"time reversal acoustics"

I found this off a link from Schneier's newsletter. Can
anybody 
comment on this?

Udhay

http://www.phys
org.com/news12093.html

>For every burst of sound, there must exist a sound that
bursts in 
>reverse, according to the theory of time reversal
acoustics. From 
>their discoveries of some surprising characteristics of
mediums and 
>frequencies, scientists have gained insight into
acoustic time 
>reversal, which could impact applications such as
sending 
>confidential messages in cryptography.


-- 
((Udhay Shankar N)) ((udhay  pobox.com))
((www.digeratus.com))


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On 2006-04-22, Udhay Shankar N wrote:

> I found this off a link from Schneier's newsletter.
Can anybody 
> comment on this?

Passive, linear, time-invariant, nondissipative systems are
essentially 
allpass networks which simply redistribute energy in time
and outputs. 
Like all LTI systems they can be completely specified in
terms of their 
response to impulses but in this particular case the system
is also 
extremely easy to invert: you just reverse both the system
(outputs 
become inputs) and its impulse response (first sample
becomes the last). 
Systems which respond approximately this way abound in
acoustics, 
optics, circuit design, digital signal processing and who
knows where.

In time reversal acoustics the input is typically some
pointlike 
acoustic phenomenon like a spark, the output is a set of
microphone 
signals and the system is some propagation medium with
little absorption 
but an unlimited amount and complexity of internal
reflections. The 
response of such a system in time and space can be
accurately compared 
to what spread spectrum modulation does to a signal in the
frequency 
domain: energy is spread in a complex but essentially linear
fashion. 
Inverting the system by time reversion recompacts the energy
at the 
original source so that the reverse system can be used as a
spatially 
and temporally randomized focusing device/lens despite its
complexity. 
And since there is no essential limit to the complexity,
area or (to a 
lesser amount) temporal extent of the system response, the
achievable 
spreading can be large enough to hide the decompacted signal
under the 
local noise floor of the surrounding acoustic environment
while still 
allowing recompaction at the focal point. Such
spatial-temporal 
de/spreading is probably the cryptographic application that
was 
imagined; its prime weakness is probably in the linearity of
the 
primitive.

So in a sense this is nothing new. Direct sequence spread
spectrum has 
been used for decades now, systems as different as synthetic
aperture 
radars, beamforming sonars and laser pulse compressor
gratings do energy 
compaction all the time, and holographic crypto -- another
highly 
dispersive physical system with a unitary response -- exists
as a 
concept in literature. At least to me it seems that the most
interesting 
parts of this are the realizations that a) highly dispersive
media can 
be used as somewhat lossy collectors/focusing lenses with
any point as 
the focal point provided their response can be inverted, b)
such 
physical operations are available in the acoustic domain too
and c) it 
might be possible to construct such media in a difficult to
duplicate, 
one-shot fashion and then use them as computationally heavy
black boxes 
in algorithms. (The latter part is analogous to efforts at
creating 
physical one-time tokens for watermarking purposes.)
-- 
Sampo Syreeni, aka decoy - mailto:decoyiki.fi,
tel:+358-50-5756111 
student/math+cs/helsinki university, http://www.iki.fi/~dec
oy/front 
openpgp: 050985C2/025E D175 ABE5 027C 9494 EEB0 E090 8BA9
0509 85C2

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