These are baby audio monitors. This is basically an one-way, always-on intercom system. It doesn't need to be used just for baby monitoring. I'm disabled and live in a two floor house. I cannot get upstairs, so I use this system to easily call people upstairs for help. IT works well without a hitch.
It offers good reception up to 400 feet from transmitter to receiver. I wouldn't say crystal clear but not too bad. There is a volume control because it can be loud; we keep at 50%. Sometimes it picks up radio signals but not much. However, if you turn just the transmitter off, you will think ghosts are in your house because it will pick up tons of outside signals and you will hear countless voices of people using other monitors and cell phones nearby. If you're bored and nosy, this can come in handy!
They operated at 49Mhz and have two channels to minimize interference. NOTE: both transmitter and receiver need to be set to the same channel.
The transmitter and receiver both come with AC electric adaptors. Or they can use batteries but power will be used up relatively quickly. (A belt clip allows for convient portability.) Only use batteries when necessary. A low battery indicator warns when to replace batteries. A power-on LED indicator tells you that the unit is on.
The monitors are not that big. With antennae, the transmitter is 8.5"H x 4"W and the receiver is 9.2"H x 2.8"W.
These are a great, low-cost way to monitor your babies, children, disabled, or elderly people.