This month, as an example of measuring vibrations from a familiar device, we will introduce a case study of measuring vibrations from the vibration function of a mobile phone.
When installing acceleration detectors or measuring instruments for vibration measurement, or when checking and investigating their operation in the event of a malfunction, it would be ideal to have an object to measure, but often this is not possible, or even if an object is available, it cannot be operated. If a vibrator or vibration calibrator is available, accurate operation can be checked, but if that is not possible, I think many people resort to methods such as shaking the acceleration detector or tapping near the detector.
In such cases, if you have a mobile phone, you can use its vibrator. Of course, it cannot be used for calibration or accuracy checks, but it can generate a more stable vibration than methods such as shaking or tapping, so it can be used for simple operational checks. In addition, the vibrator of a mobile phone can be used as a training exercise for vibration measuring instruments, or for comparative verification of multiple detectors and measuring instruments.
This time, I'd like to introduce some examples of measuring vibrations from mobile phones, hoping they will be helpful as a reference for applications like this.
Example of a measurement system
- ONO SOKKI NP-3211 Preamplifier-Integrated Accelerometer
- ONO SOKKI DS-3000 Series Data Station
- ONO SOKKI DS-0321 FFT analysis function (software)
- PC
Measurement method
- Place your cell phone on a folded towel.
- Attach the accelerometer to the center of the back of the mobile phone using double-sided tape or wax.
- We will make a phone call to a mobile phone and measure the vibration of the vibrator when the call comes in.
If you are concerned about the effect of double-sided tape or wax on your phone case, please have an old phone ready to use as a test.
Measurement conditions
- Frequency range: 4kHz
- Sample score: 2048 points
- Voltage range: 31.6mVrms
Measurement result-1
Figure 1 shows the time-domain waveform with the voltage value displayed before unit and calibration. The sensitivity of the NP-3211 accelerometer is 1 mV/(m/s²), so 9.15 mV (0-peak, single amplitude) corresponds to 9.15 m/s².
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Figure 1: Measurement Results 1 - Time-domain waveform (voltage value) -
Measurement results-2
After performing unit and calibration, the time-domain waveform and power spectrum are shown in m/s². The peak value of the time-domain waveform was 9.90 m/s² (0-peak, single amplitude), and the RMS value was 6.45 m/s². The overall value was 16.2 dB, 6.46 m/s² (rms, effective value), which is in close agreement with the RMS value of the time-domain waveform. Also, since the time-domain waveform is slightly distorted from a sine wave, the peak value of the power spectrum (220 Hz) is slightly smaller than the overall value. Note that the dB value is based on a reference value of 1 m/s².
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Figure 2: Measurement Results 2 - Time-domain waveform and power spectrum (acceleration values) -
Figure 3 shows the power spectra of velocity and displacement values after single and double integration. Although the peak position has shifted to 215 Hz due to the integration, it can be seen that the relationship between velocity = acceleration/2πf and displacement = velocity/2πf, where f = 215 Hz, is almost true.
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Figure 3: Measurement Results 2 - Time-domain waveform and power spectrum (acceleration values) -
Measurement results-3
To investigate the effect of the mass of the accelerometer, vibration acceleration was measured using two accelerometers with different masses. It was found that the vibration acceleration of the NP-3130 was very small because its mass is not negligible compared to the mass of the part of the mobile phone that vibrates due to the vibrator.
| Model name | Sensitivity | Weight | Acceleration value (effective value) |
| NP-3211 | 1.02 mV/(m/s²) ±15% | 0.5 g | 6.34 m/s² |
| NP-3130 | 10 mV/(m/s²) ±1 dB | 46 g | 0.699 m/s² |
As described above, using a mobile phone's vibrator allows you to observe basic vibration phenomena (relationship between time-domain waveform and spectrum, relationship between vibration acceleration, velocity, and displacement). On the other hand, the amplitude is affected by the detector's mass, and although we couldn't confirm this time, there are likely differences depending on the mobile phone model and individual phone. Therefore, it cannot be used for precise verification purposes, but measuring the vibration of your mobile phone once might be useful.
(Excerpt from the email newsletter issued on February 23, 2012)
