If we refer to Dictionary.com, the meaning of eavesdrop was defined as follow:
-verb (used without object)
1. To listen secretly to a private conversation.
-verb (used with object)
2.Archaic. To eavesdrop on.
3. Water that drips from the eaves.
4. The ground on which such water falls.
Make yourself familiar with this topic by learning some basic knowledge of eavesdropping.
The most common and simplest solution to eavesdropping is to install spy equipment which we call it a bug. A bug is a device that receives audio information and broadcasts it through the air, usually via radio waves. Some bugs have tiny microphones that pick up sound waves directly.
Just as in any microphone, this sound is represented by an electrical current. In a bug, the current runs to a radio transmitter, which transmits a signal that varies with the current.
The Spy sets up a nearby radio receiver that picks up this signal and sends it to a speaker or encodes it on a tape.
A bug with a microphone can pick up any sound in a room, whether the person is talking on the phone or not. But a distinctive wiretapping bug doesn’t need its own microphone, since the phone already has one. If the Spy hooks this bug anywhere along the phone line, it receives the electrical current directly.
Often, the Spy will hook the bug up to the wires that are actually inside the phone. Since people very rarely look inside their phones, this can be a superb hiding spot. Of course, if someone is searching for a wiretap, both the Spy and his / her bug will be exposed.
This is the best sort of wiretap for most Spies. Bugs are so small that the subject is unlikely to notice them, and once they are installed, the Spy doesn’t have to return to the scene of the crime to keep them running.
All of the complex recording equipment can be kept away from the phone lines, in a hidden location. But since the radio receiver has to be within range of the transmitter, the Spy must find a hidden spot near the wiretap. The traditional receiving spot is to use any suitable vehicle parked outside the subject’s home.
Lastly, if you are not a legal Spy, don’t install any spy equipment such as electronic listening, or recording device to someone’s phone, or you will be breaking Federal Laws.Unless you can somehow manage to obtain a legal warrant to do so from a judge.
You may or may not be breaking the law by eavesdropping by any other means, but you certainly will be morally bankrupt if you do. Try to be a good citizen and mind your own business.