Projection engineering (Jan 1932-Mar 1933)

FEBRUARY, 1932 Page 19 The buttonhole microphone By W. C. Jones* A Mike fastened to coat lapel enables speaker to move around on rostrum MANY speakers, unfamiliar with the use of the conventional stationary microphone, mar the public address programs in which they take part by straying away from the instrument. To permit the speaker to move about more freely, a microphone has been developed which is worn on his clothing and moves with him. This microphone is an adaptation to public-address use of the essential elements of a transmitter recently developed for operators. It is only about an inch in diameter and weighs about one and one-half ounces. A thirty-foot length of flexible cord provides *Transmission instruments development engineer, Bell Telephone Labs. ^INSULATING HOUSING Fig. 2. The lapel microphone is of the granular carbon type, with an unstretched cone-shaped diaphragm of aluminum. The two electrodes are stationary and are electrically insulated from the diaphragm. Fig. 3. Associated with the lapel microphones, an input circuit attenuates the lower frequencies which the microphone picks up from the speaker's chest, and prevents clicks when the microphone is cut in or out of circuit. the connection to the amplifier of the public-address system. The diaphragm is made of thin aluminum, formed into a cone to provide sufficient stiffness to cause it to vibrate as a unit throughout the frequency range of interest. A number of impregnated paper rings, about four tenthousandths of an inch thick, support the edge of the diaphragm. The dimensions of the recess into which these rings fit are such that the rings separate slightly from one another. This not only provides a resilient support for the diaphragm, but also adds a certain amount bf damping which reduces the effect of -resonance and improves the response characteristics. Unlike the carbon microphones now in use in public-address work, in which the diaphragm forms one of the electrodes, the diaphragm of the lapel microphone is insulated by a coating of phenol varnish from the granular carbon. Electrical connection to the carbon is made through two stationary cylindrical electrodes, insulated from each other by a ceramic barrier which also serves to define the current path through the carbon. In order to increase the life of the microphone and reduce the noise which results from moving it about, the space between the electrodes is filled practically full of carbon. A rubber covering for the microphone eliminates the disturbing noise which would otherwise result from rubbing against the speaker's clothing, or would be picked up through the clip which is provided for attaching it to the clothing. The clip is so arranged that the microphone can be attached to either the inside or outside of the breastpocket of a coat, or to the lapel. The latter position is preferable, for it brings the microphone closer to the speaker's lips and increases the intensity of the speech without adding to the interfering noise. In the case of a woman, the clip can be fastened to her dress with a pin. The frequency response characteristics of the lapel microphone compare favorably with those of the stretched diaphragm type now in use. Owing to the smaller size of the lapel model, less sound field distortion is introduced. When worn on the speaker's clothing, however, the instrument picks up sound as the result of chest vibration in addition to that reaching it through the air. The part due to chest vibration is rich in low frequencies, and if unattenuated results in a "deep" unnatural quality of transmission. A special input circuit has therefore been provided for connecting the microphone to the Fig. I. H. L. Lundberg demonstrates how, when the lapel microphone is in use, the jack and plug are placed in the coat pocket.