In order to improve channel sounding performance, this paper discusses channel sounders employing modified maximal length (M) sequences for acoustic communications in severe time and frequency selective environment, i.e., doubly selective environment. There still exist the following issues in the conventional channel sounders employing M sequences: 1) M sequences have side-lobe for their auto-correlations; 2) they output acoustic signals in undesired frequency band; 3) estimated channel impulse responses are time-varying due to sampling frequency deviation between Tx/Rx sides. This paper employs the modified M sequences, band pass filters in Tx/Rx sides and over-sampling signal processing employing interpolation for the items 1), 2) and 3), respectively. Next, computer simulation results and experimental results confirm that the proposed schemes improve channel sounding performance. Finally, this paper evaluates actual acoustic channel environments using the proposed channel sounders.
