I recently found this article published in an Asian magazine, which claims the possibility of turning ultrasonic waves into audible sounds after "hitting" an object.
A Japanese enterprise specialized on highways and roads is supposed to commercialize an ultrasound system to warn drivers and to avoid crashes.
"Speakers about 130 centimeters high and 60 cm wide will be installed at intervals along expressways. They issue ultrasonic waves, which cannot be heard by humans due to high frequencies, toward the expressways.
When the ultrasonic waves hit windows and bodies of moving cars, the shapes of the waves change, enabling drivers to hear the sounds inside their vehicles."
I have no idea how can this be possible. Ultrasonic waves can become audible after "hitting" some object?