Several companies offer audio surveillance devices for sale online. But are they legal?

© Michael Duva/Getty Images

Listening Devices

­Of course, listening is one of the best ways to gather information. We rely on our hearing to communicate with each other, and a conversation between two people can reveal any number of facts.Overhearing an exchange between suspects, such as an important plan, location or details of a past event, can strengthen a case against a criminal, and recording these exchanges can prove even more valuable for future evaluation or evidence.

In espionage, covert listening is one of a spy's most important skills. Using the right kind of listening device, you can gather sensitive information. The longer you stay undetected, the more information you'll be able to collect.

As far as commercially available audio surveillance gadgets go, parabolic microphones are some of the most popular listening devices available to the public. Nature photographers and bird watchers often use these types of microphones to locate animals that are too difficult to see in their natural environments, but they've proven useful in the spy world, too. Shaped like satellite dish antennas, these special microphones amplify sound coming from an area that's too far away to hear clearly without the aid of a listening device. By pointing the device at a desired location, sounds from as far as 300 yards (274.3 meters) away can seem like they're right next to the listener. Simple headphones are typically plugged in, and recorders can be hooked up to the device for documentation. Because of their long range, listeners can remain undetected at a safe distance.

This type of gear looks to be readily available. You can visit several Web sites, search through a large variety of listening devices marketed specifically for surveillance and have one delivered right to your house. In a matter of days you could be spying on your neighbors, right? But is it even legal to use any of these devices to listen to other people?

U.S. law prohibits anyone other than law enforcement officials from installing, using and maintaining listening devices to hear or record someone's conversation without their permission [source: Advanced Surveillance Group]. Any type of audio surveillance requires a warrant. Before you ever consider setting up any kind of audio surveillance, it's a good idea to talk to a lawyer, especially since some state laws can vary slightly.