[--JW]

I saw this site that says that you can build a laser listening device (Sounds Fancy huh?) by bouncing a laser beam off of a window to a CdS photocell. I was wondering if this is indeed a feasible task?

Here's the link:
http://www.ehow.com/...ing-device.html

[mooeypoo]

I'm not sure about the engineering aspects of it (as in "is it powerful enough" or "accurate enough" to achieve this) but theoretically speaking, yes, it's absolutely possible.

The concept this is going after is the fact that sound "rattles" the air, and also rattles windows as a result. In fact, the window's glass will amplify the sound of the air, so theoretically speaking, if you could get the vibration of the glass and translate it to audio, you could "listen in" on others.

the main problem I can see with something like this is that it can't really cancel out noise. There is a lot going on in a room not just people talking - air conditioning, for example, will likely shake a bit of the glass as well, and it probably will vibrate it much stronger than people's voices will, so you will end up getting the noise of the AC rather than people's actual conversation.

If noises are constant, there are computerized algorithms that can eliminate them (that's what "noise cancelling" devices) do. That's likely quite expensive.. but I'm not sure. As I said, I'm less knowledgeable about the engineering of this device.

Hope that helped.

~mooey

[TonyMcC]

It certainly is "doable". Not only that you can buy devices to combat it if you think you are being affected.
I don't want to advertise any particular device but if you google -bugging lasers - and then chose "Laser surveillance on eBay" you should find reference to an Anti-Ease Dropping electronic device. (I think they mean Anti - Eaves Dropping). The description of how this device works also gives a brief description of the device it combats, which is based on pointing a laser at a window.

[Joshua201]

mooeypoo, on 10 April 2011 - 06:05 AM, said:

I'm not sure about the engineering aspects of it (as in "is it powerful enough" or "accurate enough" to achieve this) but theoretically speaking, yes, it's absolutely possible.

The concept this is going after is the fact that sound "rattles" the air, and also rattles windows as a result. In fact, the window's glass will amplify the sound of the air, so theoretically speaking, if you could get the vibration of the glass and translate it to audio, you could "listen in" on others.

the main problem I can see with something like this is that it can't really cancel out noise. There is a lot going on in a room not just people talking - air conditioning, for example, will likely shake a bit of the glass as well, and it probably will vibrate it much stronger than people's voices will, so you will end up getting the noise of the AC rather than people's actual conversation.

If noises are constant, there are computerized algorithms that can eliminate them (that's what "noise cancelling" devices) do. That's likely quite expensive.. but I'm not sure. As I said, I'm less knowledgeable about the engineering of this device.

Hope that helped.

~mooey

I couldn't agree more. Even in theory, you still have to cancel other sounds which constitute about 99% of the entire input

[TonyMcC]

As someone who has worked with military electronics I think you might be surprised about the possibility of recovering intelligence through a background of noise. My experience mainly concerned radar equipments which worked with very small signals often accompanied by noise which was larger in amplitude than the signal itself.

This post has been edited by TonyMcC: 15 April 2011 - 02:44 PM

[Stefan-CoA]

Most certainly I think, otherwise all the SplinterCell video games have taught me wrong!

No, but I'm fairly certain that since sound is a wave and light is a wave (sometimes) the two should be able to interact somehow.