Schneier on Security

Clive Robinson • August 9, 2014 5:59 AM

@ Bruce

The problem with saying “plastic bag” is there are many types of plastic bag, most won’t work for this unless they are mechanicaly stiffend in some way. Crisp bags are generaly a bag made of a plastic suitable for this as the surface holds ridges and folds and thus has edge stiffened areas (and it’s the edges they are working with).

There is a bunch of physics behind this that appear in materials science. Importantly there are non lineararities which makes easy comprehension of what would and would not be suitable for this difficult as normal real world experience conflicts. It’s over a third if a centuy since I had a need to use the physics in anger so I’m assuming it has moved on a lot in that time.

In some respects it’s similar to the use of glass inside hard drives for the platters, most outside the industry would assume that some metal would be more appropriate due to their real world experiance.

@ me,

There are various ways objects behave under the influance of the machanical vibration caused by the pressure changes that are sound. Any object will try and absorb the preasure changes by moving to the sound source on low preasure and away on high preasure. The stiffness and thickness of the objects surface facing the source will determine the amount of the sound energy that is absorbed, reflected or transmitted through the object. If the dimensions of the object are less than 1/16th of a wavelength of the sound then the object will react more as a solid that moves in it’s entirety than having resonance or flexure in the surface. However round thin walled objects like plastic cups exhibit complex and often unexpected modes of oscillation, which you can see in slow motion films of wine glasses building up resonant energy befor shattering under the influance of a trained singers voice. You can see that the frequencies it resonates at are not related to the simple measures of hight or width of the object.

@ Carl,

If you think about a video signal it has three limiting frequencies of interest, the frame rate, line rate and pixel width bandwidth.

From the article you will see they are using visual knife edges on the object that move in sympathy with the audio and thus change the luminance or colour (or both) between adjacent pixels in a given scan line. If you only compare on the frame rate your sampling frequency will be at the frame rate or double that if the video is interlaced. However if you use the signal from several lines then you get bursts of sampes at the line rate, where the bursts are seperated by the frame rate.

Further if you have several visual knife edges you can get a multitude of bursts of signals, if these can be appropriately aligned in time then you start to have more bursts and fill in the holes.

There is a problem though… you are using a sampled signal. The output of a sampled signal suffers from the problem of spectrum folding where a 1.25KHz signal sampled at 1KHz will appear as a 0.25KHz signal in the sampled output, the same with 2.25KHz, 3.25KHz signals.

I can think of some tricks where by the frequency folding can be partialy unfolded with the use of the line frequency sampling, my brain is nolonger up to doing the maths side in anything close to real time 😉 So you would be looking to find a researcher in the likes of radar signal processing to give you a good starting point.

I would note that both the NSA and GCHQ have been recruiting signal processing researchers and specialists for well over a quater of a century so I suspect they already know how to do this from other related systems.

@ Bob S,

Due to all the crumples and folds in your tinfoil hat, the number of visual knife edges would be very large, thus an almost ideal target for this type of evesdroping.

@ All,

If you think about it due to the use of visual knife edges, these would also make ideal “reflection targets” unlike other methods, as the curved surface of many common objects would magnify the knife edge rather usefully…

Which also means you could using further signal processing take the sound source from several points in the room giving in effect a stereo or better signal, this can be used to give the equivalent of VLB measurments which can be used to much more easily pick out desired signals from undesired signals.

Thus picking up the sound of you pressing keys on your keyboard even in a room with a radio on and people talking. Oh and also easily overcome the “running water” sound masking you see in spy movies…

Clive Robinson • August 11, 2014 5:02 PM

@ vas pup,

Is white noise generator placed inside the room sufficient protection by overriding all informative sounds by wide noise OR it is possible to filter out multiple vibration superimposed on the plastic bag based on the video captured?

A white noise generator only works against a single microphone ” sometimes”…

To see why a simple thought experiment,

Take a room of the usual 8ft high 16ft long and 12ft wide or thereabouts. Place a 1Khz omnidirectional source at the top end of the room and a 1.2KHz generator at the bottom end, both producing identical amplitude sine waves. With a suitable program it’s easily possible to make a predictive plot of exactly what pattern would be picked up in the room by an omnidirectional microphone.

What the plot will show is that the relative amplitude of the two signals is quite predictable and experimentation generaly verifies the plot.

What is obvious is that there are very few points where the amplitudes are the same and boundry diagrams can be quite easily drawn.

Multiple microphones on the boundries won’t pick up amplitude differences but they will pickup delay differences that with sine waves will be seen as phase differences. Thus by delaying the signal from one mic you can arrange for one of the two signals to be out of phase with itself, thuss adding the two signals together will cancel the out of phase signal leaving the vector sum of the other signal.

Obviously two mics not on a boundry will need both amplitude correction and dely correction to achieve the same effect.

This gets more complicated with broadband signals where both amplitude and phase may change differently for two disimilar microphones. However modern software available for PCs for studio work have sufficient signal processing ability to correct this.

So with this camera system the output from two different plastic diaphragms in different parts of the room will overcome a single white noise source.

So you use two or more noise sources in different parts of the room… However this turns into the old ECM ECCM ECCCM game of diminishing returns for both parties.