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SOUND REPRODUCTION 277
properties of the upholstery, are arranged to be as nearly as possible equal to the sound absorbing qualities of persons sitting on these seats. It is rather a sad thought to realise that the delightful cushions in some theatre seats are designed for our hearing comforts and not our physical ease !
Sound Fading
Returning now to projectors, it will be obvious that the change-over from one machine to the next, in order to maintain a continuous performance, must be accomplished without any peculiar noises occuring due to the imput signals to the main amplifier being transferred from one pre-amplifier to another. This is accomplished by using a sound fading device consisting of two potentiometers, connected in parallel, and each controlling the volume from one sound head. Both units are 'ganged together' and operated by a single control knob. Should the wiring break or burn out, an automatic auxiliary resistance may be immediately brought into service which will maintain a constant volume, corresponding to the centre step on the fader setting scale.
Sound films contain a printed instruction to the operator indicating the correct setting to use with any particular film. For instance, the leader film may contain the following information 'Play sound eight or nine Db above M.G.M. average'. This would indicate to the operator that, assuming he has made a careful check or has been told the average fader setting to use with M.G.M. tracks in his theatre, he must run this particular film with the fader set above this average. There is a 12 decibel difference between fader steps 1 and 2, a 6 decibel difference between steps 2 and 3 and also between 3 and 4; all other steps cause a difference of 3 decibels. Assuming his normal setting to be step 9, it will be necessary to increase this to step 12 when running a print requiring 8 or 9 Db above this average. The fader is so designed that rotating the control reduces the volume from one machine to zero and, by still rotating the control in the one direction, then increases the volume from the second machine. If, therefore, the first reel is to be run at step 9 and the second at step 12, the operator will be required to move the fader setting from position 9 on the one half of the scale smoothly down to zero and up to setting 12 on the opposite half of the scale, and at the moment when the two machines are changed over.
Having mentioned decibels once more it may be as well to refresh our memory regarding these units. As we know, air pressure waves are created by sounds and, if we consider two sounds, the