Car Window Water Leak Detection With An Ultrasonic Transmitter?

The ratio of wavelength to aperture size is not relevant for sound waves. Sound waves consist of longitudinal vibration, i.e. vibration in the direction of travel, not transverse vibration as in the case of electromagnetic radiation, and so are not affected by this ratio.

Which velocity are you using--that of light or of sound? That will have an effect on wavelength....

In air, sound travels about 1000 ft/sec. The wavelength of 1 KHz is about 1 foot, and 40 KHz a little larger than 1/4 inch. A hole that big you could probably find with your finger.

just use a water hose and a paper towel.

Oh, hose outside, paper towel inside.

Or drive through a car wash...

Smoke Wizard....

https://www.youtube.com/watch?v=WlckHdd5WoE

Using high tech on a low tech problem is over complicating the problem.

This is another situation where K.I.S.S is the better approach. Water requires a substantial gap to leak through. You can have "wind noise" coming through a window seal at highway speeds with no water leaks.

To keep it simple, I've always used a tick that was passed down to by a master mechanic 50 years ago. And that is to close a (US) $ dollar bill in between the window glass and seal, it should have a substantial resistance when you trying pulling out the bill out. Like using a "feeler gauge", do this all the way round the window and seal. This also works on rear hatches and trunk (UK-Boot) seals

Thanks for the replies.

Please understand that my question is not about how to find water leaks. The question is, how is the ultrasonic sound produced on the other side of a hole, much smaller than the sound wavelength, and not necessarily in a straight path, when using an ultrasound transmitter?

Is turbulent flow somehow created? If so, how? Does the sound just travel thru the hole? If so how, when the hole size is a small fraction of the sound wavelength and not in a straight path? Or is some other process going on?

I believe Rixter answered this in post #1.

Sound is a pressure wave in the air, with rarefaction and compression. As long as there is passage for the compression and rarefaction to move molecules through, the sound will exit. The only time the hole won't pass is when it is smaller than the air molecules.

For a science fair in high school I used a round split plate neon bulb and modulated it with recorded sounds. The image was reflected by a spinning mirror, giving close and farther apart circles in a train effect. This is close to a sound wave from a round speaker. (it didn't spread out as it moved away, but still was a pretty good image to imagine the appearance of a sound wave) Actually got a scholarship from that turkey.

The ultrasonic detection principle you're attempting to use is applicable only where the fluid's flow regime is turbulent. In other words, water would have to be surging through a hole in the windshield (under substantial pressure) to generate this noise. The slow seepage of water through a typical seal leak or hole would definitely be a case of "laminar" flow rather than turbulent, and completely silent.