One might say sound is 'vision' underwater, as far as the sonar of man is concerned. The same applies to the echolocation of toothed whales, such as the orca (opposite, with Prof. Leighton).

Part (I) described the refraction and scattering of the feeding calls of humpback whales within bubble nets produced by these creatures.

However many more species exploit bubble clouds to feed underwater. A superb example can be found in the BBC/Discovery series 'The Blue Planet'. Stills taken from episode one of that series (Figure 1, below) show common dolphin exploiting bubble nets to catch fish.

Not only that, but diving gannets create a wealth of bubble plumes which must complicate the acoustic environment, not only generating loud noises as each plume is formed, but also scattering the ocean sounds (including dolphin signals). This could well startle the fish and cause them to school, aiding feeding as described in Part (I). However whether this is intentional or uinintentional, is unknown.

To dolphins, sonar in a sense provides their 'vision' underwater. Yet such dense bubble clouds as they produce would be impenetrable to  human sonar equipment. This raises the interesting dilemma. When they generate bubble clouds to hunt, are dolphins really nullifying their most spectacular sensory apparatus, in an environment so visually complex that successful echolocation would be very beneficial? Are we to accept that, in making such bubble nets, the dolphins are 'blinding' their most impressive sensory capability? Or have the dolphins adapted their sonar to work in this environment?

Let us say the problem is to detect a linearly scattering target (the 'target') which is difficult to detect because it is immersed in a cloud of bubbly water. Such a target might be a fish in a dolphin bubble net  - even with a swim bladder, the fish would produce ostensibly linear scatter from dolphin echolocation because the gas is driven so far from resonance.  Alternatively, it might consist of a military mine which is a hazard to landing craft because it is hidden from sonar by breaking waves. 

Consider if the emitted sonar signal were to consist of two high amplitude pulses, one having reverse polarity with respect to the other (Figure 2, top line).  Linear reflection from the solid body is shown in Figure 2b(i). The bubble generates nonlinear radial excursions (Figure 2a(i)) and emits a corresponding pressure field (Figure 2a(ii)). Whilst the pressure emitted by the bubble may contain linear and odd-powered nonlinearities, it is the even powered (e.g. quadratic) nonlinearities which will be insensitive to the sign or the driving pulse, and hence which can be used to enhance the scatter from the target over that from the bubbles. It is these quadratic (and high even-powered components) which will be discussed in Figure 2, and below.

Normal sonar would not be able to detect the signal from the solid (Figure 2b(i)), as it is swamped by that from the bubbles (Figure 2a(ii)). If however the returned time histories are split in the middle and combined to make a time history half as long, enhancement and suppression occurs. If the two halves of the returned signals are added, the even-powered nonlinear components of the scattering from the bubble are enhanced (Figure 2a(iii)), whilst the scatter from linearly scattering target is suppressed (Figure 2b(ii)). This can be used to enhance the scatter from biomedical contrast agents. If however the two halves of each returned signal are subtracted from one another,  the even-powered nonlinear components of the scattering from the bubbles is suppressed (Figure 2a(iv)) whilst the reflections from the solid body are enhanced  (with the usual constraints imposed by increased signal-to-noise ratio) (Figure 2b(iii)).

If echolocation is the equivalent of vision underwater, then switching from linear to nonlinear sonar in bubble clouds might find analogy with driving through fog. 'Linear headlamps' would provide the familiar backscatter from the fog, making detection of targets difficult (analogous to the intense sonar backscatter from bubbles). However switching to nonlinear sonar might be equivalent to turning on 'nonlinear headlamps' in a car, which backscatter far less from the fog and so make driving easier.

The hypothesis is therefore, is this the way dolphins think?

For associated journal publications, click on the links below:

Leighton, T.G. From seas to surgeries, from babbling brooks to baby scans: The acoustics of gas bubbles in liquids, Invited Review Article for International Journal of Modern Physics B, 18(25), 2004, 3267-314

Leighton T G, ‘Nonlinear Bubble Dynamics And The Effects On Propagation Through Near-Surface Bubble Layers,’ High-Frequency Ocean Acoustics, Eds. M.B. Porter, M. Siderius, and W. Kuperman, 2005, American Institute of Physics, Melville, New York (in press) (2004)

Leighton T G, Meers S D and White P R, Propagation through nonlinear time-dependent bubble clouds, and the estimation of bubble populations from measured acoustic characteristics. Proceedings of the Royal Society A, 460(2049) 2521-2550, 2004