Our ears are amazing devices; they can pick up sounds
literally as quite as a pin dropping to a jet aircraft taking off close
by. When processed by our brain we have the ability to filter out lots of
unwanted sounds and even the ability to work out that a sound is coming
from left because is reaches your left ear a moment before the right.
Microphones are not intelligent; they pick up the wanted sound and the unwanted sound. Our job is to make sure they pick up mostly the wanted sound and avoid the unwanted sounds.
When working outside the unwanted noise is normally the background noise of everyday life - traffic, chatter, animals and aircraft. Indoors we have a similar set of problem sounds to avoid but we also have to contend with another - echo or reverberation. This is the wanted sound but at the wrong time.
The only way to record good sound outside of a recording studio is top get the microphone close to the wanted sound. Closer means it will be louder which means you will need to turn down the level of the recording to record it at the same level. Turning down the level reduces all of the sound, including the unwanted background sounds. The result is an improved ratio of wanted sound to unwanted sound. Job done!
Top tips for recording speech and dialogue:-