What is a Microphone's Sensitivity Rating?
A microphone's sensitivity rating is the specification that comes with a microphone that serves to tell you the microphone's gain. Different microphones have different gains in response to input audio signals. For a given signal, one microphone may record the signal at a low level; thus, it has a low sensitivity rating. For that same signal, another microphone may record the signal at a loud level; thus, it has a high sensitivity rating. Another may record a signal with a volume level in between those two extremes. Thus, the sensitivity rating serves to tell you what level of sound, or gain, the microphone records at.
The sensitivity rating of a microphone is given as a negative number in decibels (dB) many of the times. So, for example, the sensitivity rating of a microphone may be -40dB. Why is it given as a negative number? Remember, that the unit of decibels is a ratio. It always gives the value of something only when compared to something else. In the case of microphone sensitivity, it gives the value of the gain of the microphone at mic level compared to the value that is needed to boost it up to line level which is at 0dB. So if a microphone has a sensitivity rating of -35dB, for example, it needs an amplication of 35dB to bring it up to line level (at 0dB). A microphone with a rating of -60dB needs an amplification of 60dB to bring it up to line level.
When we compare microphone sensitivity in regard to dB value, the microphone with the value closer to 0 has the higher gain, or sensitivity, than a microphone with a number farther away. Thus, a microphone with a sensitivity of -40dB has a much greater gain than a microphone with -60dB.
Another way in which microphone sensitivity is measured or expressed is in volts/pascal. Pascal is the air or sound pressure that a microphone is subject to. The resulting output voltage that a microphone produces in response to that sound pressure is its gain or sensitivity. Stated over in plain terms, a microphone is exposed to a certain level of sound (sound is produced by air pressure which is why it is measured in pascal). The amount of voltage that a microphone produces in relation to that sound is its gain, or sensitivity. If two microphones are exposed to the same exact sound pressure level, the microphone with the higher sensitivity will produce a greater output voltage in response.
Microphone sensitivity ratings provide a great way to compare microphone output levels. They allow you to choose the correct microphone that is needed for a microphone circuit based on the gain needed for the circuit. For an application with a boom microphone, where a user is talking one inch from the microphone, a high sensitivity microphone may not be needed. Thus, a microphone with a sensitivity rating of about -60dB can suffice just fine. However, in a case with an application where a microphone is needed to pick up faint sounds from far distances, a high sensitivity microphone is needed, such as a microphone with a sensitivity rating of -35dB. You can pick and play and match with microphones until you get the right gain. Say you have a microphone with a sensitivity rating of -50dB and you feel that it's not providing enough gain; you can just swap it out for a microphone with a higher sensitivity rating, such as -40dB or whatever microphone would suit you best. Sensitivity ratings of microphones, thus, are very valuable for comparing microphone outputs and getting the right gain needed for a microphone circuit.
Keep in mind, though, that with sensitivity ratings, not all manufacturers of microphones use the same reference frequency. So be careful to check the manufacturer's specifications for precise details. Also microphone sensitivity ratings, just like all other electronic components, are not exact. Usually when a specification is made for a microphone, the value of the sensitivity is given and how much dB it can vary by, its tolerance. For example, a microphone may have the rating of -40 ±3dB. Thus, its sensitivity can range from 37dB to 43dB. This is usually not significant being that it only accounts for small changes in sound recording.
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What is Microphone Sensitivity?