RCA Varacoustic

Introduced in the mid-to-late 1940s, the RCA Varacoustic was designed as an adaptable ribbon microphone for broadcast, recording, and public address applications. Its distinctive feature was the the mechanically adjustable polar pattern which offered sound engineers a practical and straightforwardmeans to control pickup directivity.

Manufactured both in the United States and in the UK, RCA produced versions under model numbers including SK-50. MI-6203, MI-6237-D, MI-12034 and MI-12034-A.

Though the US and UK-made units have identical internal components, the casing shows slight differences. The American version features two bolts centrally located on the casing’s rear for assembly, whereas the UK version uses three bolts, two at the top corners and one centered at the bottom, each secured from the front. The British made version carries the Master Index no. MI-6203-CX

The better known RCA 77 D series, used the same motor design but improved on the polar pattern adjustment.

From Hi-Fi Yearbook, UK, 1959:

RCA Great Britain Limited, Lincoln Way, Windmill Road, Sunbury-on-Thames, Middlesex. Tel. : Sunbury-on-Thames 3101 . 

Varacoustic LMT. 6203C. Ribbon. 

Response 60-10,000 Hz. 

Source imp. 50, 250 and 600 ohms. 

30-ft., 2-conductor shielded cable. Stand as required. Price £22. 10s.

Further manufacturer specifications for each of the varacoustic models can be found in the original technical data sheets, linked in the Further Reading section. These documents have been generously scanned and made publicly available by Stanley Coutant on his excellent reference website, www.coutant.org.

The microphone features a mechanical shutter mechanism at the rear of the microphone body enabling the user to adjust the microphone’s polar pattern characteristics. The shutter provides adjustment between marked positions labeled P (Pressure), U (Unidirectional), and V (Velocity), with additional intermediate settings in between.

This system consists of a variable aperture to an acoustic labyrinth that can be adjusted to partially or fully restrict airflow to the rear of the ribbon. By altering the acoustic impedance at the rear entrance, this effectively changes the phase and amplitude of the sound reaching the back side of the ribbon. In theory this adjusts the microphone’s directivity from bi-directional towards a more unidirectional response, however in practice the effect is less distinct. This adjustment also effectively alters the microphone’s frequency response and can therefore be used for tone control as well as for adjusting directivity.

Internally, the Varacoustic shares its ribbon motor design with the later RCA 77-D microphones, though it uses a different pattern control mechanism.

A detailed post about the Varacoustic at the Coutant website

A post about the Varacoustic at Vintage Microphone World

The Xaudia blog discussed RCA Master Index numbers.