Engineering Units (EU) and Unit Calibration - Part 6: Microphones and Sound Level Meters, Part 2 -

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.

A sound level meter is a measuring instrument used to measure sound pressure levels, and its structure integrates a microphone, amplifier, and sound pressure level calculation/display unit. Sound level meters equipped with an AC output terminal can be connected to analysis devices such as FFT analyzers to perform sound analysis.

Last time, we introduced the calibration procedures for our DS-2000 series' DS-0221 FFT analysis software and DS-0250 throughput disk software. This time, we will introduce the calibration procedures for the DS-3000 series' acoustic vibration analysis software and the CF-9200/9400 portable FFT analyzer. Please note that the screen may differ in older versions of the DS-3000 series software.

Sound pressure and sound pressure level

Sound is a type of wave that travels through air and other materials, and its magnitude (sound pressure) is measured in pascals (Pa). However, the unit used to express the loudness of sound is usually called sound pressure level. The unit of sound pressure level is decibels (dB).
The sound pressure level Lp of sound traveling through the air is defined by the following formula:

(dB)

p: Measured sound pressure (effective value of instantaneous sound pressure)
p0: Reference sound pressure (20 μPa)

For example, if the effective value of the instantaneous sound pressure is 1 Pa, the sound pressure level Lp will be approximately 94 dB, as shown in the following equation.

(dB)

The DS-0221 and DS-0250 software, which we introduced last time, do not allow you to specify a reference sound pressure value (20 μPa). Therefore, while the correct sound pressure level is displayed when displaying the power spectrum in decibels, the sound pressure value displayed in time-axis waveforms, etc., is not the correct value, and instead displays a value 50,000 times the value in Pascals (Pa) (the reciprocal of 20 μPa).

The product we are introducing today allows you to specify a reference sound pressure value (20 μPa), so you can set it to display both the sound pressure level value, such as the power spectrum, and the sound pressure value, such as the time-domain waveform, with the correct values.

Figures 1 and 2 show the time-domain waveforms of sound signals (1 kHz sine wave, RMS value: approximately 1 Pa) measured using the method introduced previously, which does not specify a reference sound pressure (unit: spl), and the method introduced this time, which specifies a reference sound pressure (20 μPa). In Figure 1, the amplitude of the sine wave is a very large value of 70,845 (equivalent to 50,100 RMS value), but in Figure 2, the amplitude of the sine wave is 1.417 Pa (equivalent to 1.002 Pa RMS value).

Connection of sound level meter and analysis device

Figure 3 shows an example of connecting a sound level meter and an 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 to AC.

Our sound level meters LA-1410/1440/4440 and LA-3260/3560/3570 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 of the analysis device using the internal calibration signal of the sound level meter

The procedure for calibrating the analysis device using the internal calibration signal of the sound level meter is as follows:

1. Press the A/C/Z keys on the sound level meter to set the frequency weighting to C-weighted.
2. Press the FAST/SLOW key on the sound level meter to set the time weighting to F.
3. Press the LEVEL UP/DOWN button on the sound level meter to adjust the level range to the setting you will actually be using.
4. Press the CAL button on the sound level meter to output a calibration signal.
5. Calibrate the analysis device by referring to the calibration procedures for each product described below.
6. To stop the calibration signal, press the CAL button again to stop the internal calibration signal.
7. Press keys such as A/C/Z or FAST/SLOW to return the sound level meter to its actual usage settings.

How to configure the DS-3000 series acoustic vibration analysis software.

1. Set the frequency range to the same range you will actually use for 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).
2. Go to [Input/Output Settings Menu] → [Input Settings] and set the analog filter to Z (FLAT).
3. Navigate to [Input/Output Settings Menu] → [Units, Calibration Settings], and in the Calibration dialog, under the Units/Calibration tab, turn on Calibration ① and enter "Pa" for Unit Name ②. Select "V/EU" for EU Type ③. Enter "2E-5" (20 μPa) for 0 dB Reference Value ④. Leave the EU value as 1.
   
4. Display the EU/S.P tab in the calibration dialog. Confirm that the search cursor is at the far right of the graph and that "X:OverAll" is displayed below the graph. Set voltage range ⑤ to match the magnitude of the calibration signal. Our sound level meters typically have an internal calibration signal of 0.7 Vrms, so the appropriate voltage range is the 1 Vrms range.
   
5. Press the AVG button ⑥, then press the Start button ⑦ to perform the averaging calculation. After the averaging stops, enter the calibration signal value (e.g., 94.0 dB) displayed on the sound level meter into the Calibration Value box ⑧, and press the Set button ⑨. Then press the Start button ⑦ again to perform the averaging calculation and confirm that the Y value is the same as the calibration signal value.
   
6. We will perform the same calibration on other channels as well.
7. Switch to the Units/Calibration tab, verify that the EU value is set to an appropriate value or approximately the same value as when it was last calibrated, and then click the OK button to close the dialog box.
8. (1) and (2) will be reverted to the settings used for the actual analysis.

How to configure the CF-9200/9400 portable FFT analyzer.

1. Set the frequency range to the same range you will actually use for 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).
   
2. Navigate to [HOME] ⇒ [Input] ⇒ [Input Cond] and set the Filter to Z (FLAT) in the Input Signal Conditions dialog box.
   
3. Navigate to [HOME] ⇒ [Input] ⇒ [EU], and in the calibration dialog's Unit/Calib. tab, turn on EU① and enter "Pa" in Unit Name②. Select "V/EU" for EU Type③. Enter "2E-5" (20 μPa) in 0dB Reference④. Leave EU Value as 1.
   
4. Display the EU/S.P tab in the calibration dialog. Confirm that the search cursor is at the far right of the graph and that "X:OverAll" is displayed below the graph. Set voltage range ⑤ to match the magnitude of the calibration signal. Our sound level meters typically have an internal calibration signal of 0.7 Vrms, so the appropriate voltage range is the 1 Vrms range.
   
5. Press the AVG button on the main unit's hard keys, then press the START button to perform the averaging calculation. After averaging stops, enter the calibration signal value (e.g., 94.0 dB) displayed on the sound level meter into the Calibration Value box ⑥, and press the Execute Cal button ⑦. Then press the START button to perform the averaging calculation again and confirm that the Y value is the same as the calibration signal value.
   
6. We will perform the same calibration on other channels as well.
7. Switch to the Unit/Calib tab, verify that the EU value is set to an appropriate value or approximately the same value as when it was calibrated previously, and then close the dialog by clicking the × button in the upper right corner.
8. (1) and (2) will be reverted to the settings used for the actual analysis.

summary

This time, we introduced the procedure for performing unit calibration using the internal calibration signal of a sound level meter when connecting it to an analysis device such as an FFT analyzer, using the DS-3000 series acoustic vibration analysis software and the CF-9200/9400 portable FFT analyzer.
Next time, we'll show you how to calibrate microphones using an acoustic calibrator.

(Excerpt from the email newsletter issued on August 21, 2014)