Using a laser to overhear a room conversation

Question

Movies with science based tricks and gimmicks are generally silly and sometimes even annoying. The science based trick that I don't know enough about to judge is the following (and I have seen it in a few movies, although maybe each movie just copies the earlier ones):

A movie character walks into a hotel room, takes out a roll of sticky tape, and pastes a big X shape on the windows. The reason is rarely explained but I had always assumed that it was to dampen or modify the vibration patterns of the window glass, thus preventing the character's protagonists from listening to the room conversation by using a laser to monitor the glass vibrations.

I am never sure why the blinds are not drawn or the curtains pulled, the tape idea looks more dramatic I suppose.

Two questions:

• Is this listening in using a laser actually feasible? The only reason I ask this question is because I think it probably is.

• Putting tape over the windows to prevent vibration, is this a feasible method to stop the conversation being overheard using the laser? Surely the laser could be tuned accordingly to allow for this?

EDIT As Ernie says below:

If the tape is composed of mass-loaded vinyl, or if it is lead tape, it might reduce vibrations enough to stymie a laser, but only if it nearly covers the window. The way to attenuate sound waves is to trap them in an enclosure that doesn't vibrate much. The way to reduce vibrations is to make sure the vibrational medium is so heavy and dense that sound waves from a voice are not energetic enough to move the material.

Answer 1

The underlying principle is to use interferometry and the Doppler effect to remotely measure the velocity of a reflecting surface.

When a moving object is illuminated with coherent light it reflects it with a wavelength shift proportional to its velocity. This is the well-known Doppler effect.

The frequency shift relates to the source's velocity as follows:

$$f = \left( \frac{c + v_\text{r}}{c + v_\text{s}} \right) f_0 \,$$

If the object moves back and forth very quickly, such as in an acoustic vibration, the way to detect the wavelength shift is to merge the reflected beam with the emitted one to create interference.

The most frequent use of that principle is called laser Doppler vibrometry and is routinely used in precision manufacturing, experimental acoustics, etc. For its specific use as a remote listening device see:

Li, R., Wang, T., Zhu, Z., & Xiao, W. (2011). Vibration characteristics of various surfaces using an LDV for long-range voice acquisition. Sensors Journal, IEEE, 11(6), 1415-1422.

For the movie setting, I think there's a bit of artistic license because generally the technique is limited by the maximal distance at which you can focus the laser beam but more importantly glass windows are bad targets for laser vibrometry. The reason is that they're very smooth and thus do not scatter the reflected light in many directions. The small portion of the light that gets reflected has a very low chance of going towards the sensor.

But it's true that if you play the vibrometer's signal with a speaker, you can hear the vibration of the surface you're targeting, i.e. sound.

As per the purpose of the tape, it's unclear whether it's supposed to show an attempt at dampening the vibration of the window or to prevent reflection of the laser beam.

Answer 2

Additionnaly to Cape Code excellent answer:

In theory, the position of the reflected laser beam will vary according to the vibration of the window. It would be possible to send the laser beam with an angle and place a sensor on the expected reflected beam position. The variation of the position of the received beam will contain the actual sound information.