Formula Equation (3) gives the power spectral density of the four noise components in decibels re�� Pa per Hz as a function of frequency (f) in kHz. In addition, s is the shipping factor, which ranges from zero to one for low and high activities, respectively, and w is the wind speed in m/s:10logNt(f)=17?30logf10logNs(f)=40+20(s?0.5)+26log(f)?60log(f+0.03)10logNw(f)=50+7.5w1/2+20logf?40log(f+0.4)10logNth(f)=?15+20logfN(f)=Nt(f)+Ns(f)+Nw(f)+Nth(f)(3)As introduced in previous works [3,10], the overall effect of the transmission loss (A(l, f)) and the noise density (N(f)) are evaluated as the narrow-band signal-to-noise ratio (SNR) for different distances and different frequencies. The results shown in Figure 1 are useful for estimating the optimal frequency band, attending to the area that the network should cover.

For example, larger areas should be covered using low-frequency acoustics (below 10 kHz, as expressed in [3]), but for medium or short distances in high-density networks, frequencies around 80 kHz are preferred.Figure 1.Narrow-band signal-to-noise ratio (SNR; 1/(AN)) as a function of frequency for varying transmission distance. Environmental factors used: practical spreading, k = 1.5; wind speed, w = 3 m/s; and moderate shipping activity, s = 0.5.2.1.2. MultipathMultipath in underwater channels is mainly caused by two relevant factors: wave reflection at the surface, bottom and any object and sound refraction in the water [5].The reflection coefficient is equal to ?1 under ideal conditions, while bottom reflection coefficients depend on the type of bottom (hard, soft) and the grazing angle [5].

The second effect is a consequence of Snell’s law. The propagation speed of an acoustic signal underwater is, on average, approximately1,500ms; however, the actual value depends on the salinity, temperature and pressure of the medium, among other factors. A complete nine-term equation for calculating propagation speed is defined in [11], as shown in Equation (4), where T denotes the temperature in degrees Celsius, D, the depth in meters, and S, the salinity in parts per Entinostat thousand of the water.c=1448.96+4.591T?5.304��10?2T2+2.374��10?4T3+1.340(S?35)+1.630��10?2D+1.675��10?7D2?1.025��10?2T(S?35)?7.139��10?13D3(4)Figure 2 shows an example of how sound speed propagation changes along a water column placed in the Pacific ocean.

As can be seen in subfigure (a), the speed rapidly decreases until approximately a 600-m depth, remaining nearly constant around 4 in deep oceans waters. In subfigure (b), where the individual contributions of temperature, salinity and pressure to sound speed are displayed, shows that deep water sound speed variation mainly depends on the increasing pressure.Figure 2.Temperature, salinity data and their impact on the sound speed profile. Data retrieved from the Papa station placed in the Pacific ocean (39��N, 146��W) in August, 1959.