Product Notes...
"Works on wooden walls, doors, windows, steel plates, etc. Highly Sensitive, carefully adjust audio slowly, as not to cause discomfort to your ear-buds. This product is being sold as an investigative tools for law enforcement or licensed investigators. Anyone else ordering this device should only be ordering it as a simple toy since MANY COUNTRIES STRICTLY PROHIBIT OWNERSHIP OF SPY DEVICES." (more)
Why do I mention it?
So you will know what you are up against.
Want to build one yourself? (start here)