Western Electric No. 618A
moving coil microphone
Instructions for Use
In radio broadcasting, sound recording, and public address systems, a microphone is needed that has the utmost fidelity of pick-up, inherent quietness, stability, and mechanical ruggedness. These requirements are met by the Western Electric No. 618A Moving Coil Microphone. This microphone has a low electrical impedance, and may be connected to its amplifier by means of a suitable transformer; it may be used at a considerable distance from an associated amplifier. The microphone may be mounted in a suspension, table, or floor type of mounting.
The microphone has a thin duralumin diaphragm of low mechanical stiffness, which is clamped around its outer edge. To this diaphragm is attached a coil of aluminum ribbon. Motion of the diaphragm causes the coil to move in the magnetic field in which it is located. The magnetic field is maintained by a permanent magnet made from a high grade cobalt steel alloy. The use of the permanent magnet eliminates the necessity for supplying a polarizing voltage to the microphone.
Motion of the coil in the magnetic field, caused by sound pressures acting upon the diaphragm, induces a voltage in the coil that is proportional to the velocity with which it moves. A number of air chambers and slot openings connecting them have been associated with the diaphragm in order to obtain substantially uniform response over a frequency range from 35 to 9,500 cycles per second. One of these acoustic elements, in addition to exerting a control on the motion of the diaphragm, allows air to be transferred from the front to the back of the diaphragm. This eliminates effects due to changes in barometric pressure.
The design and principles of operation of the No. 618A Microphone are such that it is not affected appreciably by any changes in temperature likely to be encountered in its use. The electrical impedance of the No. 618A Microphone at low frequencies is approximately 28 ohms with a phase angle of nearly zero degrees. The impedance rises gradually to about 38 ohms with a phase angle of about 20 degrees at 10,000 cycles per second. The efficiency of this microphone is higher than that of the conventional form of condenser microphone. For the condition of a person speaking with normal conversational intensity at a distance of three feet from the microphone, an output level of −84 dB relative to a zero level of six milliwatts is representative.
All text above this point is from the Western Electric Company and Bell Telephone Laboratories Bulletin entitled Moving Coil Type Microphone and Associated Apparatus No. 618A Instructions for Use, which can be downloaded at the bottom of this web page. Considerably more information is contained in the download than is shown on this web page.
Additional information and technical details can be seen near the bottom of the BBC-Marconi Type AX Microphone page, which presents technical specifications for the STC 4017 Moving-Coil Microphone that are related to the Western Electric No. 618A microphone shown here.
Dennis Schrank comments:
The Western Electric No. 618A is an important historical mic. It was the world’s first dynamic mic that was developed by Bell Labs in the early 1930s. It quickly became a favorite of broadcasters everywhere. It was a great all-purpose studio mic and was the standard 1930s news conference mic that was used by FDR for his fireside chats.
The following eight images were obtained via the kindness of Mr. Schrank.
Download the Instructions for this mic.