Sound bends it's direction when it pierces levels with different sound speeds above and below it. So if the sound speed is reducing with depth then each feet of depth it penetrates it slightly bends downward towards the lower speed region. If the ray is already pointed downwards then it goes steeper down. If it is pointed upwards then it turns shallower.
If the sound speed increases again with depth then every feet of water it passes the soundrays bend toward the lower speed region and thus upwards. This can create a pattern of sounds trying to climb to the surface and keep bouncing down at the water-air interface. This is a surface duct, and increases the detection range. But it is very much dependant on the surface state. Stormy surface doesn't reflect the noise very nicely.
If you have a minimum soundspeed at some depth then the soundrays start oscillating around it and create a (deep) sound channel. But you need quite a depth for this to happen, as sounds hitting the bottom will distort it.
As far as I understood, soundspeed layers are not really reflective or like a opaque curtain, as Hollywood would like to make us believe. Maybe more so with extreme salinity and temperature changes will it have reflective properties. Sound will still penetrate it, but it will have less intensity. And due to the bending it will head the other way again within some distance and never reach the listener until it is too close. This is where our tactical advantage comes in when staying below the layer. The trick is to be in a depth in which your sound doesn't reach your target, and most of his does reach you.
|