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Do ultrasonic sensors sense through transparent materials?

In my case, I have to read if there's something inside a transparent acrylic bottle.

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  • No, ultrasonic sensors will work only in gas environments (typically, in air atmosphere).
    – jfpoilpret
    Sep 24, 2014 at 19:31
  • Only through materials which are transparent to ultrasound. I think rubber or polyurethane work. See for example the material that covers the reverse sensors in cars. @jfpoilpret I don't think they will work in e.g. helium gas.
    – Gerben
    Sep 24, 2014 at 20:46
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    The issue is not transparency to ultrasound (sound will travel within hard plastic for some distance quite well), but rather presenting the same impedance at the ultrasonic frequency as the intervening air does. Since most solids do not, the majority of incident energy reflects off the discontinuity, and only a small amount transfers through.
    – Chris Stratton
    Sep 24, 2014 at 20:48

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I'm assuming you mean a "normal" sensor, like this:

enter image description here

Ultrasonic sensors work by sending out sound waves. Obviously, sound waves don't care whether a material is transparent or opaque, so they are blocked just as easily by acrylic as by a brick wall.

Side note: If you look on the front of an ultrasonic sensor, you will notice that it is covered by a screen, not glass or plastic. This is why.

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  • Which means that it would sense through grids, meshes, nets or veils, and may even sense through thin but opaque fabrics. Fabrics can cause strange readings because they do not provide a solid enough block. Visible light opacity has nothing to do with it.
    – Danny Staple
    Sep 27, 2014 at 23:28
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Ultrasonic sensors detect echoes of high frequency sounds bouncing off of a boundary between air and some other substance.

As others have pointed out, the light transmission properties of an object (transparency to light) has nothing to do with its transparency to ultrasonic sound waves.

It's possible to project ultrasonic sounds through solids or liquids, but that's not what these sensors are designed to do.

Trying to send ultrasonic pulses through an acrylic bottle will no doubt mess up the signals. Try pointing one of these sensors at an acrylic bottle in a controlled environment (fixed distance, mounted firmly and the same way each time, a solid back-stop at a fairly large, fixed distance behind the bottle to avoid varying echoes from behind the bottle) and check the distance measurement with the bottle full and empty. If you detect a difference in readings, then use those 2 different distance values for full and empty. (But make sure your production environment is the same as your test environment, including the area around and behind the bottle you're measuring)

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Wanting to use HC-SR04 to measure water level, but protect the SR04 from moisture I tried covering with cling film, the most minimal film I could find. No - the sensor sees it and reports 1cm distance. So these sensors have to be left exposed - probably won't last long at top of my water tank.

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  • I'm unsure how useful answering a question (that has received multiple answers) from 2014 with something that's much more of a comment is. I think there are much more applicable sensors for this
    – Paul
    May 20, 2019 at 8:19
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Well, you can visualize babies in a womb using ultrasonic sensors. So why should it be a problem to "hear" inside a bottle? Maybe not with the sensors you can afford, and maybe not with an arduino, but basically it is possible, even if the bottle is not transparent.

if the bottle is open, and the content is liquid, a simple approach could be implementing a kind of ultrasonic range finder. This can be done with comparatively cheap hardware.

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    Ultrasonic sensors sense the reflection off an impedance discontinuity. For the "in-air" type of sensor most likely being discussed, that would be the transition between air and a solid or liquid surface, and their receivers typically only report the first major reflection. For a medical sensor, the sensor is coupled to the body with gel to improve the impedance match, and a much more sophisticated receiving system picks up partial reflections (at multiple depths, from multiple directions) off of density variation of tissues and fluid within the body.
    – Chris Stratton
    Sep 24, 2014 at 20:46
  • @Chris Stratton: Sure! But the question was generally about "ultrasonic sensors", and not what's "most likely" used. Nevertheless my answer includes a solution for the "most likely" sensor, thus a downvote is not justified in my opinion. But... feel free!
    – mic
    Sep 25, 2014 at 6:08
  • I did not downvote your answer, though I do think it's main point remains a based on a lack of understanding.
    – Chris Stratton
    Sep 25, 2014 at 13:07

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