1, listening through the wall: this kind of equipment is more expensive, but the effect is better.
2. Use a cup or bowl: Fix the cup or bowl on the wall, and then stick your ear to the bottom of the bowl. Low input cost and convenient use.
3. Use a stethoscope: stick one end of the stethoscope on the wall and wear the other end on your ear.
4. Use the microphone receiver: disassemble the microphone, take out the receiver and install it directly on the wall to achieve the radio effect.