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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
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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.

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  • $\begingroup$ " 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. $\endgroup$ – Krishnanand J 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. $\endgroup$ – Krishnanand J Jul 27 '18 at 6:50
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When an ultrasonic wave 'hits' the vehicle, a part of it bounces off and a part would pass through. The waves lose some energy in the process of 'passing through' the vehicle boundaries. The energy of a wave is directly proportional to the frequency.

Building on @Farcher's comment,the incident sound may get inside the car through various sites of different thickness. Each of the sites become different sources.Since the ultrasonic sound is of high energy, most of the sites would permit the sound inside. These mixed sound sources may interfere and produce audible beats.

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Audible sounds are of relatively lower energy. During normal circumstances, only the thinnest sites let through the sound, and the others die out. The possibility of noticeable beats is less as most of the far off frequencies are of low intensity.

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  • $\begingroup$ Thanks @Krishnanand J, it seems to be a plausible answer. Could you please provide some references to study this phenomenon? $\endgroup$ – Luis M Gato Jul 23 '18 at 14:59
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    $\begingroup$ Yet this does not happen with normal sound waves - the loud music from the car next to you does not change frequency if you open your window or not... $\endgroup$ – Jon Custer Jul 23 '18 at 15:08
  • $\begingroup$ Yes, actually it's common sense. If you listen to music, no matter the distance and thus the wave's energy, you won't expect the pitch to change with the distance. $\endgroup$ – Luis M Gato Jul 23 '18 at 15:23
  • $\begingroup$ @LuisMGato Edited my answer. $\endgroup$ – Krishnanand J Jul 23 '18 at 15:37
  • $\begingroup$ @Krishnanand J, beats produce a wave with varying amplitude, but they don't produce new frequencies. The frequency difference is not heard, it just acts as the envelope of the resulting wave. See: en.wikipedia.org/wiki/Beat_(acoustics) $\endgroup$ – Luis M Gato Jul 23 '18 at 15:56

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