An STC moving coil mic developed circa 1935, some years after the STC 4017A
microphone.
The internal acoustical arrangements were less elaborate in the 4021 than
in the 4017 but the response at the upper part of the range was less dependent
on the angle of incidence of the sound, which was an advantage as both mics
were intended to be omni-directional and the 4021 was indeed more omni-directional
at HF than the 4017.
Because of its shape it was known as the 'apple and biscuit' mic. Some non-BBC
writers have claimed it was called the 'ball and biscuit' mic within the BBC
- this is ill-informed nonsense!
The mic was designed to be used with 'biscuit' horizontal, but there were
few studio applications for an omni-directional mic. (Though Gerald
Jackson reports that local radio used the 4021 in the middle of the studio
table for 'round table' discussions - "we had nothing else!", he
says.) Useful outdoors as an effects/ambience mic, indoors it was mainly used
in echo rooms and as a talkback mic. As a talkback mic, it was never used
as the manufacturers intended; it was either fitted flush to a panel on the
mixer or used with the 'biscuit' vertical on a swan neck that could barely
support its weight.