Radiated Sound Power from Near-Surface Acoustic Sources

Abstract

This article investigates the radiated sound power from idealized propeller noise sources, characterized by elemental monopole and dipole acoustic sources near the sea surface. The free surface of the sea is modeled as a pressure-release surface. The ratio of sound power of the near surface sources to the sound power from the same sources in an unbounded fluid is presented as a function of source immersion relative to sound wavelength. We herein show that the sound power radiated by submerged monopole and horizontal dipole sources is greatly reduced by the effect of the free surface at typical blade passing frequencies. By contrast, the sound power from a submerged vertical dipole is doubled. A transition frequency for the submerged monopole and horizontal dipole is identified. Above this transition frequency, the radiated power is not significantly influenced by the sea surface. Directivity patterns for the acoustic sources are also presented. The principal sources contributing to underwater radiated noise (URN) over a wide frequency range are propellers and onboard machinery (Urick 1983; Ross 1987; Collier 1997; Carlton 2007). Propeller sources are highly complex, but simplification is possible at low frequencies where the wavelength of underwater sound is much larger than propeller dimensions. The propeller may then be regarded as a set of fluctuating forces at the propeller hub and a stationary monopole source that represents the growth and collapse of a cavitation region as each blade passes through the region of wake deficit. This type of model was used by Kinns and Bloor (2004) to examine the net fluctuating forces on a cruise ship hull due to defined propeller sources. The nature of the monopole source was considered by Gray and Greeley (1980), who focused on singlescrew merchant ships where cavitation is dominant at operational speeds. Nonuniformity in the wake, as well as static pressure that falls toward the sea surface, causes this monopole source to be located near top dead center, closer to the surface than the propeller hub. It introduces cyclic components at multiples of propeller blade passing frequency (bpf) as well as broadband noise over a wide frequency range. These components create a pressure field that acts on nearby hull surfaces, but the URN is controlled by the presence of the pressure release surface that corresponds to the free surface of the sea. The aim of this article was to investigate how idealized propeller noise sources are influenced by the surface of the sea.