Water leak location can be determined in a car by placing a 40KHZ ultrasonic transmitter in the car and using an ultrasonic leak detector with its microphone to probe the outside to find where the sound exits.It is my understanding that ultrasonic leak detection is based on the turbulent flow of fluids and gases. Turbulent flow has a high content of ultrasound. Turbulence must occur for a leak to be detected.
When using a transmitter, I thought the sound waves just traveled thru the hole. However, I am not sure that is what happens. The leak detector seems to be able to find leaks much smaller than the 40KHZ wavelength. At 20 degrees C, the wave length of 40KHZ in air is about 8.6mm (larger in glass, metal, or gasket material). This is a very large hole. Yet these detectors seem to find leaks a very small fraction of the 40KHZ wavelength.
So how do these things actually work when using a transmitter? Is a turbulent flow created when using an ultrasonic transmitter? If so, how? Or is some other process going on?