Propagation#
Unlike in the vacuum of space or the air of our atmosphere, light does not travel very far in the underwater environment. It is strongly absorbed by the water molecules themselves and scattered by suspended particles and material in the water column. In a crystal clear lake, you might be able to see out to about 200 feet (60 m) but usually it is much less. Sound, on the other hand, can travel much, much further. Sound is a traveling wave of compression and rarefaction of the water itself. While the water molecules will also absorb some of it’s energy, the absorption of sound is weak as compared to that of light and in fact, low frequency sound in the ocean can travel half way around the world. It could go further if not for the continents that tend to get in the way.
In this section, we will explore some of the concepts behind sound propagation and explore those aspects of the underwater environment that affect the ability of sound to travel long distances. While we will begin by revisiting the concept of simple sound sources and how the sound generated by these sources interacts with one another, the majority of this section will explore how the underwater environment affects the propagation of sound. The key aspects of the environment that we will consider are the sea surface, the seafloor, and the properties of the water itself. We will conclude this section by considering how the combined effects of the environment can either enhance or diminish the ability of sound to travel long distances in the ocean.
Learning objectives
Understand the factors that impact sound propagation through water.
Understand how sound reflects from the sea surface and seafloor.
Become familiar with how the environment as a whole affects propagation through the ocean.
Before diving into this section, we will touch briefly on how sound propagation enters into the sonar equation.
Transmission Loss#
Transmission loss (TL) is the ratio of the recieved sound to the transmitted sound (detailed discussion is later in this section) and is a measure of the amount of sound that has been lost to the environment. When we say “lost,” we are referring to not just sound that has been lost due to absorption or reflection from the seafloor, but also the reduction in sound level due spreading from the source or refraction away from the receiver. While the refracted sound isn’t lost (we might even know where it went!), the key point is that it doesn’t contribute to the signal measured at the receiver. Hence it is “lost” from the receiver’s point of view.
In our orca example, when Ola is purely listening,
\(RL = SL - TL,\)
the TL is the difference in decibels between the source level and the receive level (which is the literal definition of transmission loss). However, in our echolocation example, when Ola is pinging and listening,
\(RL = SL - 2TL + TS,\)
the transmission loss is twice as much as the purely listening case. The doubling of the loss is due in part to the fact that the sound travels to the target and then back to the reciever. This is also due to the fact that sound propagation paths are reciprocal, in that if you transmit sound back along the path from the receiver to the sound source, the reduction in sound level will be the same as occured going from the source to the receiver. We won’t go into why that’s the case, but it’s an important concept and one that is captured in the sonar equation.