Transfering data or voice through walls and windows without drilling

Hi, many times I come accross a difficult problem. I need to transfer voice or data from inside the house to outside and vice versa. The problem is that drilling a hole through the wall is unacceptable. On the other hand, no RF must exist inside the house, apart from the through-wall point (or very close to it) where the inside and outside devices communicate. In other words, if RF is to be used it must be constrained only to the path through the wall and not the surroundings.

How yould you solve such a problem?

Some ideas:

Magnetic coupling? (AM ferrite loop antennas)
Some kind of very directive RF?
Hypersonic?
Vibration?

Wouldn't be surprised if products are available which can do this via x10.

You'll have to explain the "no RF" part of the problem. Unless you live in a copper box or in a cave there's no way your house is RF free. A simple low power AM transmitter/receiver could do the job, with an "RF" frequency around 1MHz.

I doubt ultrasound will travel through walls well at all.

This could be the same as low frequency acoustic. The stethoscope principle could work here. Sound can be conveyed by solid objects, if firmly attached together. To broadcast, a loudspeaker pressed snug against each side of the wall. They can also serve as a microphone.

You mentioned windows, which might make an optical system practical.

Direct magnetic coupling of audio works pretty well, a large multi turn loop fed by an audio power amplifier will generate the magnetic field. A ferrite rod with many hundreds of turns will receive that, although mains hum can be a problem, depending on the particular location.

How come electrical coupling always gets overlooked :[. Place two metallic plates on opposite sides of the wall. This forms a capacitor. The bigger the plates, the bigger the capacitor, and the lower the series impedance will be. There's an added bonus that the plates can be flush-mounted and can even be painted over on top.

Of course, this does depend on the frequency you're using, as you don't want it to be a resonant size -- as well as the thickness and material of the wall.

RF Free? Your mobile phone or the radio is quiet inside? How about using a laser beam? It is simple and can sustain high data rates.

I am thinking about this electrical (plate) system, how can this transfer data without a ground reference?

Would you need the same DC reference on both sides?

If you really wanted a return path, you could have two adjacent sets of plates, though of course this would decrease the capacitance by half.