Transmission Loss

Transmission in the ocean has probably been the subject of more interest than any other topic in underwater communication, since it is the parameter that is the least predictable and the least capable of being influenced.

The sound from a source can travel through the water both directly and by means of multiple bounces between the surface and seabed. Sound may also travel sideways through the rocks of the seabed, re-emerging back into the water at distance. Refraction and absorption further distorts the impulse, leading to a complex wave arriving at a distant point, which may bear little resemblance to the wave in the vicinity of the source. Finally, sound may be carried with little loss to great distance by being trapped in sound channels.

Predicting the level of sound from a source is therefore extremely difficult, and use is generally made of simple models or empirical data based on mesaurements for its estimation.

Estimates of Transmission Loss

Transmission Loss is a measure of the rate at which sound energy is lost, and is defined as:

• TL = 20 log(P₀ / P_R)

Where:

• P₀ = The pressure at a point one metre away from the source
• P_R = The pressure at range R metres from the source

The usual method of modelling the Transmission Loss is from the expression:

• TL = N log(R) - αR

Where:

• SPL = The peak sound pressure level of the noise in dB re 1 µPa
• SL = The Source Level of the noise source in dB re 1 µPa
• R = The range from the source
• N = The coefficient of geometric spreading
• α = The coefficient of absorption

High values of N and α relate to rapid attenuation of the sound and limited area of environmental effect and low values to the converse. For ranges of < 10 km, the linear attenuation term α can in general be ignored; a value of N = 20, corresponding to spherical spreading of the sound according to the inverse square law, is often assumed.