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Medical stethoscopes are sensitive and may be able to hear through a wall, while the less sensitive mechanical stethoscopes might work for a thin wall but there’s no guarantee. If using an unmodified stethoscope, you’ll need to experiment a bit, such as having a friend talk in the other room and seeing if you can hear what they’re saying while moving the scope around until the sound is clearest. However, as explained in the above article, for a stethoscope to be most effective at hearing through walls, it requires modification by adding microphones to amplify sounds muffled by the wall. There are spy stethoscopes, which are expensive, but are modified already to hear through walls; do your research if getting one. Finally, the materials and thickness of the wall will make a difference to whether or not you can hear through it with a stethoscope, modified or not.