Application of built-in microphone of smartphone for inspecting unbonded length of fully grouted rock bolt

Jong Sub Lee, Jung Doung Yu

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Article number105646
JournalTunnelling and Underground Space Technology
Publication statusPublished - 2024 Apr

Bibliographical note

Publisher Copyright:
© 2024 Elsevier Ltd


  • Predominant frequency
  • Rock bolt
  • Smartphone
  • Sound waves
  • Unbonded length

ASJC Scopus subject areas

  • Building and Construction
  • Geotechnical Engineering and Engineering Geology


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