Impedance is very important because selecting the wrong impedance mic can cause immediate and sometimes serious problems.
Other factors, like output noise and maximum sound pressure, are important only in very specific situations when the production requires specific sound pickup other than typical voice or music, or in situations with a lot of interfering signals or sounds.
Microphone impedance refers to the amount of resistance a signal encounters in a microphone circuit. The more impedance in a circuit, the less signal will flow out of it. Therefore, all other things being equal, low-impedance microphones are more efficient in signal flow and therefore produce more signal than high-impedance microphones. Most professional mics have low impedance, which allows long audio cable runs without significant loss of signal. Their higher level of signal, relative to high-impedance mics, gives better rejection of hum and other types of interference. Also, low-impedance mics are compatible with almost all professional audio and video equipment.
Although impedance levels are rated as high or low (corresponding to the designations high Z or low Z), mics can be found that are somewhere in between. Table 1 lists typical microphone impedance levels and their corresponding measurement in ohms, the unit of electrical resistance.
The impedance of most professional mics is usually 150 ohms; it is measured as —60 dB, which is referred to as mic level. After an audio signal has been passed through a mixer, recorder, microwave transmitter, or any other processing device the signal coming from that device is usually sent at a medium impedance of 600 ohms and is measured as 4 dB, which is referred to as line level. When audio signals are called high or low in professional situations, that usually means the line level is high (more signal) and the mic level is low (less signal). In other words, a mic level signal is weak and a line level signal is strong. For a mic level signal to reach line level, it must be amplified, which occurs when the low-impedance signal is processed by a mic preamplifier.
When mics are bombarded with more audio signal than they can process accurately, the mic is overloaded. Dynamic mics are very hard to overload. They have very low distortion across their entire 140-dB dynamic range. The same cannot be said for condenser mics. At high acoustic levels, the output signal of the capsule can overload the impedance converter circuit in the microphone. Some mics have built-in pads (an inline device that can change the signal that passes through it) that can reduce the signal level, sometimes at the cost of adding noise to the sound, thus reducing quality.
The sound recordist must also be cautious not to overload the microphone preamplifier that boosts the electrical signal for recording. If a grating or distorted sound is heard that is not an accurate reproduction of your sound source, it must be determined which link in the chain is overloading. It could be caused by the microphone or the mic preamplifier, or by a problem with the mic cable.