1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is it possible to identify something with sound?

  1. Jun 11, 2014 #1
    Is it to identify something (not necessarily very accurately but at all) with sound? Also, i know its possible with light but i don't exactly understand how, can someone explain?
  2. jcsd
  3. Jun 11, 2014 #2

    Larry Gopnik

    User Avatar
    Gold Member

    Identify something with sound? You mean... hear something with your ears?
  4. Jun 11, 2014 #3
    No, more like using the souls spectrum (if that's the proper word for it) to identify something like a substance, say stuff in the blood or dirt in a glass if water you can't see, etc, not hearing
  5. Jun 11, 2014 #4
    Of course you can. Bats and dolphins do it all the time! I won't go all national geographic on you though cause frankly, I'd suck at that.

    If you closed your eyes and yelled into a cave as opposed to an open field or concert hall, you'd be able to identify the general structure of the space just from the echo you get back (or in the case of the open field, no echo at all). Our ears aren't so incredible that we can discern very much, but a device built to pick up the echo certainly could. Every time sound bounces, it'll lose a little bit of energy (and therefore frequency, so the pitch would drop a bit), and by knowing where the sound wave came from and comparing to waves from all sides, you'd be able to compute where solid objects are.

    Hope I answered the right question.
  6. Jun 11, 2014 #5
    What about identifying what a substance is? Like going back to my dirt in the cup example, can we identify that t is dirt using sound? Or can we only tell that something is there?
  7. Jun 11, 2014 #6
    The more specific you get, the more refined your method needs to be. Simply yelling at a cup...probably won't work in that case, but equipment has already been built for the job. For example, sonar uses sound waves to identify objects in water. It doesn't get confused between the ocean floor, a whale, or a ship. Depending on the structure of the object, the response will be slightly different. You'll need something quite sensitive to figure out the difference between the different kinds of impurities in water though. However, at that point, the budget might rise above what you'd normally spend for a casual experiment. You're better off taking it to a chemistry lab and using the gazillion techniques they have for determining purity.
  8. Jun 11, 2014 #7
    What about light? I know we use the light spectrum to identify things and it's decently acurate, how does that work exactly? Supposedly there is some advanced math involved?
  9. Jun 11, 2014 #8
    Or could we find the natural frequency of the dirt and match it to increase the amplitude and then measure the amplitude increase to determine the ammount of dirt
  10. Jun 11, 2014 #9
    Light, I'm much more familiar with; I'm doing a lot of MRI work right now which utilizes that to an extent.
    The math... does tend to be fairly complex, but the basics can be..well...basic. Think of a single ray of light. You hit it at something and it bounces. Your detector picks it up and tells you what angle it came at. Some simple geometry and you now know the angle the surface forms to produce the deflection you observed. Do the same with many rays of light and you have a coherent view of the object. Our eyes do this all the time (though only superman actually shoots the light beams). Light scattering would be a very good way of fulfilling your requirements. Any small particle can be used actually, though light may be easiest to think about.

    I'm reluctant to respond to that second part. Yes, everything has a natural frequency it oscillates at, but that property is difficult to make use of in small particles like that. Resonance is explained through circuit theory, for the most part, and requires two particular typical elements: an inductor and capacitor. All things contain traits of these parts, but not necessarily to a noticeable extent. I can't say for sure, but what you say is theoretically possible, but practically very very very difficult.
  11. Jun 11, 2014 #10


    User Avatar
    Science Advisor
    Gold Member

    I designed a system to sort scrap metal with ultrasound - makes use of the differing acoustic impedance of different metals. The tests we conducted showed that it was feasible, but it was shelved in favor of other techniques.
  12. Jun 11, 2014 #11


    User Avatar
    Science Advisor
    Gold Member

    Raman spectroscopy can identify molecular composition; you shine a laser on the substance, and collect the back-scattered light ... filter out the laser line, and examine the remaining spectrum. This can be matched to a library of spectra of interest.

    Raman spectroscopy: https://en.wikipedia.org/wiki/Raman_spectroscopy

    We used this in the lab to test carbon nanotube structure; it is sensitive to more than just the molecular composition, but also (sometimes) the physical properties as well.
  13. Jun 11, 2014 #12
    I think the OPer is talking about line spectrum.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook