Legal Listening Devicesby Peter Timm
Listening devices can vary from pocket recorders to mini dishes that let you listen to conversations from a distance. Many of these devices are legal to own but can easily be used illegally. Generally, if you want record a conversation between you and someone else, you can legally do so--provided that it's for your own records. However, if you use these devices to record the conversations of people who have no idea that they're being recorded, that's illegal.
Long Range Listening
These devices are shaped like mini satellite dishes and allow you to listen in by pointing the dish where you want to hear. These devices are readily available at most "spy" stores that specialize in surveillance equipment, and cheaper versions are even available in toy stores. These devices typically can hear up to 100 yards.
Acoustic Leakage Microphones
These microphones allow you to listen through walls. This microphone is mounted against a solid surface with special putty and amplifies all sound vibrations up to 100 times. These microphones can also be used for listening to pipes--for leakage--or for checking the walls for pests.
Covert Listening Devices
These devices are the typical listening "bug" commonly featured in spy movies. They can be concealed and remotely activated. They can legally be used to listen in to your home to allow you to monitor was going on there. However, placing these devices on property that is not yours to listen to conversations where people have the expectation of privacy is illegal.