The professional listening device is a highly sensitive contact and probe microphone amplifier system that can hear sounds through concrete, metal, wood, brick, or any solid surface. Both the microphone and amplifier are built into a strong metal housings for durability which is backed by our lifetime guarantee. With high quality earbuds, a built in lithium battery, and all accessories included, you will be sure to achieve the job with this device. With the highest sensitive contact microphone available on the market, the ultimate professional listen through wall system can hear sounds through concrete, metal, wood, brick or any solid surface. Simple to use, place the sensitive contact microphone to the surface of interest and listen with crystal clear audio. It also has variable gain and volume settings to make the audio even clearer. You may need to experiment further with the position for better results and we can not guarantee results the first time every time. We guarantee it will hear audible sounds through any door or single brick/cavity walls. HIGHEST SENSITIVE LISTEN THROUGH WALLS / CEILING CONTACT PROBE MICROPHONE SET ON THE MARKET Features and Specifications: Penetrable surfaces: Cement, Metal, Wood, Iron, Glass, Plastic, Brick, Walls Working time: Continuous monitoring up to 24 hours Headphone output: 3.5mm stereo @ 250mW Four audio channel: 1200Hz, 800Hz, 200Hz Maximum gain amplification: 20000 times Battery type: Rechargeable 3.7v lithium battery Microphone sensitivity: -70db Current consumption: 10mA Record line output: 2.5mm stereo Jack socket Battery capacity: 250mA Package includes: Contact microphone Probe / Needle microphone Microphone / Headphone Amplifier 2.5mm-3.5mm stereo record lead Carrying case Disclaimer: Some surfaces work better than others and density/thickness can also restrict its performance. We have received varied results as it depends entirely on the surface you use and many other factors such as distance and volume.