Valid terms and regulations for studio microphones are standardized in the DIN-IEC 268-4.
The acoustic operating principle
1a. Pressure transducer
Pressure transducers expose only the front of the diaphragm to the sound source. The output produced by the microphone depends only on the sound pressure in front of the diaphragm. It is therefore to a great extent non-directional (omnidirectional characteristic). Depending on the size of the diaphragm high frequencies are transmitted with increasing directivity, deviating from the omnidirectional characteristic.
1b. Pressure gradient transducer
Pressure gradient transducers expose both sides of the diaphragm to the sound source. In case of an ideal pressure gradient transducer the voltage transmitted by the microphone depends only on the difference of the pressure in front of and behind the diaphragm. A figure-8 characteristic is the result. Through the constructive design of the microphone capsule with delay lines one-sided directional characteristics may be obtained, as, for example, wide-angle cardioid, cardioid and hypercardioid.
1c. Interference transducer
The interference transducer features in front of the capsule a tube with lateral sound permeability (shotgun). Sound incidence from the side is partially obliterated through multipath cancellation. This results in a very directional shotgun polar pattern. The interference principle functions only down to a certain cut-off frequency depending on the length of the tube. Below this frequency the microphone operates as a pressure gradient transducer with hypercardioid characteristic.
Polar Pattern
Depending on their acoustic principle microphones respond with varied sensitivity to sound sources coming from different directions. Pressure transducers have a more or less non-directional sensitivity (omnidirectional characteristic). Pressure gradient transducers can feature several polar patterns: wide-angle cardioid, cardioid, hypercardioid or figure-8. Interference transducers further increase the directivity of the polar pattern (shotgun characteristic).
The dummy head is an exception: It has the directional pattern of the human ear.
Due to physical reasons pressure transducers are either diffuse-field or free-field equalized.
- Diffuse-field equalized
A diffuse-field equalized pressure transducer is tuned in such a way that the frequency response is flat in case of diffuse (all-round) sound incidence. Frontal sound incidence causes a high frequency rise as a result of the pressure build-up in front of the diaphragm.
- Free-field equalized
A free-field equalized pressure transducer is tuned in such a way that the frequency response is flat in case of frontal (0-) sound incidence. Diffuse sound incidence results in a high frequency roll-off.
Transmission Range
The transmission range indicates the frequency range defined for each microphone by the manufacturer. For studio condenser microphones it usually ranges from 20 Hz to 20 kHz.
Sensitivity (at 1 kHz)
The sensitivity indicates the RMS voltage a microphone generates when exposed to 1 Pa = 94 dB sound pressure under free-field conditions. The value refers to a frequency of 1 kHz and a load impedance of 1 kohm. The values are slightly higher with no-load operation. For studio condenser microphones the free-field sensitivity usually ranges from 8 mV/Pa to 40 mV/Pa.
Rated impedance
The rated impedance is the complex output resistance of the microphone. The input impedance of the following microphone pre-amplifier should be at least three times higher.
Rated load impedance
The rated load impedance is the smallest recommended impedance of the following microphone pre-amplifier to ensure the nominal values of the microphone.
The rated load impedance should always be at least three times higher than the rated impedance of the microphone.
Equivalent loudness level due to inherent noise
Apart from the audio signal the output signal of each microphone always contains a low noise signal. To indicate the extent of this noise voltage it is given as a fictitious sound pressure level. With an ideal noise-free microphone a sound pressure level of this value would result in an output voltage equivalent to the inherent noise voltage.
The inherent noise is measured and weighted according to CCIR 468-3, also DIN/IEC 651 (A-weighted) in order to correlate the result with the sensation of the human ear.
For studio condenser microphones the equivalent noise level usually ranges from 20 to 30 dB (CCIR), or 10 to 20 dB (A).
S/N ratio
The S/N (signal-to-noise) ratio is the difference between the 94 dB = 1 Pa reference sound level and the equivalent noise level.
For studio condenser microphones the signal-to-noise ratio usually ranges from 74 to 64 dB (CCIR), or 84 to 74 dB (A).
Max. SPL
The maximum sound pressure level indicates the limits of the electrical circuitry of the microphone. The distortion of the microphone amplifier is then k < 0.5 %. The reference frequency is f = 1 kHz.
If the microphone has a preattenuation switch, this value can be increased by 6 to 10 dB.
Maximum output voltage
The maximum output voltage is reached when the microphone is exposed to the maximum sound pressure level. The distortion of the microphone amplifier is then k < 0.5 %.
In case of transformerless circuitry the maximum output voltage reaches 2.5 V RMS corresponding to +10 dBu. (rel. 0.775 V).
Dynamic range
The dynamic range is the difference between the maximum sound pressure level and the A-weighted equivalent noise level (weighted according to DIN/IEC 651). It characterizes the dynamics of the microphone between inherent noise and distortion.
Studio condenser microphones operate with a dynamic range of more than 130 dB.
Power supply
Almost every studio microphone needs 48 V ± 4 V phantom powering (P 48, IEC 268-15). The phantom power is fed symmetrically to both modulation leads and returned via the cable shield.
Some microphones can also be operated with batteries or a plug-in mains unit.
Current consumption
Almost every studio microphone needs 48 V ± 4 V phantom powering (P 48, IEC 268-15). To ensure perfect functioning of the microphone the employed power supplies need to provide at least the indicated current.