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Microphone Choices: The Bullet Train

"At The Harmonica Microphone Bench" with Fritz Hasenpusch

by Fritz Hasenpusch

Shure Bros. of Chicago got on board the Bullet Train in 1939 and they're riding it still today. It's fair to say there have been some serious hairpin turns and even gaps in the track along the way. But, let's take a look at a major fork in the road of Shure microphone development that was to yield two parallel yet distinctly different paths of travel: Introduction of the previously mentioned "magnetic unit," or the Controlled-Reluctance (CR) and Controlled- Magnetic (CM) element. I'd considered putting a "/" between those two names rather than "and" as they are units of the same design operating under the same principles, but sharing two labels. Players seem to perceive of the two differently, so it's "and" unit further notice. Since the CR label appeared first, we'll use it here…

Shure's "fork in the road" occurred in 1949 with the introduction of their wartime-developed Controlled-Reluctance element into civilian use via a number of their economy-minded communications mics, most notably the new 520 "Green Bullet." Although Shure had strayed from the crystal path two years earlier when they marketed the model 52 "Econodyne," which incorporated a massive dynamic element (YES! A Shure bullet, factory-equipped with a dynamic element!), the great majority of their La Salle Headlamp Factor mics contained the crystal elements licensed from the Brush Development Corporation. Introduction of the CR-equipped 520 provided a wise choice in applications where a fragile crystal dare not tread. The 1949 Shure brochure entitled "Controlled- Reluctance… A New Microphone Principle", refers to its ruggedness and economy by suggesting its suitability for "low-cost Southern or tropical installations."

The Shure CR may have constituted a new approach to microphone design, but take a look at its inherent features and you'll find striking similarities to a much earlier audio development: The cone speaker. The Western Electric 540, 560, and the similarly designed Crosley Super Musicone speakers share many of the physical properties utilized in the Shure CR. Unlike modern speakers that are driven by a circular voicecoil positioned within a magnetic gap and attached to a speaker cone (the reverse application of a dynamic mic's construction), these early cone speakers possessed a central single armature to drive the cone and reproduce soundwaves. With the Shure CR element, the "cone" is the collector diaphragm, and the attached central armature's resulting movement causes variations in the unit's magnetic field, thus producing an electronic signal that can then be amplified. Pretty clever, very rugged. And it just so happens that the CR's design possesses other properties that harp players have found to be advantageous as well: Output nearly as hot as a crystal (-52.5db) and a frequency response that, while designed for human speech, is right in the harp's power alley (100HZ-9000HZ) without adding problematic treble frequencies. Shure's fork in the road unknowingly put another weapon into the harpster's arsenal.

Some thoughts on the numbers game of response curves, applied audio perceptions and comparisons, next time on the MIC BENCH.

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For pictures and descriptions of most of the microphones listed visit http://www.harmonicamasterclass.com/vintage_collection.htm

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To contact Fritz for his Custom Mics or Repair email him at
harpmicman@earthlink.net