SOFAR channel in sound

When sound spreads out evenly in each and every one directions, the intensity drops in proportion to the inverse square of the distance. However, in the ocean there is a layer also called the 'deep sound channel' or SOFAR channel which can imprison sound waves at a particular depth, allowing them to travel much further. In the SOFAR channel, the speed of sound is lesser than that in the layers above and below. Just as light waves will refract towards a section of higher index, sound waves will refract towards a region where their speed is reduced. The result is that sound gets small in the layer, much the way light can be confined in a sheet of glass or optical fiber.

A similar consequence occurs in the atmosphere. Project Mogul successfully used this consequence to detect a nuclear explosion at a considerable distance.