Engineering Units (EU) and Unit Calibration - Part 8: Microphones and Sound Level Meters, Part 4 -

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 or sound level meters are used as sensors. At this time, the analysis device is calibrated to ensure that the analysis results are displayed with the correct sound pressure level values.

When using a sound level meter for analysis, if an acoustic calibrator is available, use it to calibrate both the sound level meter and the analysis equipment. An acoustic calibrator is a device that generates a specific sound pressure level (such as 114 dB or 94 dB) and is inserted into the microphone at the tip of the sound level meter.

This time, we will introduce the procedure for calibrating a sound level meter and analysis device using an acoustic calibrator.

Connection of sound level meter and analysis device

If the sound level meter has both AC (alternating current) and DC (direct current) output terminals, connect the signal from the AC output terminal to the analysis device. If the sound level meter's settings allow you to switch between AC and DC for the output signal, switch it to AC.

Our sound level meters LA-3260/3560/3570 and LA-1410/1440/4440 can also output an AC-Z signal at the output terminal. This function outputs a signal weighted by Z characteristics (FLAT characteristics) at the output terminal, regardless of the frequency weighting (A-weighting/C-weighting/Z-weighting) selected on the sound level meter. This can be used when you want to display the A-weighted sound pressure level on the sound level meter, but analyze the Z-weighted (FLAT characteristics) signal on the analysis device.

Calibration procedure for sound level meter and analysis device

The procedure for calibrating the sound level meter and analysis device is as follows. For detailed instructions on operating the sound level meter and analysis device, please refer to their respective instruction manuals. Also, for instructions on operating the acoustic calibrator, please refer to the acoustic calibrator's instruction manual.

1. The sound pressure level (calibration value) of the acoustic calibrator can be confirmed using the included test report or the markings on the unit. If atmospheric pressure correction is required for the sound pressure level, measure the atmospheric pressure and use the value corrected for atmospheric pressure as the calibration value.
2. If a correction value is specified for the sound level meter when using an acoustic calibrator, the calibration value should be the value obtained by adding the correction value to the value in (1). The correction value for our sound level meters LA-3260/3560 is -0.1 dB, and the correction value for L-3570 is +0.2 dB. No correction is required for sound level meters that do not have a specified correction value.
3. Turn on the acoustic calibrator with the microphone disconnected and check its operation. After checking, turn off the power.
4. Set up the sound level meter.
   The frequency weighting characteristic is set to C or Z (FLAT). The level range is usually set to the lowest level range within which the calibration value falls (for example, if the calibration value is 114 dB, the range is 50-120 dB). If there are two types of level ranges, wide range and normal range, select the same type of range that will be used during the measurement.
5. 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 at least twice that frequency (2 kHz). Set the analog filter to Z (FLAT) or C. Also, set the averaging time to 20 seconds.
6. In the unit calibration settings of the analyzer, set the unit name to "Pa", the EU type to "V/EU", the 0 dB reference value to "2E-5" (20 μPa), and the offset to "0 dB". For analyzers that do not allow setting a 0 dB reference value, set the unit name to blank or to "spl".
7. Slowly insert the microphone at the tip of the sound level meter into the acoustic calibrator.
   Insert the microphone precisely all the way in. Wait at least 30 seconds for the pressure inside the calibrator to stabilize.
8. Turn on the acoustic calibrator.
9. Adjust the voltage range of the analysis device to match the magnitude of the calibration signal. Typically, the appropriate voltage range is 1 Vrms.
10. 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.
11. Search cursor overall (in the case of a real-time octave analyzer)
    The unit calibration is performed to match the all-pass value so that the value becomes the calibration value.
12. Press the start button on the analysis device again and check if the overall value (or all-pass value) matches the calibration value.
13. Check and record the EU value (V/EU value) in the unit calibration settings of the analysis device. Also, confirm that it closely matches the EU value calculated from the level range of the sound level meter (described later).
14. Turn off the acoustic calibrator and remove the sound level meter.
15. Reset the sound level meter settings.
16. The frequency range, analog filter, averaging time, voltage range, and other settings of the analysis device will be returned to their original settings.
17. The sound emitted from the object is actually measured, and the sound level meter's level range is set to the smallest range that does not cause an overload.
18. The difference between the upper limit of the sound level meter level range value (17) during measurement and the upper limit of the level range value (4) during calibration is set as the offset for the unit calibration setting of the analysis device.
    If the level range during measurement is 20-90 dB and the level range during calibration is 60-120 dB, set the offset to 90 - 120 = -30 dB.
    If the analysis device's unit calibration settings do not include an offset function, the EU value will be corrected by the amount the level range has been changed. If the level range during measurement is 10 dB lower than the calibration value, the EU value (V/EU value) will be multiplied by 3.1623. If it is 20 dB lower, multiply by 10; if it is 30 dB lower, multiply by 31.623; and if it is 40 dB lower, multiply by 100.

Method for calculating EU values from the level range of a sound level meter (assuming the 0 dB reference value is set to 20 μPa)

The EU value obtained after performing the unit calibration described above can be calculated from the AC output specifications of the sound level meter. However, since there is an error in the output voltage of the AC output, the EU value calculated using this method is an approximate value.

Table 1 shows the formulas and calculation examples for EU values (V/EU values) and EU values (EU/V values) when the 0 dB reference value of the analysis device's unit calibration setting is set to 20 μPa. The calculation example is for a sound level meter that outputs 0.707 Vrms to the AC output when a sound at the upper limit of the level range (full scale value) is input. The unit of the obtained EU value (V/EU value) is V/Pa, indicating how many V is output per 1 Pa. The unit of the EU value (EU/V value) is Pa/V, indicating how many Pas correspond to 1 V.

Table 1 Example of EU value calculation (when the 0 dB reference value is set to 20 μPa)

Method for calculating EU values from the level range of a sound level meter (when a 0 dB reference value cannot be set)

The EU value obtained after performing the unit calibration described above can be calculated from the AC output specifications of the sound level meter. However, since there is an error in the output voltage of the AC output, the EU value calculated using this method is an approximate value.

Table 2 shows the formulas and calculation examples for the EU value (V/EU value) and EU value (EU/V value) when a 0 dB reference value cannot be set in the unit calibration settings of the analysis device, or when the 0 dB reference value is set to 1. The calculation example is for a sound level meter that outputs 0.707 Vrms to the AC output when a sound at the upper limit of the level range (full scale value) is input. The unit of the obtained EU value (V/EU value) indicates how many V is output per 20 μPa. The EU value (EU/V value) is its reciprocal.

Table 2 Example of EU value calculation (when a 0 dB reference value cannot be set)

(Excerpt from the email newsletter issued on December 18, 2014)