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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 (7 - 20 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: 7 - 15 dB(A) • Tube models: 17 - 23 dB(A) • Capacitor mics are 10 - 12 dB greater than EIN of good console or preamp. • Dynamic 10 - 12 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.