The Controlled Reluctance microphone (aka Controlled Magnetic) was developed by Shure during World War 2 to fulfil the need for a battle announce microphone that could operate reliably at extremes of temperature and humidity. (Subsequently also perfect for Korea and Vietnam!)
Although the Controlled Reluctance design was in some respects similar to a conventional dynamic in other ways it was significantly different.
The 2 diagrams below illustrate the differences.
Here, in response to an email enquiry, is an explanation from Shure of the working of this microphone
On 3 May 2017 15:06, “Shure Europe” <firstname.lastname@example.org> wrote:
Response By Email (Michael P) (05/03/2017 09:06 AM)
Controlled reluctance is a variation of a dynamic mic. The controlled reluctance mic diaphragm connects to a small lever made from ferrous material. The other end of this lever is positioned inside of a stationary coil of wire. Surrounding the coil of wire is a stationary magnet. As the ferrous lever is moved by the mic diaphragm, the lever disturbs the magnet field. This induces an AC signal (the audio signal) in the coil of wire.
This type of mic was originally designed for military applications.
The CR/CM elements have a high output (making them suitable for transmission over large distances) and require no additional transformer. They are therefore cheaper to manufacture than a conventional dynamic.
After WW2 Shure introduced several models for the civilian market, including the famous Green Bullet (still popular with harmonica players to this day). These were mostly budget PA mics intended for speech applications such as paging and announcements. Whilst not being particularly noted for high quality audio, their main selling points were cheapness and reliability. It is therefore not hard to see why these sturdy, affordable mics soon found favour with musicians and singers.
Apart from being highly profitable, Shure’s military communications contracts had the additional spin-off of enhancing a lasting reputation for reliability. Indeed, the company have often boasted that all of their products are tested to military standards (MILSPEC). Even though the Model 430 was made for the civilian, domestic market it nevertheless trades on its military heritage with the name ‘Commando’ and it’s distinctive, camouflage green head!
Data Sheet for the Shure Commando series.:- us_pro_415_ug
Shure Microphones :- 1957 Catalogue
So what does it sound like? CLICK HERE for a short clip of spoken word and Blues Harmonica.