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."

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I have no idea how can this be possible. Ultrasonic waves can become audible after "hitting" some object?

Reference: http://www.asahi.com/ajw/articles/AJ201801050049.html

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    $\begingroup$ Under normal conditions I think this is impossible. Maybe some special material (highly non linear) could do something like this, but not the widows of a car. Another way to do this could be the Doppler effect, but the ultrasonic waves should be sent from the back of the cars and not from the front as in the link you posted. Also the speed of the cars should be quiet high for the effect to become appreciable, I think. $\endgroup$ – user171780 Jul 23 '18 at 14:46
  • $\begingroup$ Thanks @user171780, I agree with you. However, somehow this is supposed to be a commercialized product. Although the article has no technical information, it claims a fact. I could not found any additional information about this specific research or product. $\endgroup$ – Luis M Gato Jul 23 '18 at 14:49
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    $\begingroup$ Superposition of waves at the incident frequency and the reflected waves which are Doppler shifted would produce a beat frequency in the audible frequency range? $\endgroup$ – Farcher Jul 23 '18 at 15:08
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    $\begingroup$ @Farcher, I don't see how is that possible. The beat would only produce a wave on the ultrasonic sound, with an envelope determined by the difference in frequency. The frequency difference modifies the amplitude of the ultrasonic signal, but it would not make it audible. $\endgroup$ – Luis M Gato Jul 23 '18 at 15:31

Another possibility is that the ultrasound is used as a carrier modulated by an audible frequency. In this case, non-linearities in the windows or other parts of a car would cause demodulation and the resulting low frequency component would be audible.

Here is a related article about a potential use of AM modulated ultrasound as an alternative to hearing aid. Among other things, it states that such signal won't be perceivable by humans if transmitted through air.

  • $\begingroup$ " It also has the added benefit of the warning sounds being inaudible to residents living near expressways. "- The houses near expressways can also have non-linearity, so they too could demodulate the ultrasound making it audible to the residents. $\endgroup$ – AlphaLife Jul 23 '18 at 19:33
  • $\begingroup$ @KrishnanandJ Ultrasound is easy to direct and it decays faster than audible sound. $\endgroup$ – V.F. Jul 23 '18 at 19:38
  • $\begingroup$ Mine was an obvious answer. Yours is a genius answer. $\endgroup$ – AlphaLife Jul 27 '18 at 6:50

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