Microphones

Acoustic Musician’s guide to Sound Reinforcement and Live Recording
Ch 2, Microphones

Microphone - a sound instrument that transduces, or converts sound pressure waves into a voltage.

Diaphragm - thin circular piece of metal, plastic, or paper that moves back and forth with the incoming sound waves

Transducer - connected the diaphragm, converts the mechanical motion into electromagnetic using a moving coil, ribbon, or electret (condenser).

Moving Coil - simple construction used in dynamic microphones, a moving coil of wire attached to a diaphragm induces a voltage of about 1 millivolt.

Ribbon - movement of a thin paper ribbon in between a magnetic field induces a very low voltage.

Electrostatic (condenser) - the diaphragm and backplate of the transducer create a capacitor. A voltage is supplied to them. Movement of the diaphragm causes variances in the capacitance which causes changes in the voltage. Q=CV Q is the constant charge in coulombs. An amplifier is necessary close to the transducer because of the very high impedance that is susceptible to induced noise from power lines or other interference.

Electret - have a permanent charge on either the diaphragm or backplate. removes the need for polarizing voltage across capsule, but still needs lower voltage supply for the pre-amplifier.

Phantom Power - 48 volt standard for microphone powering. It is supplied through the two positive leeds in a balanced microphone cable with the negative coming through the cable ground wire.

Pickup Patterns or Polar Response - allows for discrimination of specific sounds or frequencies in the recording process.

Omnidirectional - responds well to sounds coming from all directions. slight shadow in high freq from behind mic, because of physical shape of mic.

Figure-of-eight - two touching spheres front and behind, sounds from the sides are discriminated against.

Cardioid - heart shaped pickup pattern with lowest response to sounds behind the capsule. Tunes in recording by 20dB more to sounds infront of capsule. Allows for noise or unwanted sounds to be reduced in the recording.

Hypercardioid (shotgun) - very directional microphone used for outdoor news gathering, or film work. Slits along the side cause sounds that come from behind the capsule to be out of phase and cancelled out.

directional microphones are increasingly more susceptible to exaggerated bass response as you decrease the distance from the mic. Most cardioids need the sound source to be 30 - 50 cm away from the mic, or more for hypercardioids.

Parabolic - an omnidirectional microphone is placed at the focal point of a parabolic dish. Extreamly directional.

Pressure Zone Microphone (PZM) - Omnidirectional microphone that uses the surface it is mounted to to act as the sound gathering source. The mic is mounted 1mm above the surface so that phasing of the sounds is eliminated. Used for room recordings, certain instruments, theatre

Microphone Placement - some typical stereo micing pair arrangements: xy (american) - 90 degree cardioids 1" apart; N.O.S. (Dutch Radio) - 90 degree cardioids 7" apart; and ORFT (French Radio) - 110 degree cardioids 9" apart.

Handling Microphones - don't drop or get wet. Condenser mics should not be handled when recording, very loud handling noise. Dynamic mics more suitable for hand held recording.

Discussion about noise, eleminating reverberation, and vibrations in recording. Proper placement relative to the subject.

Mic placement - important, do what sounds best, but make sure mic is pointed at the intended subject.

Use the right mic for the situation.