One of the most common issues during a live mix for some instruments or a band is the mic leakage or also called bleed. It can be the drums or onstage amplifiers leaking into the vocals microphone. It can be the leakage of crash into the guitar cabinet mic placed next it and picking up every time the crash is hit. There are some very useful ways to rectify this problem.
Once you are done with the sound check of individual instruments like kick, snare, tom, over-hats, guitar, vocals, tabla, synth, etc. Now be sure to a PFL(Pre fade listen) check of each microphone as the full band is playing. While listening to the primary sound of each microphone, also listen to what the other sounds are bleeding through.
Sometimes, it is like impossible to completely fix the leakage that can be due to many reasons like- small stage size, artists being more comfortable to stand close for communication but we can always reduce it by repositioning instruments, amplifiers and microphones around the stage by taking advantage of the polar patterns and rejection offered by the directional mics. Most of the live sound mics or dynamic mics offer cardiod polar pattern and reject sound well from the back.
Other way that can further help you, is by making a judicious use of EQ and Gates to filter the unwanted sounds bleeding into your microphones.
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