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30i CINEMATOGRAPHIC ANNUAL
stances and are due to the production of a new tone having a frequency equal to the sum of the frequencies of the original tones.
VARIATION OF INTENSITY WITH DISTANCE
In free air the intensity of sound falls away in proportion to the square of the distance. Thus the intensity at 50 feet is onefourth of that at 25 feet. But in rooms there is a marked tendency for reflections from floor, walls, ceiling, etc., to maintain the intensity at remote points.
INSULATION, TRANSMISSION AND ABSORPTION
Sound is transmitted best by homogeneous media. If air is non-homogeneous due to clouds, fog, irregular currents, etc., sound transmission will be interfered with, and the same action takes place in solids. Insulation of sound is thus enhanced by discontinuities such as alternating layers of solid and air, and hard and soft materials alternately in contact in all type of construction. Very solid and heavy construction will serve to prevent sound from setting up vibrations.
Absorption takes place where material is porous and considerable air friction is produced. It also takes place when inelastic masses are set in motion by the sound.
Recent tests have shown improved absorption of walls when a perforated front wall is set in front of a rear surface of felt, an inch or so of air space being left between. The air holes and pockets cause friction and also produce absorption due to certain resonant actions.
CONVERSION OF SOUND INTO OTHER FORMS
Sound has been described as a moving pressure wave, or series of waves, in the air (or in liquids or solids). It thus involves a mechanical motion of the medium in which it travels.
When a sound strikes the diaphragm of a transmitter or microphone it makes the diaphragm move to and fro in the same wave form as the sound itself, and the arrangement in the microphone is such that this motion of the diaphragm causes an electric current to vary, in exactly the same way, so that we now have the variations of the electric current substituted for the original sound.
This varying electric current can be transmitted over wires