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SPEAKERS 585
promptly rushes, when it moves inward. (See Page 414.) In consequence, the vibrations of any unsteady current supplied to the coils of the receiver are converted, by the receiver action, into similar vibrations in the nearby air, which the ear will apprehend as the sound of words or music.
This instrument, however, is not capable of more than ear-volume unless it is equipped with a horn. Without a horn, high volume merely makes it rattle. The reason is not hard to understand.
When the receiver is operating under normal conditions, without a horn, air pushed out of the way when the diaphragm moves outward is pushed in all directions, and therefore offers comparatively small resistance to the diaphragm's motion. Similarly, when the diaphragm moves inward air rushes from all directions into the vacuum it leaves behind, and there is no very great disparity of air pressure on the two sides of the diaphragm to hinder its free motion. In short, the diaphragm, without a horn, is a motor moving without a load, and it moves easily and freely. If the volume is made high, the diaphragm will strike against the pole pieces each time it moves inward, and thereby add a rattling noise to the sound it creates.
When a horn is added the motion of the diaphragm meets greater resistance, since the' neck of the horn confines the surrounding air and limits its freedom of motion. The diaphragm is then a motor operating under load. It moves through a smaller distance but imparts motion to a greater quantity of air, since the air immediately in front of it cannot slip away to either side. Therefore it creates a higher volume of sound (imparts greater energy to the air) without moving far enough to strike against its pole-pieces. In just the same way a motor running at high speed without load may wreck itself in doing less work than it would running safely at low speed with a full load.
Tim: Magnetic Loud Speaker
I 2 i Magnetic loud speakers are used in theatres as