Microphone Measurements, Standards, and Specifications Eargle, Ch. 7
Primary Performance Specifications Directional properties Data in polar form, or set of on- and off-axis frequency response measurements. Frequency response measurements Usually at 0° axis, and 90° (plus others) Output sensitivity Often stated at 1 kHz, measured in free field Output source impedance Equivalent self-noise level Maximum operating SPL for a stated % of THD
Frequency Response Should always state measuring distance (to access proximity effect), or assume 1 meter. Can be shown with tolerance limits, but if so, not the measurement of a specific, actual mic Sometimes, other distances included to show proximity effect. Response with low-cut (high-pass) filter
Polar Response Eargle states that due to microphone symmetry, hemispherical plots are sufficient (180°). Most manufacturers include full sphere.
Mic Sensitivity Pressure Sensitivity Output rms in response to 1 kHz, 94 dB SPL, in free field. Output Power Sensitivity Measure the output when mic is loaded with an impedance equal to its own internal impedance
Microphone Source Impedance Almost all professional mic’s today are low impedance. Typically 50 to 200 ohms Typical input load impedance in 2k to 5k ohm range. Output impedance rating of little consequence.
Normal Mic Design Sensitivity Reference output sensitivity matches intended use of microphone Mic’s intended for strong sound sources will need less output sensitivity. Distant pickups require more. Close-in (2 - 8 mV/Pa) Distant ( mV/Pa)
Mic Equivalent Self-Noise Level Rating Stated in dB(A), representing the signal that would be picked up by an ideal mic in the rated acoustical sound field. Capacitor mics: dB(A) Tube models: dB(A) Capacitor mics are dB greater than EIN of good console or preamp. Dynamic dB less than capacitor.
Distortion and Dynamic Range Studio mics: 0.5% THD at 1 kHz. Require signal levels in the 130 to 140 range. Dynamic Range is range specified in dB between noise floor and 0.5% THD level.