This experimental study investigates the underground sound generated by different kinds of rock motions that occur in debris flows. The experiments were divided into two parts. The first part consisted of measuring the main characteristics of the underground sound caused by friction of a rock rubbed against a bed of gravel and of that caused by a free-falling rock hitting a similar bed of gravel. In the second part a hydrophone was installed at the bottom of a laboratory channel to measure the underground sound of debris flows in that channel. The sound signals were analyzed using both the fast Fourier transform and the Gabor transform to represent the signals in both the frequency and time-frequency domains. The measurement results reveal that the frequency of the sound generated by the rock-gravel bed friction is relatively low, being mostly between 20 and 80 Hz. In contrast, the frequency range of the collision sound is relatively higher, between 10 and 500 Hz. Finally, the frequency of the underground sound caused by the debris flows in the flume is in the range of 20--300 Hz.