Propagation of 127‐, 400‐, and 1700‐Hz Acoustic Energy in the BIFI Range

Comparisons are made of propagation loss and frequency smear at the three frequencies. In a previous paper, it was noted that in the summer the measured propagation loss values at 1700 Hz exceeded the predicted values and were independent of sea state. The current summer tests at 127 Hz showed that while the losses are still independent of sea state, they are close to the predicted values. Ray tracing showed that at 127 Hz the ray undergoes considerably fewer bounces than at 1700 Hz, thus explaining the relative magnitudes of the losses. Application of Rayleigh's criterion showed that, even at the highest sea state encountered in the tests, the surface was smooth for the lower frequencies, thus making the loss independent of sea state. The effect of surface roughness was also apparent in the frequency smear measurements which showed higher dispersion at the highest frequency than at the lower ones. An asymmetry noted in all dispersion spectra is explained by means of diffraction theory.