Just to clarify: Some people are confused about a "boom mic". The
"boom" is a piece of equipment - most often a pole of some kind -
that the microphone is attached to.
The further away the mic is from the actors, the higher the volume
needs to be. The higher the volume, the more “noise” you get. Your
goal is to have a very high signal to noise ratio - more signal (the
dialogue) less noise (the background). Even a very good, expensive
Sennheiser mounted on the camera will pick up a lot of background
ambiance because it’s far away from the actors.