Engineering Units (EU) and Unit Calibration - Part 7: Microphones and Sound Level Meters, Part 3 -

Sound is a type of wave that travels through air and other materials, and its magnitude is observed as a change in pressure (sound pressure). The unit of sound pressure is the pascal (Pa), but the unit of sound pressure level is usually used to express the loudness of sound. The unit of sound pressure level is the decibel (dB).

When analyzing sound using an analysis device such as an FFT analyzer, microphones and sound level meters are used as sensors. In this process, the analysis device is calibrated so that the analysis results are displayed as the correct sound pressure level values.

When using a measurement microphone, the microphone and analysis equipment are calibrated using an acoustic calibrator. The acoustic calibrator is a device that generates a specific sound pressure level (such as 114 dB or 94 dB), and is used by inserting the tip of the measurement microphone into it.

This time, we will introduce the procedure for calibrating a measurement microphone and analysis device using an acoustic calibrator.

Connection of measurement microphone/preamplifier and analysis device

If the measurement microphone and preamplifier support a power supply method called CCLD (constant current drive), then the microphone/preamplifier can be directly connected to the analysis device if the analysis device also supports CCLD. An example connection is shown in Figure 1. If the measurement microphone and preamplifier do not use a power supply method other than CCLD, or if the analysis device does not support CCLD, then a separate microphone amplifier or similar device will be required.

Calibration procedure for measuring microphones and analysis equipment

The procedure for calibrating the measurement microphone and analysis device is as follows. For detailed instructions on operating the analysis device, please refer to the analysis device's instruction manual. Similarly, for instructions on operating the acoustic calibrator, please refer to the acoustic calibrator's instruction manual.

1. Check the sound pressure level generated by the acoustic calibrator using the included test report or the markings on the unit. If atmospheric pressure correction is required for the generated sound pressure level, measure the atmospheric pressure and correct the generated sound pressure level accordingly.
2. Turn on the acoustic calibrator with the microphone disconnected and check its operation. After checking, turn off the power.
3. Set up the analysis device.
   - Set the frequency range to the same range used for actual measurements. However, if the frequency range is similar to or lower than the frequency of the internal calibration signal (usually 1 kHz), set it to a range of twice that frequency (2 kHz) or higher.
   Set the analog filter to Z (FLAT) or C.
   The averaging time is set to 20 seconds.
4. In the unit calibration settings of the analyzer, set the unit name to "Pa", the EU type to "V/EU", and the 0 dB reference value to "2E-5" (20 μPa). For analyzers that do not allow setting a 0 dB reference value, set the unit name to blank or to "spl".
5. Slowly insert the microphone into the acoustic calibrator. Make sure to insert the microphone all the way in. Wait at least 30 seconds for the pressure inside the calibrator to stabilize.
6. Turn on the acoustic calibrator.
7. Adjust the voltage range of the analysis device. Set it to the smallest possible voltage range without causing an input overload.
8. Once the acoustic calibrator's output has stabilized, press the start button on the analysis device and perform 20 seconds of averaging. Note that some acoustic calibrators will automatically shut down if not operated for a certain period of time; if this happens, please restart the process.
9. Set the search cursor to Overall (or All-Pass in the case of a real-time octave analyzer), and perform unit calibration so that the value matches the sound pressure level generated by the acoustic calibrator.
10. Press the start button on the analysis device again and check if the overall value (or all-pass value) matches the generated sound pressure level.
11. Check and record the EU value (V/EU value) in the unit calibration settings of the analyzer. Also, My
    We will confirm that it closely matches the EU value (described later) calculated from the Kurohon sensitivity.
12. Turn off the acoustic calibrator and remove the microphone.
13. The frequency range, analog filter, averaging time, voltage range, and other settings of the analysis device will be returned to their original settings.

Method for calculating EU value from microphone sensitivity (assuming a 0 dB reference value of 20 μPa)

The microphone sensitivity and preamplifier insertion loss are confirmed using test reports for the microphone and preamplifier, and the overall sensitivity is calculated. If test reports are unavailable, the values specified in the catalog are used.

Table 1 Example of calculating overall sensitivity

The EU value (V/EU value) can be calculated using the following formula:

The EU values for the microphones and preamplifiers shown in Table 1 are 0.031261.

If you set the EU type to EU/V, the value will be the reciprocal of the V/EU value, which in the example above would be 31.989.

How to calculate EU value from microphone sensitivity (when a 0 dB reference value cannot be set)

The microphone sensitivity and preamplifier insertion loss are confirmed using test reports for the microphone and preamplifier, and the overall sensitivity is calculated. If test reports are unavailable, the values specified in the catalog are used.

Table 2 Example of overall sensitivity calculation

The EU value (V/EU value) can be calculated using the following formula:

The EU values for the microphones and preamplifiers shown in Table 2 are 6.2522 × 10⁻⁷.

If you set the EU type to EU/V, the value will be the reciprocal of the V/EU value, which in the example above would be 1.5994 × ^10⁶.

(Excerpt from the email newsletter issued on October 23, 2014)