2024 is the 150th anniversary of the moving coil dynamic microphone.
In January 1874, Ernst Werner Siemens, a citizen of Berlin, Germany, filed an application for a U.S. patent, entitled “Improvement in Magneto-Electric Apparatus.” This patent, No. 149,797, was granted April 14, 1874.
Yes, this is the person who founded the Brobdingnagian company bearing his name – Siemens Global, an employer of over 320,000 people worldwide. Its 2023 revenues were nearly seventy-eight billion dollars.
E.W. Siemens, 1885
Every Tech Tip reader has used, sold, touched, or heard a Shure moving coil dynamic microphone. Our livelihoods rely, in some part, on this microphone type. Today, we examine the wellspring of moving coil mic design – das Patent von Herrn Siemens.
U.S. Patent 149,797
Siemens refers to his invention as an apparatus, not as a microphone, for by doing so the patent’s scope would be limited. The following text, selected from the patent, has been condensed and simplified.
A permanent magnet can be constructed with a very narrow space between its two polar surfaces [north pole & south pole].
Introducing an electric circuit [coil of wire] into the narrow space and moving the circuit to and fro will set up electrical currents in the circuit. [A moving coil microphone]
Conversely, when electrical currents are sent through the circuit [coil of wire] it is caused to move. [A moving coil loudspeaker/earphone]
My invention applies either for obtaining electrical currents from the mechanical movement of an electrical coil [a microphone], or from obtaining the mechanical movement of an electrical coil from electrical currents transmitters through it [a loudspeaker].
Taken from the Siemens patent, the following diagram is a cut-away side view of his apparatus.
Siemens patent drawing, 1874
The image below is based on the Siemens patent but rotated 90 degrees left from the drawing above.
Moving coil mic element
Telefunken, a manufacturing firm located in Berlin and funded by Siemens, produced one of the first professional moving coil microphones, the Model ELA 230; it dates from the early 1930s.
Telefunken Model ELA 230, circa 1930
Impeding the development of moving coil mics were the high cost of magnets and the anemic flux strength of magnets from this era. As prices declined and magnets improved, moving coil mics became viable. Elegant in its simplicity and exceptionally reliable, the moving coil dynamic mic has been a staple of the Shure product line for over eight decades.
Reader Quiz: What is the model number of the first Shure moving coil mic? The answer is found at the end of today’s Tech Tip.
Related information: Ernst Werner Siemens
Arcana – mysterious or specialized knowledge
Circa 1827, Sir Charles Wheatstone coined the word microphone. A combination of the Greek for micro [small] and phon [sound], the new word then described a purely acoustic device that augmented faint sounds, much like a stethoscope. Wheatstone’s microphone consisted of two thin rods that conveyed vibrations to the ears.
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Catalog description of the first Shure moving coil microphone: Very latest in dynamic microphone design – the first high-quality, low-cost moving coil type dynamic with true cardioid unidirectional characteristics. Eliminates feedback, audience and background noise. Greatly reduces reverberation pickup. Gives smooth, wide-range reproduction from 40 to 10,000 cycles over a wide angle at the front yet is dead at the rear. Solves many troublesome sound pickup problem in public address recording and broadcasting. Rugged shockproof construction with specially suspended, double windscreened, moving coil-system. Practically unaffected by heat and humidity. Ideal for severe outdoor as well as indoor service. Ultra-modern, streamlined design. Finished in beautiful satin chrome head. Built-in cable connector.
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Dynamo is early terminology for an apparatus that converts mechanical energy to electrical energy using some form of magnet force. Dynamic as a synonym for moving coil has etymological roots in dynamo.
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Answer to the Reader Quiz
Below is the first Shure moving coil microphone.
Shure Unidyne I Model 55, 1939
I choose to believe that every Tech Tip reader already knew the answer to the quiz.
If not, don’t tell me…
