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9.1 - SENSITIVITY

The starting point for this series of calculations is the “Output Sensitivity” of the microphone. The “Output Sensitivity” measurement indicates the level of electrical signal that will be produced by a specific microphone in the presence of a standard acoustic stimulus - it is the key link between the acoustic domain and the electrical domain. This measurement is normally specified as the number of millivolts generated by the microphone when the diaphragm of the microphone is subjected to an acoustic pressure of 1 Pascal. This value is shown in the data sheets in :
measurement value of mV per Pascal

This is the standard convention that has been adopted by Rycote in the presentation of the microphone data sheets. The acoustic pressure value of “1 Pa” can also be expressed as a Sound Pressure Level of 94dB SPL. Sound Pressure Level is calculated with respect to a reference acoustic pressure of 2 x 10-5 Pa. This is considered to be the average threshold of sensitivity of the ear at 1000 Hz.

Reference level for the acoustic signal used for measurement of microphone characteristics

1 Pa of acoustic pressure is also equivalent to 10 ¼bars or 10 dynes/cm2, but nowadays the measurement units of Pascals and dB SPLs are by far the most frequently used. If you compare the Rycote data presentation with the manufacturer’s microphone specification sheets, you will see that some manufacturers still specify the output sensitivity measurement with respect to a standard acoustic stimulus of 1 ¼bar.

equivalent values for one microbar

The sensitivity measurement at 1 ¼bar is therefore 20dB lower than measurements made at 1Pa. In this case the output sensitivity will be shown as:

measurement value of mV per microbar

The sensitivity or output voltage Vm from the microphone can be also be expressed in ‘decibels’ with respect to a reference voltage Vref value using the formula:
Formula for determining the output sensitivity in decibels using 20 log Vm divided by V ref

This reference value ‘Vref’ of 0 decibels (0dBm) is normally accepted as “775 mV into a load resistance of 600 ohms” (or 0dBu if the termination impedance is disregarded). However this reference value can also refer to “1 volt” in which case the suffix is dBV - 1mW is also 1V into a load resistance of 1000 ohms. If we take an electrostatic microphone with a typical sensitivity of about 7.75mV/Pa as an example, we can establish the correspondence between the other measurement units, be it in the acoustical or the electrical domain. In this example for a typical electrostatic microphone:

An acoustical stimulus of 1 Pa or 94dB SPL
will produce an electrical output of 7.75 mV or – 40dBu

Output sensitivity in the acoustical and electrical domaine

This can be compared with the typical sensitivity of a moving-coil electro-dynamic microphone of about 0.775 mV/Pa:
1 Pa or 94 dB SPL will produce 0.775 mV or – 60 dBu

Output sensitivity in the acoustical and electrical domaine

This shows that the “dynamic” microphone is about 10 times or 20dB less sensitive than the “static” microphone. Select a few microphones from the Rycote “Microphone Data” and compare the sensitivity values for the different types of microphones – dynamic, ribbon, condenser and electret. This calculation of “Output Sensitivity” in both the acoustical and electrical domain and in the various measurement units, will help us to “position” the other microphone characteristics such as “noise floor” and “maximum Sound Pressure Level”.