[Scott Dorsey]

John Williamson wrote:
Not really. What I call EMI is usually low frequency, directly induced
in the coils, while RFI is a modulated higher frequency, which normally
only affects circuits with a semiconductor or other rectifying component
in them, such as a preamp.

EMI is everything under the sun. E-field coupling, B-field coupling,
RF coupling. Anything that isn't directly acoustic is EMI.

RFI is RF coupling, one specific kind of EMI.

I find it a useful distinction to draw, as the cures are different.

The OP describes a hum or a buzz, rather than the typical "chirping"
sound of a cellphone being picked up, so I was guessing at LF, possibly
from a choke, fluorescent light or (Unlikely) a SMPS near the
microphone, which doesn't have a humbucking coil. It's hard to get
interference from cellphone transmissions, even from base stations,
more than a few feet from the microphone due to the low power and
inverse square law of propagation, though an experienced movie sound guy
can often tell what make of cellphone is about to ring on set.....

What he describes is consistent with magnetic pickup. Which is one very
specific case of EMI... but EMI is really a catch-all term.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."