origin upward, and the sound level is indicated
roughly by the density of the trace—the blacker the trace, the more
intense the sound at that frequency and at that moment of time. Ran-
dom noise on such a plot shows up as a gray, slightly mottled rec-
tangle as all frequencies in the audible range and all intensities are
represented as time progresses. The snare drum approaches random
noise at certain points, but it is intermittent. The “wolf whistle”
opens on a rising note followed by a gap, and then a similar rising
note that then falls in frequency as time goes on. The police whistle
is a tone, slightly frequency modulated. Each common noise has its
spectrographic signature that reveals the very stuff that character-
izes it.
The human voice mechanism is capable of producing many
sounds other than speech. Figure 5-2 shows a number of these as
revealed by sound spectrograms. It is interesting to note that har-
monic trains appear on a spectrogram as more or less horizontal lines
spaced vertically in frequency. These are particularly noticeable in
the trained soprano’s voice and the baby’s cry, but traces are evident