## 3 System Test and Application
### System Test  
The noise test conducted experiments by playing an audio file in AAC format based on the ISO standard, with a sampling rate of 44.1 kHz and a bit rate of 192 Kbps. The two measuring instruments recorded two data every second, and the experiment time was 150 s. Among them, Figure 4 shows the data comparison curve diagram of the two measuring instruments in this test, and Figure 5 shows the relative error with interval distribution. In Figure 4, the data trends of the two curves are roughly the same. The data fluctuation of the mobile phone measuring instrument is slightly lower than the standard value of the professional instrument, and the amplitude is generally slightly lower. The reasons for generating the lower fluctuation may include the following:  
1. Hardware: the limitations of the type and quality of built-in MEMS microphones in smartphones (small size, circuit board position, dynamic range and signal-to-noise ratio responsiveness, etc. );
2. Software: the correction method of the application, the time delay of data processing, and the influence of software running in the background;
3. Others: the influence of various obstacles (phone protection covers, microphone openings clogged by dust), different operating systems, different mobile phone brands, and environmental vibration. Figure 4: Comparison of noise data. It can be seen from Figure 5 that the relative errors of the two groups of data are mainly distributed between 1.5% and 4.5%; the maximum error is 7.83%, and the overall error is within a reasonable range. In the acceleration test, the mobile phone measuring instrument and the professional measuring instrument were fixed on a wooden partition, and the vibration environment was created by artificial shaking. For the convenience of comparison, two data were recorded every second, and the experiment time is 120 s.  
Figure 6 shows the relative error distribution interval diagram of the three-axis acceleration, and the experimental data include a total of 256 points. As shown in this figure, the relative error of the three-axis acceleration is generally distributed between 2% and 10%; the individual data error is greater than 14%, and the minimum error is less than 2%. In the three-axis data, the relative error data distributions of the y-axis and the x-axis are relatively concentrated, mainly distributed in 2-8%, and the z-axis data distribution is relatively uniform, mainly distributed in 0-10%. The Sperling index test adopted the same method as the acceleration test, and the vibration test environment was built by artificial shaking. The Sperling index of the mobile phone measuring instrument was extracted from the database, and the Sperling index of the professional acceleration instrument was calculated from its acceleration value through MATLAB. The sampling frequency of the two instruments was 50 Hz. A Sperling index  
Figure 5: Distribution of relative error intervals. Figure 6: Distribution of acceleration relative error. value was calculated for every 1000 acceleration data, and the experiment time was set to be 55 s. There are 10 sets of data in total, as shown in Table 2.  
In Table 2, there is little difference between the Sperling index value calculated by the mobile phone and the Sperling index value calculated by MATLAB; i.e., the lateral error is less than 5%, and the vertical error is less than 3%. The data comparison shows that there is still a certain error between the mobile phone and the professional tester. The reasons for generating the above errors may include the following:  
1. On the whole, due to the limitations of mobile phone cost, sensor volume, and built-in location, the measurement accuracy and sampling performance of the built-in acceleration sensor and microphone in mobile phones are not as good as professional equipment, but the gap is not obvious;
2. The computing power of the mobile phone CPU is uneven, which leads to a certain delay in the A-weighting calculation and Sperling value calculation.