Sometimes you want to hear what's going on in the room next to you without entering the room. If you think people are exchanging important yet secret information, or you have a nosy streak, you can increase your ability to listen through walls by modifying common household items. Keep in mind that invading someone's privacy may anger them. If you use any information you've gathered by spying, people will know you've been listening in on their conversations.
Figure out where the voices meet the wall best. If you know people are speaking on the couch and that the couch is at the back of the room, you want to use the spot on the wall that best corresponds to the back of the other room.
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Take a clean wine glass and press it to the wall. The cone shape helps pull sound and channel it through the handle. Use a thin stemmed glass for best results.
Place an earplug in your free ear to block out ambient noise.
Put on a stethoscope and place the round, metal end to the wall. A stethoscope should work well as a listening device since it is designed to hear the heartbeat from within the thick chest cavity.
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These devices work best on thin to moderately thick walls.