The most enlightening comment there was the one that contained this:

"Still I ended up north of $3k for hardware, and several hundred projected work-hours. A lot of the cost comes from needing 50-100 transducers to get a useable image."

https://news.ycombinator.com/item?id=13231831

So you can make a very low cost one, but you trade off what use cases it's good for.

Ah, that makes a lot of sense - I was trying to find information on how ultrasound machines worked awhile ago, and most of the information I found just said that a ceramic piezo is used to emit the sound and measure the time until it returned. I could not fathom at the time what kind of math must be required to get a 2D image out of a single data point like that, and I couldn't find much elaboration on the subject.

But if it's just a few hundred piezos in an array, now THAT makes a lot more sense.

It's not an array of piezos (or at least, not a one-per-column array of transducers). It's a phased array which they use to rapidly scan the volume, iirc something like 32 emitters and a single receiver in a commercial unit.

Source: I looked into building one last time we had a baby but didn't get around to it.

Yep. Array is the keyword that makes it easier to Google...phased arrays, curved arrays, etc. And of course, with a bunch of piezos, your signal processing problem is larger.

The point being that on these experiments, the sensor isn't an array of piezos, rather a single-element piezo. Cuts the costs (a fab invoices around 100-200e for such a sensor), but lowers the overall image quality.

I wonder if two sensors placed at a distance can make a huge difference, and how/if the image quality converges as you increase sensors.

One of the next experiment is to try with 8 small elements ;)

Before trolling a bit, I remember the main costs don't come from hardware (though it definitely costs something) - rather for r&d, patents, and most of all proper certification, since, in the end, it's a matter of life and death. That being said, this experiment is mostly a dev kit to help curious tinkerers explore the topic :)

> in the end, it's a matter of life and death

But also if you can't afford one.

You don't go to jail for not building one.

Am I allowed to built an ultrasonic probe the way I want if I don't use it on anybody else but me ?

Yes. Just come up with a non-medical/safety use for it and you are good. Check your car's engine to see if it will last through the next race - a very useful thing to do and nobody will think twice. Check a boiler to be sure it won't explode - you better get this right or people will die so don't talk about potential uses here.

Yes, more and more (since I started reusing old devices), I wished I could inspect things non invasively. To find locking or friction points.

I guess nothing prevents you from doing so :)