Study on sound wave characteristics of directional drill rods in coal mines

Acoustic transmission technology offers the advantages of a stable channel, high transmission speed, and independence from other media,making it a promising direction for future underground drilling measurement in coal mines. This study examines the acoustic transmission performance of various structural directional drill pipes used in underground coal mines by applying the transfer matrix method to the continuous drill string wave equation. Key parameters,including different drill pipe lengths (L),joint lengths (2l+b),drill pipe wall thicknesses (R₀ -r),joint wall thicknesses (R₀ -R₁),and drill rod cascade numbers (N),were used to construct a frequency response characteristic calculation matrix. An acoustic transmission test bench was also built for verification. Analysis of 1, 215 parameter combinations of drill string frequency spectrum characteristics revealed the following findings:(1) The length of a single drill pipe significantly impacts the signal passband. As the pipe length increases,the passband narrows, the stopband widens, and the passband period shortens. (2) Longer joint lengths result in narrower passbands and wider stopbands,affecting signal transmission. (3) Changes in wall thickness affect the signal channel due to abrupt changes in cross-sectional area. An increased ratio of the cross-sectional area of the straight pipe section to that of the joint section reduces transmission coefficient fluctuations,stabilizing the transmission coefficient within a single passband and improving suitability for signal transmission.