Averaging procedure, which is one of the data smoothing methods, is generally used to obtain a fine-quality lofargram. However, the procedure causes to spread impulse noise over time-axis on the lofargram. In this case, it is difficult to distinguish target signal from received signal on the lofargram. In this paper, we propose Variable Time-Resolution Lofargram using Morphological Opening Filter to suppress the impulse noise on lofargram instead of averaging procedure. The computer simulation results in this paper shows effective improvements of processing gain and time-resolution as compared with that of averaging procedure.
To clarify the propagation mechanism of underwater hearing, threshold levels were carefully measured as a function of submerged skull and ear canals by means of a tank with low background noise level. From several experimental results, it is concluded that the mechanism of underwater hearing is primarily due to bone conduction but the external ear canals also play an important role in total sensation as listed below. (1) At frequencies above about 1 kHz, underwater hearing is primarily bone conduction via skull and more sensitive around the ear than at top of head. (2) Water in the ear canal acts as a loss to bone conduction. (3) At frequencies below about 1 kHz, the ear canal conduction gradually makes greater contribution to the auditory sensation and total characteristics slightly vary depending on the ear canal conditions.