This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
When analyzing sound signals recorded with microphones or sound level meters using analysis equipment such as FFT analyzers or data stations, acoustic calibration is performed using an acoustic calibrator or the REF signal (CAL signal) of the sound level meter.
Acoustic calibrators and REF signals are signals at specific sound pressure levels (e.g., 114 dB, 94 dB). By checking the magnitude of the voltage observed by the analysis device when this signal is input, the sensitivity (mV/Pa) of the microphone or sound level meter can be determined. This also serves to verify the operation of the microphone, sound level meter, and analysis device, and corrects for sensitivity deviations due to environmental factors (temperature, etc.) and aging, as well as deviations in the input characteristics of the analysis device. Therefore, this type of acoustic calibration is usually performed when measuring sound.
However, if acoustic calibration is performed without noticing setting errors, equipment malfunctions, or broken cables, the displayed values may appear correct, but in reality, the measurements are not accurate. This time, we will introduce several examples of incorrect acoustic calibration.
Normal calibration signal
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.
Figure 2 shows the 114 dB, 1 kHz calibration signal measured by connecting the AC-Z output of our LA-4440 sound level meter to a DS-3000 data station.
The sound level meter has a level range of 60-120 dB, and the DS-3000 has a voltage range of 1 Vrms. At the upper limit of the level range (120 dB, 20 Pa), the sound level meter outputs a voltage signal with an effective value of 0.707 V, so the sensitivity is 0.03535 V/Pa.
The magnitude of the calibration signal is 114 dB (10 Pa), so the voltage of the calibration signal is 0.3535 V in RMS and 0.5 V in amplitude. Looking at the upper part of Figure 2, we can see that a voltage signal with an amplitude of 0.5 V is observed. The sound pressure signal in the middle part is a signal with an RMS of 10 Pa and an amplitude of 14.1 Pa. Acoustic calibration was performed with this signal applied, so the 1 kHz component and overall value of the power spectrum in the lower part are 114 dB.
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Figure 2. Normal calibration signal (Top: Voltage waveform, Middle: Sound pressure waveform, Bottom: Power spectrum)
If an input overload occurs (1)
Figure 3 shows the calibration signal measured with the DS-3000 voltage range set to 0.316 Vrms. The calibration signal voltage is 0.3535 V RMS and 0.5 V amplitude, so the 0.316 Vrms range results in a slight input overload. The voltage waveform (top) and sound pressure waveform (middle) are almost sine waves. Acoustic calibration was performed with this signal input, so the 1 kHz component and overall value of the power spectrum (bottom) are 114 dB. However, looking at the power spectrum, there are many peaks other than 1 kHz, indicating that it is not a normal signal. Also, the OVER LED next to the input channel on the DS-3000 data station unit lights up red.
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Figure 3 Calibration signal when input is overloaded (0.316 Vmrs)
(Top: Voltage waveform, Middle: Sound pressure waveform, Bottom: Power spectrum)
If an input overload occurs (2)
Figure 4 shows the calibration signal measured with the DS-3000 voltage range set to 100 mVrms. The voltage waveform (top) and sound pressure waveform (middle) are flattened and clearly not sine waves. In addition, the power spectrum has many peaks other than at 1 kHz, indicating that it is not a normal signal.
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Figure 4 Calibration signal when input is overloaded (100 mVmrs)
(Top: Voltage waveform, Middle: Sound pressure waveform, Bottom: Power spectrum)
A similar waveform will be produced if the level range of the sound level meter is not appropriate. For example, this can occur if the sound level meter's level range is set to 40-100 dB or 30-90 dB, and a 114 dB acoustic calibrator is used. The analysis device, such as the DS-3000, will not show an overvoltage, but the sound level meter will display an overvoltage (OV) indication.
Note that pistonphone-type acoustic calibrators have relatively high distortion, and harmonic components of the calibration signal may be observed. With an acoustic calibrator with a total distortion of 2.5%, harmonics 32 dB (= 20 × log10(0.025)) smaller than or equal to the calibration signal may be observed; this is normal. To distinguish this from input overload or range overload, check the overload indicator on the analysis device/sound level meter.
If the cable is broken
Figure 5 shows the calibration signal measured with the cable connecting the sound level meter and the DS-3000 disconnected. If the disconnection is only a short distance away, the signal will travel through the air, and a seemingly plausible time waveform and power spectrum will be observed. In this data, the amplitude of the voltage signal is 0.368 V, which is slightly smaller than the original value (0.5 V). If acoustic calibration is performed with this signal input, the sound pressure waveform and power spectrum will show normal values. However, looking at the voltage waveform and sound pressure waveform, it can be seen that the entire waveform is slowly fluctuating up and down. Also, looking at the power spectrum, a spectrum of 50 Hz and its harmonics, which are thought to be power supply noise, can be seen.
Such signals may also be observed if the input stage fuse is blown or the input circuit is faulty, for example, due to an overvoltage being applied to an analysis device such as the DS-3000.
Please note that the waveform shown here is just one example. If no waveform is displayed at all due to a broken wire or malfunction, you would immediately notice the abnormality. However, this is presented as an example of a waveform that may be displayed that is only slightly different from a normal waveform.
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Figure 5 Calibration signal when cable is broken
(Top: Voltage waveform, Middle: Sound pressure waveform, Bottom: Power spectrum)
Correct calibration value
To confirm that acoustic calibration was unsuccessful due to voltage range/level range overload, broken wires, malfunctions, or other reasons, you can check the calibration values (V/EU values) after the acoustic calibration.
For products like our sound level meter, which output an effective value of 0.707 V at the upper limit of the level range, the calibration values are as shown in Table 1. If the 0 dB reference value is set to 2E-5 (20 μPa), the values are in the third column; if the 0 dB reference value is set to 1, the values are in the fourth column.
Table 1 Relationship between level range and calibration value
| Level range | upper limit of the range Effective sound pressure [Pa] |
Calibration values (0dB reference: 20μPa) |
Calibration values (0dB reference: 1) |
| 60-130 dB | 63.2 Pa | 0.01118 V/EU | 2.236E-07 |
| 50-120 dB | 20.0 Pa | 0.03535 V/EU | 7.070E-07 |
| 40-110 dB | 6.32 Pa | 0.1118 V/EU | 2.236E-06 |
| 30-100 dB | 2.00 Pa | 0.3535 V/EU | 7.070E-06 |
| 20-90 dB | 0.632 Pa | 1.118 V/EU | 2.236E-05 |
| 10-80 dB | 0.200 Pa | 3.535 V/EU | 7.070E-05 |
summary
When analyzing sound signals recorded by microphones or sound level meters using analysis equipment such as FFT analyzers or data stations, acoustic calibration is performed using the REF signal (CAL signal) of the acoustic calibrator or sound level meter. If there are setting errors, equipment malfunctions, or cable breaks, a calibration signal different from a normal calibration signal will be observed.
However, simply being told that a signal is "not a normal calibration signal" can be difficult to discern, so this time we intentionally generated an "incorrect calibration signal" for demonstration purposes.
(Excerpt from the email newsletter issued on June 20, 2018)
