On small stages the guitar amp, kick, and snare can be clearly heard in the vocal mic, an input delay can 'move up' the backline. This becomes problematic when these same signals are being mic'd closely so that they are also run through the PA. Because the close mic'd signals and the natural bleed into the vocal mic are not time aligned, this can cause comb filtering that blues the mix. By delaying the backline signals to match the acoustic bleed, this comb filtering can be minimized to tighten the overall mix and give it more clarity and punch.
To begin, you must isolate the vocal mic and the signal to be delayed. In our example, we will be delaying the mic'd guitar amp signal, so we will Solo both the vocal mic and the guitar amp mic channels.
Next, we will measure the distance from the guitar cabinet to the vocal mic. Sound travels at a rate of 1,130 feet per second, this means you will be dividing the distance measured in feet by 1.13.
For our example, let's say the guitar amp is 5 feet from the vocal mic, so we will be dividing 5 by 1.13 to calculate an input delay of 4.4.
Ask the guitarist to play a staccato pulse and listen for any remaining flamming. Move up or down 0.1 ms until your hear the tightest sound.