Get this it works through: Cement walls, Steel walls, wooden walls, iron walls, glass and air brick 1.Features:  F-999B is a high sensitive partition monitor, which is perfectly constituted by metal receiver, metal handset with superior detecting ability and high-quality headphones, making it more sensitive and sound-qualified. It owns built-in Lithium battery and unique FM transmitter function, easy to be manipulated and carried  2.Usage:  Put the earplug into headphone hole, the sounder into the MIC hole, and clockwise rotate the "switch on/off" button of the volume. When the indicator light of the power works, booth the machine and adjust the volume, smoothly stick the sounder to wall the need to be monitored, adjust the gear-choice buttons A B C until you can clearly hear the sound from the room.  If you need to do the sound recording, you can use recording equipment's to go through the mouth of the recording to connect the audio cable for recording. Also, F-999B has another special FM wireless emission function, with specialized FM receiver which can receive the information afar and monitor that.  3.Cautions:  The indicator light works when it is charging please shutdown in case of damaging the machine. It needs to charge for about five hours. The sounder must smoothly stick to the wall`s surface; never move the sounder when you are sticking it to the wall in case of damaging the sounder. The effect of the monitoring depends on the thickness, materials and other features of the wall. Appropriately adjust the gear-choice button until you can get the best effect of monitoring. D is the partition monitoring boogie of the sounder is the refined part which should not be dismantled or touched discretionary.  4.Parameter:  Electric Current Exhaust: 10mA  Battery type: 3.7V lithium-battery  Continuous monitoring for 100hours  Four Audio Choices: A (200MHz) B (800MHz) C (1200MHz) D (partition boogie monitoring)  Sounder`s sensitivity: -70db, the maximum times of amplifying  the audio: 2000 times  Penetrability: Cement wall, Steel wall, wooden wall, iron wall, glass and air brick  Answering patterns: Headphones, recording, wireless FM emission