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 analysis equipment such as an FFT analyzer, microphones and sound level meters are used as sensors.
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.
In this and the next article, we will introduce a method for performing unit calibration using the internal calibration signal of a sound level meter when connecting the sound level meter to an analysis device such as an FFT analyzer.
This time, we are introducing the calibration procedures for our DS-2000 series DS-0221 FFT analysis software and DS-0250 throughput disk software. Procedures for other products will be introduced in the next installment.
Sound pressure in decibels
The sound pressure level 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)
Connection of sound level meter and analysis device
Figure 1 shows an example of connecting a sound level meter and an analysis device.
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Figure 1: Example of connection between 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:
We explain how to leave the unit name blank or set it to "spl". If you simply want to display the sound pressure level in units of "dB", set the unit name to blank. When importing the measured data into our time-series data analysis tool Oscope2, setting the unit name to "spl" will allow it to be converted and read as the correct sound pressure signal.
- Press the A/C/Z keys on the sound level meter to set the frequency weighting to C-weighted.
- Press the FAST/SLOW key on the sound level meter to set the time weighting to F.
- Press the LEVEL UP/DOWN button on the sound level meter to adjust the level range to the setting you will actually be using.
- Press the CAL button on the sound level meter to output a calibration signal.
- Calibrate the analysis equipment by referring to the calibration procedures for each analysis software described below.
- To stop the calibration signal, press the CAL button again to stop the internal calibration signal.
- Press keys such as A/C/Z or FAST/SLOW to return the sound level meter to its actual usage settings.
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Figure 2: Example of a sound level meter calibration signal output screen.
How to configure the DS-0221 FFT analysis software.
- 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 voltage range to match the magnitude of the calibration signal. Our sound level meters typically have an internal calibration signal of 0.35 Vrms, so the appropriate voltage range is the 1 Vrms range.
- Go to [Input Menu] → [Voltage Range Setting] and set the filter in the Set2 tab to FLAT.
- Navigate to [Input Menu] → [Units, Calibration], and in the Units and Calibration dialog box, under the Set tab, turn on Calibration ① and enter a blank or “spl” in Unit Name ②. For Calibration Value Setting ③, select “V/EU”. Leave the Physical Value (EU Value) as 1.
- Display the EU/SP tab in the Units and Calibration dialog box. Move the handle ④ on the bar below the graph all the way to the right so that "X:OverAll" is displayed below the graph.
- Press the AVG button on the DS-0221 to perform the averaging calculation. After averaging stops, enter the calibration signal value (e.g., 124.0 dB) displayed on the sound level meter into box ⑤ above the setting button, and then press the setting button ⑥. Next, press the START button on the DS-0221 to display the data and confirm that the Y value is the same as the calibration signal value.
- We will perform the same calibration on other channels as well.
- Switch to the Set tab, verify that the physical value (EU value) is set to an appropriate value, or approximately the same value as when it was previously calibrated, and then click the OK button to close the dialog.
- (1) to (3) are changed back to the settings used for the actual analysis.
How to configure the DS-0250 throughput disk software
- 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).
- Set the voltage range to match the magnitude of the calibration signal. Our sound level meters typically have an internal calibration signal of 0.35 Vrms, so the appropriate voltage range is the 1 Vrms range.
- Go to [Input Menu] → [Voltage Range] settings and set the filter in the Set2 tab to FLAT.
- Navigate to [Data Display Menu] → [Y-axis Scale], and in the scale settings dialog, set Y-axis LIN/LOG ① to "Log" and rms/0-peak ② to "rms". With all options checked, click the OK button to close the dialog.
- Navigate to [Data Display Menu] → [Cursor Settings] and set Search Mode ③ to “Search”. With all options checked, press the OK button to close the dialog box.
- Move the search cursor ④ (red vertical bar) to the right side of the graph so that "X:OverAll" is displayed below the graph. Press the FFT ⑤ button on the DS-0250 to switch the graph display to power spectrum and confirm that the calibration signal is displayed.
- Navigate to [Input Menu] → [Units, Calibration], and in the Set tab of the Units and Calibration dialog, turn on Calibration⑥, and enter a blank or “spl” in Unit Name⑦. For Calibration Value Setting⑧, select “V/EU”.
- Display the EU/SP tab in the Units and Calibration dialog box. Select a channel, enter the calibration signal value (e.g., 124.0 dB) in box ⑨ above the settings button, and press the settings button ⑩.
- We will perform the same calibration on other channels as well.
- Switch to the Set tab, verify that the physical value (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.
- (1) to (3) are changed back 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-0221 FFT analysis software and DS-0250 throughput disk software. Next time, we will continue to introduce the procedures for other products.
We will cover calibration methods using acoustic calibrators and microphone calibration methods in subsequent articles.
(Excerpt from the email newsletter issued on June 19, 2014)
