This study aimed to inspect the unbonded length of a rock bolt using the built-in microphone of a smartphone. An experimental study was conducted using rock bolts of different unbonded lengths. Sound waves were produced by impacting the head of the rock bolt, and the reverberation of the sound waves in the unbonded section was recorded. To compensate for the phase difference between the two signals, the stereo audio signals were extracted from the video and cross-correlation was conducted. The frequency spectrum of the cross-correlated signal shows a clear peak compared to that of original signal. Additionally, the predominant frequency decreased with increasing unbonded length. Time-frequency analysis was accomplished using a wavelet transform. Wavelet transform identified that the predominant frequency was the resonant effect of the reverberant sound waves. This study suggests that the predominant frequency of the reverberant sound waves in the unbonded section is an effective indicator for evaluating the unbonded length of the rock bolt.

中文翻译：

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应用智能手机内置麦克风检测全灌浆锚杆无粘结长度

本研究旨在使用智能手机的内置麦克风检查岩石锚杆的无粘结长度。使用不同无粘结长度的岩石锚杆进行了实验研究。通过冲击岩石锚杆头部产生声波，记录声波在未粘结部分的混响。为了补偿两个信号之间的相位差，从视频中提取立体声音频信号并进行互相关。与原始信号相比，互相关信号的频谱显示出清晰的峰值。此外，主频率随着未粘合长度的增加而降低。使用小波变换完成时频分析。小波变换发现主要频率是混响声波的共振效应。本研究认为，无粘结段混响声波的主频是评价锚杆无粘结长度的有效指标。