[hwillis]

With certain windows, for certain sounds, you can put actuators on the window itself. They vibrate the glass as a speaker. Unfortunately it is very tricky- the resonant frequency of glass panes is far lower than sound, so its very hard to make the glass all move together. Instead it prefers to take on wavy bends, which can even create their own noise.

One neat idea is electrostatic speakers! They're one of the more exotic speaker types, but have unparalleled quality at low volumes. However they are prone to distortion as volume increases and require high (100+) voltage supplies. They can be dangerous and fragile.

The idea is you stretch a mylar film very tightly into a rectangle, and then sandwich it between two metal meshes, with an air gap. By putting voltage on the meshes, static electricity pulls the mylar forward and back while keeping it almost perfectly flat.

You can put meshes (or ITO films) inside a double pane window to create a very even force over the whole window, making it an excellent transducer. It requires far higher voltages to move glass, though. Putting mylar in between the panes doesnt work (you need some free space to create proper cancellation- oversimplifying) and putting mylar on the outside is dangerous. Tricky problem.

[eternalban]

You mentioned danger twice. Can expand on this aspect of the tech?

[hwillis]

By the nature of the device the meshes need to be fairly exposed. They can be enameled so as not to shock you, but only barely. Any scratch, rubbing, or other wear can expose bare metal at >1 kV. That is a major fire risk; any spark is presumed deadly in electrical engineering. Dust accumulates everywhere, and you'd be amazed how much people mistreat electronics. Leaves, dirty laundry and dust may as well be tinder.

The even bigger concern is mostly that a moderately loud speaker system (>50-150 watts) can electrocute you. I mean that literally, as the portmanteau of electricity and execution. 1 kV is easily enough to put electricity through your heart; 50 milliamps would be lethal. 50 mA at 1 kV is only 50 watts, less than a laptop charger. A big set of electrostatic speakers can just stop your heart if you're very unlucky.