The horizontal angle (0) is measured positive clockwise from true North while the vertical angle (0) is measured positive upward from the horizontal plane. Note that no receiver beam patterns were convolved with the noise levels in Equations (7.5) and (7.6).

7.4 The RANDI model - a specific example

The research ambient-noise directionality (RANDI) model is an instructive example of an ambient-noise model. RANDI calculates the vertical and horizontal directionalities of low-frequency (10-500 Hz) ambient noise for a selected ocean environment (Wagstaff, 1973).

7.4.1 Transmission loss

RANDI utilizes one of three sources of TL inputs:

1 self-contained linear ray-trace routine;

2 input data consisting of TL versus range in addition to arrival angle arrays; and

3 deep-sound channel propagation algorithm.

7.4.2 Noise sources and spectra

RANDI considers six sources of isotropic and anisotropic surface and volumetric noise: (1) shipping; (2) wind-wave; (3) biological; (4) sea state zero; (5) rain; and (6) distant sources. A seventh source, that of a target, is also included as an option (Figure 7.2). Figure 7.3 illustrates the modeled noise field. The surface noise is generated by an infinite number of point sources distributed along a horizontal noise source plane located at a depth of about 6 m, corresponding to the nominal drafts of surface vessels. Empirical algorithms are used to calculate squared pressure spectrum levels for each of the seven noise-generation mechanisms. All of the noise (and target) squared pressure spectrum levels are then integrated over a user-specified bandwidth using an input frequency-response function. Noise level in this context refers to the mean squared spectrum level.

7.4.3 Directionality

To determine the horizontal directionality of the noise field, the ocean is divided into n wedge-shaped regions (called sectors) with the receiver at the center. The vertical thickness of the sectors is equal to the ocean depth at the receiver location. Noise calculations are performed independently in each sector defined by the user. The total squared noise pressure, as measured by an omnidirectional hydrophone, is obtained by summing the squared noise pressures for the n sectors. An example of horizontal directionality of the


System Depth Frequency Bandwidth Beam pattern

Environment S.S. Profile Bottom loss Latitude Noise sources Wind Ships







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