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Comment by orforforof | Hacker News Reader

I have some expertise here (have used sonar under breaking waves) and I'd say you're spot on. Bubbles act like acoustic black holes, they absorb sound like crazy. Whether you're trying to ping through them in the water, or also if they get stuck on the transducer. The lidar analogy seems apt. The OP speculates that wave breaking noise might be causing interference, but the noise will be way lower frequency than the sonar (ie audible frequencies) so masking by bubbles makes more sense. Any new ideas to mitigate this would be a big deal for sonar!

Also, very cool idea and project!

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> bubbles act like acoustic black holes, they absorb sound like crazy. Whether you're trying to ping through them in the water, or also if they get stuck on the transducer.

I have no idea about nautical applications but in medical imaging you use a phased array ultrasonic transducer which is really a set of single-beams with deterministic phase firing of the ultrasound.

You could probably implement a similar phased array principle by coupling multiple 'ocean grade' single beams. That would give you both beam directionality and I suspect higher immunity to bubble artefacts if you play with it in the time domain.

Trying to figure out how medical ultrasound works is a rabbit hole I'm still trying to claw my way out of. While there are phased arrays, most handheld imagers you would be familiar with are simply linear arrays of approx 100-150 transducer elements. While there is likely some simple phasing going on with adjacent elements firing together, a better model mental model for these would be the transducers firing one by one to produce a scan of individual beams. Before linear arrays, there was actually a single transducer that mechanically scanned back and forth to produce the same effect.

Air bubbles in the bloodstream show up clearly on medical ultrasound. Doctors use this to diagnose atrial septal defects.

This isn’t as relevant to bubbles but a transducer phased array is what is used in multi-beam echosounders[1], which provide a swath of depth readings perpendicular to the ship’s path.

[1] https://en.m.wikipedia.org/wiki/Multibeam_echosounder

The navy uses the effect of the bubbles being acoustic black holes in their Prarie-Masker[0] system to hide surface ship noise from submarine sonar. To sonar, the system sounds like rain hitting the surface.

[0]https://en.wikipedia.org/wiki/Prairie-Masker

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