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Microphones and Acoustics

Section I: Definitions and Terminology
Section II: Microphone Recommendations
Section III: Calibration and Testing
Section IV: Specification Clarifications
Section V: Specialty Microphone Applications
Section VI: Maintenance and Handling

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What is a temperature coefficient and how is it applied?

As temperatures change, the sensitivity of microphone can be affected. Coefficients required by the IEC 61094-4 (working class microphone) standard can be used to estimate the change in sensitivity, due to these fluctuations in temperature.

Detailed Information: The diaphragm stiffness and the air gap between the diaphragm and the backplate act like a spring (see figure below) Changes in temperature, humidity and atmospheric pressure changes can result in a difference in the tension of that spring effect, which in turn will cause a change in the sensitivity of the microphone. The sensitivity change, if not accounted for, will cause an inaccuracy in the output reading of the pressure that end users are accurately trying to measure.

The best way to account for the change in sensitivity, due to these environmental changes, is to use a hand held calibrator (example a CAL200) to measure the exact sensitivity in the test environment, after the microphone has had time to stabilize.

If you do not have a handheld calibrator (or pistonphone) you can estimate the effects by performing a calculation using the coefficients in the specification sheet. For this example we will compensate for a change in temperature.

Every calibration certificate will have the original microphones sensitivity along with the environmental conditions at the time the calibration was made (see below).

By utilizing the temperature coefficient (see table below), we can add or subtract environmental effects. A positive number equates to a positive slope (less sensitive at low temperatures, more sensitive at higher temperatures.)

In order to perform the calculation of what the sensitivity would be for the above 377B02 microphone if we wanted to test in a 100oC , take the difference in temperature from the cal cert and multiply by the coefficient:

∆T = 100 oC – 21 oC = 79 oC temperature change 79 oC * 0.009 dB/ oC = 0.71 dB

The above 377B02 example has a 49.62 mV/Pa sensitivity which equals -26.09 dB re 1V/Pa If you add the 0.71 dB from the change in temperature to the -26.09, you get the following sensitivity result:
0.71 – 26.09 = -25.38 dB re 1V/Pa

By converting that new -25.38 dB re 1V/Pa back into sensitivity in mV/Pa using the appropriate logarithmic conversion formula, you obtain an increase in the sensitivity to 53.83 in mV/Pa at the new 100°C temperature. This change from 49.62 to 53.83 is directly related to the increase in temperature.

It is important to note that even though the IEC61094-4 standard only calls for the temperature coefficient to be tested from -10°C to +50°C, through testing we have confirmed that the coefficient trend line is still valid to the 120°C capability of the microphone.

What is a temperature coefficient and how is it applied?

As temperatures change, the sensitivity of microphone can be affected. Coefficients required by the IEC 61094-4 (working class microphone) standard can be used to estimate the change in sensitivity, due to these fluctuations in temperature.

Detailed Information: The diaphragm stiffness and the air gap between the diaphragm and the backplate act like a spring (see figure below) Changes in temperature, humidity and atmospheric pressure changes can result in a difference in the tension of that spring effect, which in turn will cause a change in the sensitivity of the microphone. The sensitivity change, if not accounted for, will cause an inaccuracy in the output reading of the pressure that end users are accurately trying to measure.

The best way to account for the change in sensitivity, due to these environmental changes, is to use a hand held calibrator (example a CAL200) to measure the exact sensitivity in the test environment, after the microphone has had time to stabilize.

If you do not have a handheld calibrator (or pistonphone) you can estimate the effects by performing a calculation using the coefficients in the specification sheet. For this example we will compensate for a change in temperature.

Every calibration certificate will have the original microphones sensitivity along with the environmental conditions at the time the calibration was made (see below).

By utilizing the temperature coefficient (see table below), we can add or subtract environmental effects. A positive number equates to a positive slope (less sensitive at low temperatures, more sensitive at higher temperatures.)

In order to perform the calculation of what the sensitivity would be for the above 377B02 microphone if we wanted to test in a 100oC , take the difference in temperature from the cal cert and multiply by the coefficient:

∆T = 100 oC – 21 oC = 79 oC temperature change 79 oC * 0.009 dB/ oC = 0.71 dB

The above 377B02 example has a 49.62 mV/Pa sensitivity which equals -26.09 dB re 1V/Pa If you add the 0.71 dB from the change in temperature to the -26.09, you get the following sensitivity result:
0.71 – 26.09 = -25.38 dB re 1V/Pa

By converting that new -25.38 dB re 1V/Pa back into sensitivity in mV/Pa using the appropriate logarithmic conversion formula, you obtain an increase in the sensitivity to 53.83 in mV/Pa at the new 100°C temperature. This change from 49.62 to 53.83 is directly related to the increase in temperature.

It is important to note that even though the IEC61094-4 standard only calls for the temperature coefficient to be tested from -10°C to +50°C, through testing we have confirmed that the coefficient trend line is still valid to the 120°C capability of the microphone.