Whether they are used for personal objectives or professional services, listening devices can either allow someone to eavesdrop on conversations or monitor, record and store them. Listening devices come in a range of prices and styles, from simple spy bugs to digital cell phone/recording devices to parabolic microphones. They are all meant to be used covertly and in a legal manner.
Radio Frequency Devices
Radio frequency devices are small spy bugs that can easily be hidden inside phones to pick up both sides of a phone conversation, or strategically placed in plants or other everyday household/office items to pick up room conversations. The radio frequency sends the conversations over the airwaves to a receiver like an earpiece or even an FM radio. A professional spy, a private detective, or the police may use a wiretap instead, though. In the past, a transmitter was attached to the phone wires inside a phone, sending the conversation over the airwaves to a recording device. In the early 21st century, wiretapping is done with computers, through the phone company.
Some listening devices come disguised as cell phones. One type merely looks like a cell phone, without the cell phone innards, and acts simply as a wireless transmitter. No one would give this cell phone a second thought, unless he picked up and tried to use it. Another type of cell phone device is actually a working cell phone that can be used for eavesdropping. When someone uses the phone, another person -- the spy -- can dial a secret phone number that allows him to covertly listen in on the conversation without the caller or the person being called being any the wiser.
Long Distance Listening Devices
A common form of long distance listening device is called a parabolic microphone. It consists of a microphone placed in the well of a small satellite-dish shaped receiver. It can be aimed in any direction to pick up conversations from hundreds of feet away. While it does not require wires to pick up the conversation, the conversation must be transmitted through wires to earplugs or headphones worn by the person holding the "satellite dish." The parabolic mic can also be wired to a recording device. A parabolic mic is not small and cannot be disguised as an everyday object, so the person using this device must be hidden far away from the conversation.
Listening Through the Wall
One device available to spies is a special microphone that looks something like a small stethoscope head, which plugs into a radio receiver. The microphone is placed against the wall and secured in place allowing the listener to hear through walls with a set of earphones. These high-precision instruments can even pick up whispered conversations.
Bionic Ear Devices
Bionic ear devices are like supersonic hearing aids. The user simply places the ear buds in her ear canal and listens, as the device amplifies conversations from across the room, the adjoining room, or even down the hallway. Their biggest advantage is their small size, which can make them inconspicuous from afar.