Projection engineering (Jan 1932-Mar 1933)

Record Details:

Something wrong or inaccurate about this page? Let us Know!

Thanks for helping us continually improve the quality of the Lantern search engine for all of our users! We have millions of scanned pages, so user reports are incredibly helpful for us to identify places where we can improve and update the metadata.

Please describe the issue below, and click "Submit" to send your comments to our team! If you'd prefer, you can also send us an email to mhdl@commarts.wisc.edu with your comments.




We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.

Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.

APRIL, 1932 The microphone in public-address systems By Charles Felstead* In this instructive article Mr. Felstead goes thoroughly into the subject of single and double button, and condenser microphone construction and operation THE sound pickup device that functions to transform energy from acoustical to electrical form is one of the most important instruments in any electrical system that transmits, records, or amplifies sound. The devices now available for this purpose are not very efficient in operation, nor is their fidelity of transformation uniform at all audible frequencies. It is necessary for the engineer in the field of sound amplification and transmission to have a sound pickup device that will produce an electric current having practically the same waveform and proportionate amplitudes as the sound-pressure waves that are to be amplified and supplied to the apparatus that will record or reproduce the effect of these waves, and so it is his duty to select the type of sound pickup device that is most suitable for the work to be done. The more nearly perfect the electrical copy of the sound wave, the better will be the quality of the recorded or reproduced sound. The Types of Microphones There are two main forms of this pickup device, which is known as a microphone, or transmitter, in general use at the present time. The original type of microphone is the solid-back carbon transmitter, and is of the carbon granule type; while the newer form of microphone that is superseding the carbon transmitter for all fine sound recording and reproducing work is called the condenser microphone because of its peculiar construction. The types of carbon microphones can be further subdivided into those having single and double cells of carbon granules, and known as single-button and double-button transmitters. The transmitters used in telephones, and in other places, where a microphone having great fidelity in the transformation of energy from acoustical to electrical form is not needed, are of the single-button carbon granule type ; while the double-button carbon transmitter is employed as the pickup device in radio broadcast stations, public-address systems, and in conjunction with other equipment requiring a high-quality microphone. However, chiefly because of the fact that it has less inherent "ground" noise, the condenser microphone has lately been coming into quite wide use in place of the two-button carbon microphone in radio broadcast stations, and it is now used almost exclusively as the pickup device in sound motion picture recording installations. The Simple Carbon Microphone The single-button, or single-cell, carbon transmitter was the first form of microphone to be employed for commercial purposes, and it is still used in telephones and intercommunicating telephone systems in the sound recording studios and in large public-address systems, so it will be well to describe it briefly before going on to a discussion of the two types of high-quality microphones. This simple transmitter con Page 9 sists of an elastic metal diaphragm mounted on a rubber ring that is held tightly against the frame of the transmitter. A carbon block is attached to the center of the diaphragm, and directly opposite it another similar carbon block is fastened to the frame. A band-shaped mica washer is wrapped around these two blocks, and the chamber thus formed between the blocks is filled with many small, polished carbon granules. This construction will be evident from an examination of Fig. 1. The carbon blocks form the two terminals of the transmitter, and a battery or other source of e.m.f. is connected in series with the microphone and the outgoing circuit. For some particular types of work, the microphone and battery are connected in series with the primary of a microphone transformer, as may be seen in Fig. 2. When the alternate rarefactions and compressions of the air that constitute a sound wave encounter the flexible diaphragm of the transmitter, they cause it alternately to be sucked forward by a low pressure area in front of it and then forced backward by the following compression of the air. The vibration of the diaphragm in tune to the frequency of the sound wave varies the physical pressure on the packed carbon granules. This changes the resistance of the transmitter — the greater the pressure on the carbon granules, the lower the resistance— and so varies the amount of current flowing in the circuit in which the microphone and battery are connected, for the amount of current that flows varies inversely as the resistance of the circuit. As we have said, this single-button carbon transmitter is of the low-quality type, and therefore the current variation that is created in it by a sound-pressure wave striking the *Sound Engineer, Universal Pictures Corp. Fig. 1. Courtesy Bell Telephone Labs Cross-sectional view of the 387 type double-button carbon microphone.