Step 6: Instructions on the listening device
2.Determine where the laser reflects, and situate your photocell so the reflected beam hits the front of the cell.
3.Hit record and listen!
4.Any noise from inside the room will cause the window to vibrate, which will get picked up by the laser and photocell. You might have to process the raw input to remove unwanted noise and isolate the voices.
now here is a picture how does it work.