Proximity effect — defined as a change in the frequency response of a directional microphone as the mic is brought closer to the source — can be a great tool for tailoring the response of a microphone when recording, but it can sometimes create problems when recording vocals. Singing too close to a microphone can not only make the recording sound boomy and/or muddy, but during louder passages proximity effect can also cause overload in the mic.
Luckily, proximity effect is relatively easy to control or avoid. The key is getting some distance between the vocalist and the mic. How far varies from mic to mic. Some mics pick up voices best from 6-12 inches away, which is plenty of room to stick a pop filter in between mic and singer. For particularly loud passages or notes, the singer can either slightly turn one way or the other or back up so as not to overload the mic.
Experiment with your microphones to learn how they respond to proximity to the source they are capturing. Learn when they begin to sound muddy or boomy, and figure out whether position changes or increased distance will help to control overload and unwanted proximity effect. It’s a balancing act; you want the vocalist close enough to the mic to sound present and full, but not so close that their voice becomes too bass heavy or the mic overloads.