ProExtreme-Acoustic Leakage Probe Contact Mic

Meet the ProExtreme-Acoustic Leakage Probe: a device that can pick up sound vibrations against any surface and amplifies it up to 100 times. The sensor pick up can be mounted to any solid surface. Amplifier features volume control, background noise filter switch, and 3.5mm output jacks for headphone and recording device. The device can go for 25 hours on a 9 volt battery.

You can use this tool to determine the level of acoustic leakage from inside a structure to the outside. It can also be used for pest control, finding problems in engines, and more. Not cheap but quite helpful.