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SOUND REPRODUCTION 273
of the diaphragm and, due to its shape, the outer edges are able to oscillate in response to the impulse from the speech coil whilst the centre of the curve remains stationary. The speech coil consists of an aluminium tape wound edge-wise as shown in the illustration, and free to oscillate in the air-gap between the pole pieces. The flared throat, connecting the speaker with the horn, is so designed that the vibrations from the inner and the outer sections of the diaphragm will reach the neck of the horn in true phase relationship. The rear of the diaphragm is closed by a gauze and wool packing to reduce the possibility of back pressure from air pulses on this side. Such a back pressure might set up further sound waves which would not be in phase with those produced at the front of the diaphragm.
Where low output is required, such as in a radio receiver, the speaker unit may be directly mounted to a heavy baffle board but, where much greater output is required, such as that used in theatres, it is necessary to mount the speaker behind a correctly designed horn unit. The size and shape of such horns are of great importance if high fidelity reproduction is to be achieved. The most efficient shape is known as the 'exponential curve' which means, very briefly, that the cross sectional area of the horn is doubled for every 18-inches (45-7-cms) of its length. The length of the horn should be at least equal to the length of the longest sound wave it is designed to reproduce. For example, a frequency of 70 cycles per second has a wave length of approximately 15-feet (4-6: metres) and, therefore, the effective length of a horn required to reproduce such a note faithfully should not be less than this amount. However, it is not necessary that such a horn should be built in one straight length; it may be coiled around itself in a number of ways but, in all cases, the curvature should be as little as possible. The relatively high frequencies, such as those above 4,000 cycles, are often best reproduced from speakers with shorter horns and it is therefore common practice to install two types, the large, low frequency reproducer, and also smaller high frequency reproducers, usually affectionately known as 'tweeters' !
The Sound Screen
The banks of speakers are usually mounted near to the centre of the screen, each speaker being placed at different angles to a centre line down the auditorium to ensure balanced volume over the seating area. In the early days speakers were often placed just below the screen and in the orchestra pit, but it was soon agreed that such a position was not satisfactory and, in consequence, the
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