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INTERNATIONAL PROJECTIONIST
VOLUME XI
NUMBER 6
DECEMBER 1936
STRUCTURE AND MAINTENANCE OF SOUND-PICTURE SPEAKERS
By JESSE A. COOK
THERE is probably no part of the theatre sound system that needs such careful attention as the horn and cone speaker units. These are commonly known as horn units, receivers, or loud speakers. Throughout this article the word "receiver" will be used to describe horn units, and "speakers" to describe cone type units.
Practically every theatre that has had sound pictures for three years or more has one one or more receivers that still are working and will continue to work for years if left in service but which should have had their diaphragms replaced by a competent receiver manufacturer months ago. This article will outline conditions that operate to damage receivers before the end of their normal useful life, and will cite simple steps in the care of typical receivers and economical methods of testing to disclose just which receivers are satisfactory for further use.
A receiver is a precision piece of apparatus and should be treated as such.
Before putting a receiver into service, or before removing from service, all screws should be tightened. Also, every six months all screws should be checked for tightness.
Retaining Plate Pressure
In typical receivers [Fig. 1] the diaphragm retaining member, 3, is held tight to the case cover plate, 7, with screws. These screws determine the pressure on the fibre diaphragm insulating rings, 4. If these screws are too loose, the retaining plate which is not dowel-pinned will shift so as to bind' the speech coil; if tightened down too much, the fibre rings and diaphragm will be damaged from the excessive pressure. The screws should all be tightened a little at a time until a torque of two foot-pounds is obtained on each screw.
As torque screw-drivers are made specially for factory use, a substitute will have to be used. A two foot-pound torque can be obtained by using the full strength of one hand on a screw-driver, having
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Speakers constitute one of the most important links in the theatre sound picture equipment chain, yet comparatively little attention has been paid to the servicing of these units. The author of this article is extremely well qualified for the task by reason of a wealth of experience, including five years with Western Electric Co., eight years with E. R. P. I., and, more recently, extensive field experience with the Co-Operative Sound Service Supply Co., of St. Louis.
I. P. hopes to be able to publish many more contributions by Mr. Cook, who will appreciate expressions of reader preference as a guide for future articles on sound reproducing equipment. — Editor.
a smooth wooden handle %" in diameter; or by using the full strength of just the end of the thumb and the forefinger on a screw-driver having a handle l1/^" in diameter. Do not use the full strength of the whole hand on a large handle driver.
Briefly, a diaphragm is like a drum head and bends back and forth thousands of times per second. After long