T. C. Yang
Dirk Tielbuerger
Naval Res. Lab., Washington, DC 20375
Acoustic environment is known to influence the vertical directionality of
the ambient noise. For example, in shallow water with a downward refractive
(summer) sound-speed profile, the noise vertical directionality is expected to
show a deep (>10 dB) null in the horizontal direction for wind-generated noise
below 1 kHz. This ``horizontal null'' is a consequence of the fact that
low-order modes are weakly excited by the noise sources which are located near
the ocean surface. This phenomenon has been experimentally observed and is not
expected to occur in an upward refractive (winter) sound-speed profile
environment. It is noted, however, that when internal waves are present, the
deep null may disappear or sufficiently weaken due to the ``strong'' mode
coupling effect caused by the scattering of the noise field from the internal
waves. In the internal wave fields, the conversion of higher-order modes
(generated by the surface noise sources) to low-order modes by the internal
waves can fill in the deep (>10 dB) null which would otherwise be expected
(theoretically). Numerical calculations of the vertical noise directionality at
<500 Hz for typical shallow water environments with rock (low loss) and sediment
(lossive) bottoms are presented and the potential effects of internal waves on
the ambient noise directionality are investigated. Practical situations are
discussed for which the smearing of the ``horizontal null'' in the noise
vertical directionality can be experimentally verified. [sup