Perceptual Coherence : Hearing and Seeing

344 Perceptual Coherence
Percussion Instruments and Impact Sounds
The wide range of complex and nonperiodic sounds for the percussion instruments
follows the same vibration principles described above. All such
instruments have many periodic vibration modes, and all complex vibrations
can be understood as the summation of the vibration of the individual
modes. The amplitude of each mode resulting from striking the instrument
depends on the point of excitation (like that for a vibrating string), the area
of the impact, and the strength of the excitation. For steel drums, multiple
notes are created on the concave surface by pounding the location of each
note to a different shape and thickness and then heat treating the entire surface.
The excitation of a single note at one location excites many other
notes and that gives the drums their unique sound.
On the whole, the attack times for percussion instruments are short;
there is little or no steady state; and the decay times for each mode will
be exponential although different. What this means is that the percentage
of reduction of energy for each mode within each time period will be
constant. If the energy decays to 50% within the first 50 ms, then it will
decay another 50% from 50 to 100 ms (to 1/4), another 50% from 100 to
150 ms (to 1/8), and so on. Again, on the whole the higher-frequency vibration
modes decay more rapidly because they involve more frequent and
severe bending that creates greater internal friction.
Speaking and Singing: Air Vibrations Within Nonuniform
Hollow Tubes
The voice is our most expressive instrument; it creates human interaction.
But fundamentally, the voice is no different from any other instrument. The
air expelled through the vocal folds creates the source vibration. The source
vibration is coupled to the vocal tract, composed of the mouth, lips, tongue,
and nose. The vocal tract acts as the sound body filter that selectively amplifies
the spectrum of the source vibration. The resulting sound is then
radiated from the mouth and nose.
Source Excitation
The vibration of the vocal cords creates the frequency spectrum of the
source excitation. The speaker controls the vibration frequency of the vocal
cords by varying the tension of the cords. The speaker can also control the
spectrum of the source by varying the lung pressure. At low pressures,
the flow pattern is weak, continuous, and sinusoidal; at higher pressures,
the cords can remain closed for up to 70% of the period. The source becomes
a series of air puffs. The similarities to other instruments should be