The BBC PGS
pressure gradient ribbon microphone
and predecessor of the Coles 4038
Developments of magnetic materials made possible the PGS mic, which was introduced in 1953. This weighed only 2½ pounds, and although the ribbon was only one inch long compared to the AXBT’s 2½ inches, the output was only 4 dB lower.
One problem with ribbons is that the HF response could easily drop off. Placing a perforated fibre baffle on each side of the ribbon within the housing gradually lifts the HF by means of resonance.
The shorter ribbon allowed the mic to be tilted to a greater degree than was permissible with the AXBT, which was limited to 20 degrees. At greater angles than this there was cancellation of the higher frequencies due to the length of the ribbon being similar to the wavelength of the incident sound, although in the days before EQ was available in the studios, tilting the mic could cure a speaker of excessive sibilance. The same could not be done with the PGS.
Why PGS? It stands for Pressure-Gradient Single, the “S” referring to the single magnet.
The PGS, although an attractive-looking mic, was too expensive to manufacture commercially. Therefore a cosmetically different version was manufactured by STC and coded 4038 by them.
The PGS had a pair of perforated fibre plates located either side of the ribbon which, by resonance, maintained the HF response to around 18 kHz. For the 4038 the perforated case was recessed on both sides to perform this function, thus achieving quite a big cost savings.
—Barry Taylor, the BBC
Mr. Beardsley shares the following: “The 8A.-8 on the stem of the
mic refers to Studio 8A and Broadcasting House, London.”
Photos provided via the courtesy of Roger Beardsley
Published in December 1955, this twenty-four-page monograph is available as a downloadable PDF here:
Permission to provide this link was granted by the BBC on April 27, 2010.