Microphone Transducers

A dynamic microphone works like a small loudspeaker in reverse. Sound moves a diaphragm attached to a coil, and the coil moving in a magnetic field generates a voltage. The mechanism is simple and rugged, needs no power, tolerates heat and rough handling, and accepts very high sound pressure levels, commonly well over 140 dB SPL. These properties explain its use on snare drums, guitar cabinets and loud vocals.

A condenser microphone senses the movement of a charged diaphragm relative to a fixed backplate. An externally polarised condenser uses phantom power both to polarise the capsule and to run its internal preamplifier (impedance converter), while an electret condenser carries a permanent charge and uses the power only for the preamplifier. Condensers generally offer higher sensitivity, a faster transient response and more high-frequency detail. They also pick up more of the room, spill and handling noise, so their placement is more demanding.

A ribbon microphone suspends a thin corrugated metal foil between magnets. It has a smooth high-frequency response and is usually bidirectional (figure-8), picking up front and back while rejecting the sides. Ribbons are fragile and sensitive to blasts of air, and some older models can be damaged by phantom power, so the phantom setting is checked before one is connected.

Sensitivity, expressed in millivolts per pascal, allows the three families to be compared. A condenser can deliver about 20 mV/Pa where a dynamic delivers about 2 mV/Pa; the Neumann U 87 Ai (20 mV/Pa in omnidirectional mode) and the Shure SM58 (1.85 mV/Pa) are typical examples. This tenfold difference appears directly in the preamp gain required.

Beyond the family, the intended application narrows the choice. A studio microphone favours detail and low self-noise, a live microphone ruggedness and feedback rejection, a measurement microphone a flat, calibrated response for system tuning, a lavalier discretion, and an ENG shotgun reach and durability outdoors. The same transducer type appears in several of these roles, so the application indicates which compromises are acceptable without dictating the transducer type.