
Acoustic Prism Invented at EPFL - aethertap
http://actu.epfl.ch/news/acoustic-prism-invented-at-epfl/
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xgbi
The actual paper is here:
[http://scitation.aip.org/content/asa/journal/jasa/139/6/10.1...](http://scitation.aip.org/content/asa/journal/jasa/139/6/10.1121/1.4949544)

With pictures of the apparatus.

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teddyh
Isn’t this what the cochlea already does in a human ear?

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suprathreshold
Yes, essentially! One key difference is that frequency and intensity coding in
the cochlea is facilitated by a a built-in active physiological component
(outer hair cells) that enhances frequency selectivity and provides
compression of the structure that vibrates inside the cochlea (basilar
membrane). The outcome is that listening acuity is preserved across a large
dynamic range and variable background noise levels.

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radarsat1
What do you mean when you say the hair cells are "active"? I can see how the
hairs enhance certain frequencies, but does their tuning change dynamically
somehow?

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suprathreshold
Great question, and sorry for not being clear. It's not the frequency tuning
of a given outer hair cell (OHC) that that changes, it is their response to
the sound level (what I meant by dynamic range). OHCs are activated by basilar
membrane motion (the basilar membrane mechanically vibrates due to the
physical sound). At low sound levels, the basilar membrane itself does not
have a large displacement. The theory is that when OHCs depolarize (fire) due
to basilar membrane displacement, resulting transduction currents activate
motor proteins that change the length of the cell so that it "jumps" to
displace the vibrating membrane (basilar membrane) more.

[http://i.makeagif.com/media/10-06-2015/yxvqkm.gif](http://i.makeagif.com/media/10-06-2015/yxvqkm.gif)

So at low sound levels, OHCs create a sharper displacement (gain) where
vibration is maximal on the basilar membrane, which enhances the frequency
coding onto auditory nerve fibers (via inner hair cells).

High sound levels displace the basilar membrane more, which can lead to broad
patterns of excitement of nerve fibers coding for adjacent frequencies. At
these levels, the OHCs do not change length as much, which compresses the
membrane movement. The compression helps to maintain sharper acuity at the
frequency that is physically present, and less so at adjacent frequencies. The
function describing OHC length changes against level of sound is thus
nonlinear, with gain at low levels and compression at high levels. This
compression is not seen when the organism is dead, when OHCs are damaged, or
when OHCs are genetically knocked out.

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hantusk
I wonder how much of an improvement this will be over microphone arrays and
determining direction in software.

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alister
I've marveled at the difficulty of determining the direction and origin of
unknown sounds, esp. low pitch sounds.

For example, the people of Windsor, Ontario, Canada, have been hearing a
rumbling noise for six years and have been unable to locate it, although they
finally have a theory that it's coming from a blast furnace[1].

We're talking about a city of 210,000 where a large segment of the city have
regularly heard it over the span of 6 years: _" In 2012, more than 22,000
people dialled in to a local teleconference about the hum [to voice their
concerns]."_

I'm surprised that there isn't an instrument that you can flip on and have it
reliably locate a continuous sound that lasts for hours or days at a time. Is
there something about low pitch sounds and vibrations that make them
particularly hard for direction finding tools?

[1] [https://www.theguardian.com/world/2016/jun/07/windsor-hum-
ca...](https://www.theguardian.com/world/2016/jun/07/windsor-hum-canada-zug-
island-united-states)

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radarsat1
Don't know about the specifics of that case, but I can imagine that for low-
frequency pervasive sounds, they bounce everywhere and thus any such equipment
would measure them as "coming from everywhere", because they literally are.
There's a source, but it's masked completely by reverberation effects. At some
point with enough diffusion there is very little signal in the noise, if that
helps..

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pierrebai
I still don't understand why they could not locate it.

Sounds falls off with the square of the distance. A single, localized
measurement can have a hard time pinning a direction, but a series of sample
over an area will give you a 3D curve of intensity with a maxima near the
source...

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speps
YouTube link from the article :
[https://www.youtube.com/watch?v=6sSBPxAv2qk](https://www.youtube.com/watch?v=6sSBPxAv2qk)

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sp332
It's not loading for me, so here's a text-only version from Google.
[https://webcache.googleusercontent.com/search?strip=1&q=cach...](https://webcache.googleusercontent.com/search?strip=1&q=cache:http%3A%2F%2Factu.epfl.ch%2Fnews%2Facoustic-
prism-invented-at-epfl%2F) Sounds like a cool idea, but being able to load the
pictures would probably help.

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donquichotte
I wonder whether this has any real-world applications, since the Fourier
Transform is a well understood and easy to implement method for analyzing
spectra digitally.

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raverbashing
The question is: when can you not use the Fourier Transform (or it is really
bad)?

I can think of situations when one frequency drowns the other in your
transducer

Also this was built for the audible range (apparently), but it might have more
interesting uses in different ranges or fluids

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dharma1
They mention using it for sonar in the paper

