Theatre Design & Technology - Winter 1981 - 31

Synthesizing
Sound"
Effects

by John L. Bracewell

Anyone who has had to find the sounds of gunshots and
artillery for a play like Too True To Be Good, or the various
steam train sounds of Look Homeward, Angel, will be aware that
commercial sound effects recordings can be rather frustrating.
We have all pounded seat bottoms with belts, slapped pieces of
wood together, and even hyperventilated into microphones
trying to produce needed sounds-often to little avail. Now there
is a device which can solve a lot of those problems, although
nothing is ever perfect, of course. Electronic music synthesizers
have been used by rock bands for quite a few years, and almost
everybody recognizes their sounds; but synthesizers are good
for theatre sound, as well.
Synthesizers became practical only a few years ago, when
people like Moog and Buchla developed ways of letting small
changes in a simple DC voltage control relatively complex
parameters of sound-frequency, waveform, envelope, and
timbre. The control voltage could be supplied by a standard
organ-type keyboard with which most people are already familiar to some degree.
Synthesizers are made up of modular components including
oscillators, amplifiers, filters, and envelope-generators, all controllable by voltages from the keyboard, or from one of the
oscillators. Oscillators cannot control themselves, of course, but
one oscillator can control another oscillator or a filter module.
Envelope-generators also provide control voltages for oscillators, amplifiers, and filters; envelope-generators themselves
respond only to short pulses of control voltage from the keyboard. These pulses are called triggers.
For musical use, oscillators are the primary sources of sound.
They produce tones ranging from low to high in pitch. As in other
musical instruments, the pitch is selected by depressing a key
on the keyboard. But musical performance also needs rhythm,
so synthesizers also provide a noise source that can be shaped
by the filter to simulate sounds such as drums and cymbals.
Both the tonal and the noise sources of the synthesizer are
useful in producing theatre sound effects.

Understanding Synthesizers
The character of a sound is largely determined by its waveform, and waveform is a matter of how many individual frequency components (harmonics) a sound contains. This characteristic sound is called timbre. Waveforms may be created either by
adding individual frequencies together or by taking frequencies
away from an already complex sound. The latter is most often
used because it is far less expensive and an operationally less
complicated method than additive synthesis. Oscillators in most
synthesizers produce four waveforms: sine, triangle, sawtooth,
and square. The filter is used to subtract unwanted harmonics.
Control voltages change the pitch of the oscillator and sometimes vary the symmetry of the square wave.
Filters for synthesis are most often of the type called LowPass Filters. A low-pass filter cuts out all frequencies above a
certain point called the cut-off frequency. The cut-off frequency
can be varied up or down in pitch, either by a manual control or
by a control voltage. Usually, a selective circuit creates variable
regenerative feedback in the area of the cut-off frequency, so
that the filter can be made to ring or even to oscillate. This
feedback is called emphasis. Control for the filter can be taken
from the keyboard, an oscillator, or an envelope-generator.
The envelope-generator and the amplifiers most often function together to stop and start the sounds. Envelope is a
property of sound derived from the variation of intensity with
time. Envelope has four component parts: Attack, Decay, Sustain, and Release. The initials of these four components are
often used instead of the word envelope, so that an envelopegenerator is sometimes called an ADSR. Attack is the time
required for a sound to reach full amplitude as it begins. The
quickness or slowness of the attack determines whether a
sound seems struck, plucked, or aspirated. Decay is the time
required for a sound to drop from maximum amplitude to its
sustaining intensity. The length of the decay time plus the
amount of drop from maximum amplitude determines how
"sharp" the quality of a strike or pluck is. Sustain is the period
through which a sound holds at its sustaining intensity, and
Release is the time the sound takes to die away. Attack, Decay,
and Release are measured in anything from milliseconds to
seconds; Sustain is controlled by how long a key is held down.
Touching a key on the keyboard sends a control voltage to the
oscillator and a trigger to the envelope-generator. The envelope-generator then regulates the level of the amplifier to
produce the ADSR characteristic.
The noise source of a synthesizer simply supplies white or
pink noise. White noise is random energy at all frequencies of
the audible spectrum; pink noise is biased to produce slightly
more energy in the lower end of the spectrum. By filtering the
noise source and imposing an envelope on it, the synthesizer
creates sounds like drums, cymbals, maracas, etc., and it can
also produce sounds like steam locomotives, gunshots, jet
engines, and many others. The noise source is one of the most
useful parts of the synthesizer for theatre sound effects.
There are two basic synthesizer formats: patching types and
performance types. Patching types are made up of physically
unconnected modules which must be interconnected with patch
cords in order to set up the desired sounds. This is extremely
flexible but is too time consuming for live performance. Performance synthesizers provide the same essential modules, but are
connected through switches and knobs that can be set quickly.
Newer performance units are beginning to use memory control
to preset sounds. Performance-type synthesizers can produce
almost as many sounds as a patching type, but they are
somewhat biased in favor of tones commonly used by rock
musical performers.
Most theatre people probably ask, "How much synthesizer do
I need, and how much is it going to cost?" First of all, if you do
sound as a last minute thing after all the scenery and lighting is
finished, forget it! You'll never take enough time to learn to use
even the simplest synthesizer. If you spend more nearly adeTheatre Design & Technology

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1981