Dolphins have just the smallest of ears and the best of hearing. The hardly visible pinholes function along the length of the jaws. This seems to be nicely illustrated in the following picture, where the lower jaw and the mouth line are distorted by vibrations in the water.
Less visible is that the melon, which is known to let sound go out, is also distorted. The current expression for this is 'an acoustic lens'. This is a fatty organ in the shape of a flattened egg with the point forward. It is believed to focus sound and transports it into water with minimal resistance. Its function is not fully understood and that may partly be caused by a contradiction between the two assumed functions. The similarity between the tissue that transmits the sound and the enclosing water involves a marginal outlet that is not part of the focussed sound beam. That makes it at least hypothetically possible that part of the sound produced will spread around the head.
Are these vibe waves caused by incoming or by outgoing sounds? Dusty often makes these purring sounds, seemingly as an ongoing check on her environment. And those are the sounds we can hear. It is not unlikely that she checks other aspects around her in frequencies we cannot hear. It would be pointless to make such sounds if they were not also received, so how can this work?
When my kettle whistles, I often go get it while whistling back myself. When I tune my pitch to that of the kettle, they interfere. This is an oscillation that carries digital information.
If you assume this phenomenon in the sound production of a dolphin then the beam is sent to bounce and the marginal outlet serves to interfere with the echo. The resulting interference then is the primary info scan.
A look under water at the dolphin's head reveals a sunray projection of the surface vibrations on Dusty's melon.
This 'collar' of sound intercepts and codes the echo before it is received by the jaw. The teeth of the dolphin are known to, like antennae, resonate and thereby amplify the returning sound waves. In this the angle at which the rows of teeth stand, ±10°, plays a role, while the length of the jaw gives depth to the 'short band' sound of the interference, the secondary scan. Because the further your eyes stand apart, the more depth you can see and this goes for sound as well, be it here on a gliding scale of about 22 teeth per jaw row.
From aside on the following photos you can see that, in spite of a restless surface, the vibrations projected along Dusty's face and body are markedly stable.
Further backwards the patterns become larger and more diffuse as the intensity of the 'sound collar' decreases.
In the last picture you can see that the vibrations can differ in frequency, meaning that the pitch that produces them is also variable.
I must hasten to state that the above theory is not supported by official research and only an opinion of mine. Like a DIY science.