Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Schlieren Optics – See small changes in the index of refraction in air

youtube.com

Post reply on HN
Print view View original

21–30 of 32 posts

Re: Schlieren Optics – See small changes in the index of refraction in air

#21

Post by RobGR »

Osmium wrote: post #13

Purely out of curiosity, has anyone used this in conjunction with a linguistics study to visualise sounds made during speech? I did a quick literature search but couldn't find anything. Not sure whether if features would be too subtle to see anything interesting... Edit: Spoke too soon. Did find one paper[1] (though I can't read it) which uses it to compare the production of 's' and 'z' sounds. Would love to know if…

I helped get Doc Edgerton's Schlieren appartaus working again in a lab class in the mid-90s.

We tried to image sound waves, but the density gradient for sound is much less than that produced by changes in temperature. We attempted to make a resonant chamber and use a high-intensity ultrasonic source, and make a standing wave that we might capture photographically, and while we saw something once we could not produce it. It's likely that the ultrasonic source we were using was gradually degrading.

The research paper you link to almost surely uses the heat differences to see the jets of air coming out of the mouth, and not actual sound waves.

Re: Schlieren Optics – See small changes in the index of refraction in air

#22

Post by pierrec »

I've long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air's refraction index. I've searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.

Re: Schlieren Optics – See small changes in the index of refraction in air

#23

Post by Osmium »

RobGR wrote: post #21
Osmium wrote: post #13

Purely out of curiosity, has anyone used this in conjunction with a linguistics study to visualise sounds made during speech? I did a quick literature search but couldn't find anything. Not sure whether if features would be too subtle to see anything interesting... Edit: Spoke too soon. Did find one paper[1] (though I can't read it) which uses it to compare the production of 's' and 'z' sounds. Would love to know if…

I helped get Doc Edgerton's Schlieren appartaus working again in a lab class in the mid-90s. We tried to image sound waves, but the density gradient for sound is much less than that produced by changes in temperature. We attempted to make a resonant chamber and use a high-intensity ultrasonic source, and make a standing wave that we might capture photographically, and while we saw something once we could not produce…

Thanks for the information, that's really interesting, and a shame it didn't work out! I did see that the sound of a clap had been imaged, but I imagine that's a somewhat extreme case.

> The research paper you link to almost surely uses the heat differences to see the jets of air coming out of the mouth, and not actual sound waves.

Regarding the paper, while I can't read it myself, I imagine they chose to look at fricatives precisely because they're the result of turbulent airflow which is probably ideal for Schlieren imaging rather than a different speech sound which would be more wave-like. If the image is the result of heat differences, I wonder if it could be improved further by changing ambient temperature or temperature inside the mouth, or alternatively if the subject could be asked to inhale sulphur hexafluoride first to increase the density differential? (Edit: Or, perhaps, introduce a 'uniform' (laminar) thermal source along the direction you're interested in, so that when a sound wave propagates, the resulting density differences would be much more pronounced? I'm not sure if that makes any sense, as a layperson this is pure speculation on my part...)

I believe this is an image from the paper: http://www.sciencephoto.com/media/89151/view

Thanks again for the info though, really fascinating. Hopefully we haven't reached the limits of this technique yet, and it can still be taken further.

Re: Schlieren Optics – See small changes in the index of refraction in air

#25

Post by [deleted] »

pierrec wrote: post #22

I've long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air's refraction index. I've searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.

[deleted]

Re: Schlieren Optics – See small changes in the index of refraction in air

#27

Post by RobGR »

Osmium wrote: post #23
RobGR wrote: post #21

Earlier quoted context omitted.

I helped get Doc Edgerton's Schlieren appartaus working again in a lab class in the mid-90s. We tried to image sound waves, but the density gradient for sound is much less than that produced by changes in temperature. We attempted to make a resonant chamber and use a high-intensity ultrasonic source, and make a standing wave that we might capture photographically, and while we saw something once we could not produce…

Thanks for the information, that's really interesting, and a shame it didn't work out! I did see that the sound of a clap had been imaged, but I imagine that's a somewhat extreme case. > The research paper you link to almost surely uses the heat differences to see the jets of air coming out of the mouth, and not actual sound waves. Regarding the paper, while I can't read it myself, I imagine they chose to look at fri…

I note that in that picture there appear to be wave fronts emanating from the person's mouth. It's not obvious but there are some circular artifacts that appear to be centered on the lips / teeth. Maybe they really did capture sound ?

Re: Schlieren Optics – See small changes in the index of refraction in air

#28

Post by jleader »

Scientific American used to run a really cool "Amateur Scientist" column when I was a kid, edited by C.L. Stong. In the early 70s they ran a couple articles about people who'd built their own Schlieren optics. I was fascinated by them!

The articles appear to be pay-walled now (http://www.scientificamerican.com/article/the-amateur-scient... and http://www.scientificamerican.com/article/the-amateur-scient...).

It looks like the book that collected many of Stong's columns from the 50s and 60s predates those 2 articles (https://archive.org/details/TheAmateurScientist).

There's a CD-ROM available that supposedly contains the text & images for _all_ of the Scientific American "Amateur Scientist" from the '20s to the late '90s, which would presumably also have those articles: http://www.amazon.com/exec/obidos/ISBN%3D0970347626/scienceh...

Re: Schlieren Optics – See small changes in the index of refraction in air

#30

Post by rtb »

pierrec wrote: post #22

I've long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air's refraction index. I've searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.

Interesting. I don't think that would be possible - the technique shown here relies on a mirror to get interferometry between the two light paths. I can't see how interferometry would be available to soaring birds (between its two eyes, perhaps?). Although nature has come up with some very surprising and impressive inventions before, so I might be wrong.

I would guess they're just very good at spotting the conditions which lead to thermals.

EDIT: of course, you can directly see thermals via the shimmering / "mirage" effect. It's normally obscured by the background noise, but maybe soaring birds are attuned to that. That's much more plausible, but a different effect to that shown here.

Post reply on HN