"Is that a snooper in your pocket, or are you just glad to hear me?"

...via the seller's web site...

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're up against.