Movies for TV - Early Television Foundation

MOVIE- MAKING EQUIPMENT 81
Sound, or acoustics, to give it its proper name, is an important
and somewhat complicated subject. Fortunately, the principles of
sound waves are much more easy to understand than those of
electro-magnetic waves since it is possible to feel the physical effect
of a loud, bass note from the draught it produces. When sound is
produced, it is caused by the very rapid
movement back and forth
of some form of string or diaphragm. The movement of the vibrat-
ing object causes the air surrounding it to be alternately compressed
and rarefied. This starts a series of waves radiating from the object
in all directions. Each wave is one complete compression and rare-
fication, generally known as a cycle. If a stone is dropped into a pool
of water, waves are seen to diverge in all directions in the form of
concentric circles from the point of impact. These waves also travel
through and under the water, but with less strength and cannot be
seen. But the water behaves in the same way as the air. If there
were no air surrounding a vibrating object the "noise" it made could
not be heard. In the same way, if there were no water in the pool,
there would be no splash ! Obviously !
These waves can be reflected in the same manner as light rays,
or water waves. Most acoustic measurements are predicated on free
space conditions: that is, an infinitely large area where there are
no objects to cause reflections, and therefore all the sounds heard
and recorded are caused by the original vibration only. When a
sound is reflected, it sometimes arrives at the microphone in or out
of phase with the direct signal and blends with it in such a manner
that it reinforces and improves it or else has a deleterious effect.
Thus, although studios are designed to be acoustically "dead,"
the addition of scenery often causes unwanted reflections of sound
which ruin the quality of the recording. As a rule, it is advisable to
avoid having glossy or hard surfaces, or actors themselves, close
to the microphone, for these are among the best reflectors known.
The actual power produced at the microphone diaphragm is
almost infinitesimal, but it is sufficient to move the diaphragm
enough to produce a variation in the electrical circuit to which it
is connected. It is impossible to see the diaphragm move when in
normal use. There are many types of microphone. Among them