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duced by your equipment, however, may range from about 40 to only 8,000 cycles. If it can reproduce sound at 10,000 cycles you can congratulate yourself as being the owner of a fairly good machine. But can a single speaker do full justice to such a range of sound? The answer to that question is — very seldom.
Sound, as a natural phenomena, has a number of peculiarities, and one of these is that many of the upper frequencies are more "directional" than those lower down the scale. These frequencies can actually be "beamed" at the audience with great affect, and that, in a nutshell, is why you must tilt the loudspeaker slightly downwards. But a second speaker, for treble reproduction only, will go far towards helping you to give an excellent rendition of all the sound printed upon the sound track.
We have already learnt that the photo-electric cell produces a current which varies in step with the light modulated by the sound-track, and that this current is next given a slight increase in power by the pre-stage valve. What happens next is that each succeeding valve also gives it an increase until it is of sufficient power to work the loudspeaker. This particular function of the valve, however, must not be taken to mean that there is only one circuit carrying the current to be increased, or amplified.
Each valve has its own circuit, and each valve produces an increase in power in the circuit next to it. Amplification, then, is a matter of increasing power by stages. (Valves have other functions, of course, but further theory would be out of place in a book such as this).
The powerful current finally produced, still carrying the frequencies originally produced by the photo-electric cell, is next carried along the speaker cable to the heart of the loudspeaker.
Loudspeakers also vary in design, but each will enable the current to actuate in some way the speaker diaphragm. Sound is then produced by the diaphragm vibrating in sympathy with the current frequencies. The air carries these vibrations until they reach our ear-drums, which also vibrate and thus enable us to hear.
The efficiency of any loudspeaker, then, depends on the ability of the diaphragm to vibrate at an enormous yet varying speed to produce those waves of sound. Any damage to the diaphragm, or the adherence of dirt, for instance, may therefore seriously
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