624 BELL SYSTEM TECHNICAL JOURNAL 



partly in the underlying metal and partly in the alkali metal film, the 

 relative proportions varying with the film thickness. 



The Lapel Microphone and Its Application to Public Address and 

 Announcing Systems.* W. C. Jones and D. T. Bell. Many speakers 

 find it difficult to use the conventional type of microphone, because of 

 the restrictions that it imposes upon their freedom of movement. A 

 microphone, known as the lapel microphone, designed to be attached 

 to the speaker's clothing, has been developed for overcoming these 

 limitations. 



The vibratory structure of the lapel microphone is designed to have 

 low mass and stiffness, and to resonate at a comparatively high fre- 

 quency. The resilient support of the diaphragm adds sufficient 

 mechanical resistance to prevent the occurrence of a prominent peak 

 in the response at the resonance frequency. Means are provided for 

 reducing extraneous noise to a minimum. A part of the sound reaching 

 the microphone, due to body vibration, is rich in low frequencies and 

 must be attenuated, otherwise the quality of transmission will be 

 unnatural. This attenuation is accomplished in the coupling trans- 

 former, which, together with the apparatus required for suppressing 

 clicks, for indicating when the circuit is in operation, etc., is mounted 

 in a control cabinet. A flexible cord connects the microphone to this 

 cabinet. 



It is expected that the lapel microphone will find application in 

 theaters, churches, convention halls, lecture and banquet rooms, and 

 the like, where public address systems are now employed. It also can 

 be applied in connection with other sound recording and reproducing 

 equipment where the background noise, characteristic of carbon 

 microphones, is not a limiting factor. 



Vacuum Tube and Photoelectric Tube Developments for Sound Picture 

 Systems.^ M. J. Kelly. This paper reviews some recent vacuum 

 tube and photoelectric cell developments which are of interest in 

 sound recording and reproduction systems. An indirectly heated 

 cathode triode is described, in the output circuit of which the current 

 components due to the a-c. power supply of the heater have been 

 reduced approximately 20 decibels below previously obtained levels. 

 This tube makes it possible to use an a-c. supply in amplifiers having 

 flat frequency characteristics with over-all gains of the order of 100 

 decibels. The microphonic disturbances in vacuum tubes are dis- 

 cussed. A measuring system for evaluating the microphonic noise 



* Jour. S.M.P.E., September, 1932. 



5 Jour. S.M.P.E., June, 1932. 



