## Sonar Equation

The sonar equation is used in underwater signal processing to relate received signal power to transmitted signal power for one-way or two-way sound propagation. The equation computes the received signal-to-noise ratio (SNR) from the transmitted signal level, taking into account transmission loss, noise level, sensor directivity, and target strength. The sonar equation serves the same purpose in sonar as the radar equation does in radar. The sonar equation has different forms for passive sonar and active sonar.

### Passive Sonar Equation

In a passive sonar system, sound propagates directly from a source to a receiver. The passive sonar equation is

*S**N**R*=*S**L*−*T**L*−(*N**L*−*D**I*)

where SNR is the received signal-to-noise ratio in dB.

#### Source Level (SL)

The source level (*SL*) is the ratio of the transmitted intensity from the source to a reference intensity, converted to dB:

*S**L*=10log*I*s*I*ref

where *I*_{s} is the intensity of the transmitted signal measured at 1 m distance from the source. The reference intensity, *I*_{ref}, is the intensity of a sound wave having a root mean square (rms) pressure of 1 μPa. Source level is sometimes written in dB// 1 μPa, but actually is referenced to the intensity of a 1 μPa signal. The relation between intensity and pressure is

*I*=*p*2rms*ρ**c*

where *ρ* is the density of seawater, (approximately 1000 kg/m^{3}), *c* is the speed of sound (approximately 1500 m/s). 1 μPa is equivalent to an intensity of *I*_{ref} = 6.667 ✕ 10^{-19} W/m^{2}

Sometimes, it is useful to compute the source level from the transmitted power, *P*. Assuming a nondirectional (isotropic) source, the intensity at one meter from the source is

*I*=*P*4*π*

Then, the source level as a function of transmitted power is

*S**L*=10log10*I**I*ref=10log10*P*4*π**I*ref=10log10*P*−10log104*π**I*ref=10log10*P*+170.8

When source level is defined at one yard instead of one meter, the final constant in this equation is 171.5.

When the source is directional, the source level becomes

*S**L*=10log10*I**I*ref=10log10*P*+170.8+*D**I*src

where *DI*_{src} is the directivity of the source. Source directivity is not explicitly included in the sonar equation.

#### Receiver Directivity Index (DI)

The sonar equation includes the directivity index of the receiver (*DI*). Directivity is the ratio of the total noise power at the array to the noise received by the array along its main response axis. Directivity improves the signal-to-noise ratio by reducing the total noise. See Element and Array Radiation and Response Patterns for discussions of directivity.

#### Transmission Loss (TL)

Transmission loss is the attenuation of sound intensity as the sound propagates through the underwater channel. Transmission loss (*TL*) is defined as the ratio of sound intensity at 1 m from a source to the sound intensity at distance *R*.

*T**L*=10log*I*s*I*(*R*)

There are two major contributions to transmission loss. The larger contribution is geometrical spreading of the sound wavefront. The second contribution is absorption of the sound as it propagates. There are several absorption mechanisms.

In an infinite medium, the wavefront expands spherically with distance, and attenuation follows a *1/R ^{2}* law, where

*R*is the propagation distance. However, the ocean channel has a surface and a bottom. Because of this, the wavefronts expand cylindrically when they are far from the source and follow a

*1/R*law. Near the source, the wavefronts still expand spherically. There must be a transition region where the spreading changes from spherical to cylindrical. In Phased Array System Toolbox™ sonar models, the transition region as a single range and ensures that the transmission loss is continuous at that range. Authors define the transition range differently. Here, the transition range,

*R*

_{trans}, is one-half the depth,

*D*, of the channel. The geometric transmission loss for ranges less than the transition range is

*T**L*geom=20log10*R*

For ranges greater than the transition depth, the geometric transmission loss is

*T**L*geom=10log10*R*+10log10*R*trans

In Phased Array System Toolbox, the transition range is one-half the channel depth, *H/2*.

The absorption loss model has three components: viscous absorption, the boric acid relaxation process, and the magnesium sulfate relaxation process. All absorption components are modeled by linear dependence on range, *αR*.

Viscous absorption describes the loss of intensity due to molecular motion being converted to heat. Viscous absorption applies primarily to higher frequencies. The viscous absorption coefficient is a function of frequency, *f*, temperature in Celsius, *T*, and depth, *D*:

*α*vis=4.9×10−4*f*2*e*−(*T*/27+*D*/17)

in dB/km. This is the dominant absorption mechanism above 1 MHz. Viscous absorption increases with temperature and depth.

The second mechanism for absorption is the relaxation process of boric acid. Absorption depends upon the frequency in kHz, f, the salinity in parts per thousand (ppt), *S*, and temperature in Celsius,T. The absorption coefficient (measured in dB/km) is

*α*B=0.106*f*1*f*2*f*21+*f*2*e*−(*p**H*−8)/0.56*f*1=0.78G*S*/35*e**T*/26

in dB/km. *f _{1}* is the relaxation frequency of boric acid and is about 1.1 kHz at

*T*= 10 °C and

*S*= 35 ppt.

The third mechanism is the relaxation process of magnesium sulfate. Here, the absorption coefficient is

*α*M=0.52(1+*T*43)(*S*35)*f*2*f*2*f*22+*f*2*e*−*D*/6*f*2=42*e**T*/17

in dB/km. *f _{2}* is the relaxation frequency of magnesium sulfate and is about 75.6 kHz at T = 10°C and S = 35 ppt.

The total transmission loss modeled in the toolbox is

*T**L*=*T**L*geom(*R*)+(*α*vis+*α*B+*α*M)*R*

where *R* is the range in km. In Phased Array System Toolbox, all absorption models parameters are fixed at *T = 10*, *S = 35*, and *pH = 8*. The model is implemented in `range2tl`

. Because *TL* is a monotonically increasing function of *R*, you can use the Newton-Raphson method to solve for *R* in terms of *TL*. This calculation is performed in `tl2range`

.