gentlevoice1 said:
why ribbon microphones typically go no longer than ~15 kHz upwards in frequency. Is it because of the size of the membrane + magnet structure or is there another reason?
Size of structure plays role, but slightly more complicated.
HF roll-off of ribbon microphones is rather sharp. Sharpness sugests, that it is sound field deformation effect.
(Ribbon velocity microphone is mass-controlled and mass of the ribbon only lowers sensitivity in MF)
Another effect is HF enhancement in the condenser microphone. Deformation of sound field in front
of microphone causes velocity zero and then pressure rise at HF (theoretically 6 dB).
Frequency (and polar diaphragm) of this rising can be changed by body attached to mic (Neumann M50 have sphere,
Schoeps have disc, DPAs have set of different things...)
In the velocity ribbon microphone situation is dual, because ribbon microphone picks-up velocity.
deformation of the soundfield (caused by pole pieces) causes HF loss.
Some ribbon mics have also differences in frequency when this occurs (RCA have narrow pole pieces, EMI have holes in the pole pieces,
older RESLOs? have wide pole pieces, but modification in the field is not so simple as in the capacitor mic.
Pole pieces thickness plays opposite role and serves like concentrator.
The effects together causes that frequency modulus characteristic of ribbon mic is somewhat evenly wave-shaped at the HF.
.... added ....
Effect of sound field deformation is used also in another velocity converter
(not transducer) - hot wire microphone:
http://www.microflown.com/products/standard-probes/pu-regular.html
Here are two wires nearly together (made by some micromechanic technology). These two wires are lower resistors in the Christie (...Wheatstone) bridge. DC (may be also high frequency AC - less problems with 1/f noise) current goes through the bridge and heats the wires. Air flowing between the wires causes heath exchange and then imbalance the bridge (platinum wires have thermally dependent resistance) and output signal
is present at the diagonal.
Of course, mass of the wires makes HF range poor and then cylindrical concentrators (which are not pole pieces) are used to equalise it in some frequency band. It looks nearly like ribbon mic, only each concentrator have different collour. It is for rear/front discrimination (and also because the transducer (the wires) have base plate and thus is not perfectly symmetrical.).