4 ms·

This is very impressive. I have no expertise here, but I was wondering if the low confidence in surf is due to bubbles? (I took a quick look at the transducer

This is very impressive. I have no expertise here, but I was wondering if the low confidence in surf is due to bubbles? (I took a quick look at the transducer and software docs, but did not see anything explicit about the confidence figure.) If it is due to bubbles on the underside of the board, perhaps putting the transducer in the middle or behind would help? If it is due to bubbles in the water column, it reminds me of something I learned about lidar mapping: you may get reflections from the tree canopy, the understory or the ground, but in any small area, one can assume the longest-in-transit is probably the ground (and you can sometimes figure out the height of the other layers as a bonus.)

orforforof 5y ago
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!
- DoingIsLearning 5y ago
  > 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.
  - 0xfaded 5y ago
    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.
    - nradov 5y ago
      Air bubbles in the bloodstream show up clearly on medical ultrasound. Doctors use this to diagnose atrial septal defects.
  - hydhyd 5y ago
    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 https://en.m.wikipedia.org/wiki/Multibeam_echosounder
- dylan604 5y ago
  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 https://en.wikipedia.org/wiki/Prairie-Masker
foobarbecue 5y ago
Of course! Bubbles! I didn't even think about that. I'm going out with some goggles next time to see if I can see the bubbles.
- orforforof 5y ago
  Bubbles will come and go, unless you're really in the white water (where you probably aren't interested in depths anyway). You'll likely have success if you make a simple software filter like "take the max depth over the past 30 seconds" with some tweaking. You'll get brief glimpses through clear water that you can capture as actual depths. People do this with sonar from jet skis, so don't despair it's a surmountable problem. (Of course those jet ski systems are big and clunky which is why your stuff is cool.)

orforforof 5y ago

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!

DoingIsLearning 5y ago
> 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.
- 0xfaded 5y ago
  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.
  - nradov 5y ago
    Air bubbles in the bloodstream show up clearly on medical ultrasound. Doctors use this to diagnose atrial septal defects.
- hydhyd 5y ago
  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 https://en.m.wikipedia.org/wiki/Multibeam_echosounder
dylan604 5y ago
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 https://en.wikipedia.org/wiki/Prairie-Masker

foobarbecue 5y ago

Of course! Bubbles! I didn't even think about that. I'm going out with some goggles next time to see if I can see the bubbles.

orforforof 5y ago
Bubbles will come and go, unless you're really in the white water (where you probably aren't interested in depths anyway). You'll likely have success if you make a simple software filter like "take the max depth over the past 30 seconds" with some tweaking. You'll get brief glimpses through clear water that you can capture as actual depths. People do this with sonar from jet skis, so don't despair it's a surmountable problem. (Of course those jet ski systems are big and clunky which is why your stuff is cool.)