This "channeling" of sound occurs because there is a minimum in the vertical sound speed profile in the ocean caused by changes in the density of the water column. The density is affected by water temperature, pressure (depth), and salinity. Changes in the speed of sound in the water are largely due to changes in temperature and pressure, with salinity offering only a minor effect..

As temperature decreases, the speed of sound decreases
As pressure (depth) increases, the speed of sound increases

Idealized profiles of temperature and pressure are shown below along with their resulting sound speed profile:

This minimum sound speed at the channel axis is the result of higher temperatures toward the surface of the ocean and higher pressures toward the bottom of the ocean. At the surface, the water temperature is relatively warm; as depth increases, temperature decreases so sound speeds decrease. At a certain depth (generally, at the bottom of or below the permanent thermocline), the water temperature is fairly uniform. At this point, the increasing pressure of the water column due to depth "takes over", and sound speeds increase due to increasing pressure. At low and middle latitudes, the deep sound channel axis is between 600-1200 m below the sea surface. It is deepest in the subtropics and comes to the surface in high latitudes, where the sound propagates in the surface layer.

Sound waves can become "trapped" in the deep sound channel and propagate long distances because they experience little attenuation beyond that due to geometric spreading and minor volume scattering in the water. To simplify, think of the water column as a layer cake with different densities of water piled on top of each other. Sound waves refract as they cross between layers of water with different densities. The refraction of sound waves from higher velocities above and below the sound channel axis bend the sound back towards the axis. Sound energy is refracted towards the axis of the sound channel away from the surface and the bottom of the ocean. Because propagating waves do not interact with either the sea surface or seafloor, sound propagating in the deep sound channel does not attenuate as rapidly as bottom- or surface-interacting paths.

An example of ray paths for a source in the sound channel is shown below. Note that in this idealized situation, the sound waves do not interact with either the surface or the bottom. This is a simplified example of propagation in the sound channel.

For a brief discussion on SOFAR, visit National Academies of Science's "Sound Pipeline".