How to make your band sound clear, tight and not 'muddy'.

One of the most important things about playing in a band is understanding how your instrument fits in the mix.
As a sound engineer, one of the most common problems I hear in live bands is unclear vocals and either a muddy sound or a piercing sound that makes you want to put your fingers in your ears even when its not that loud..

This is actually a really easy problem to fix, but first you need to understand why it happens.
Take a look at this great chart...
Musical Frequency Chart

It shows all the instruments and what frequencies the instrument generates.
A band that sounds tight and 'phat' will make sure that the instruments have as little as possible overlap in terms of the frequencies they generate. This means that the bass guitar is not making the same frequencies as the rhythm guitar, and the lead guitarist isn't overpowering the vocalist.

That leads me onto the number 1 cause of unclear vocals....
If you look at the chart the first thing you notice is the electric guitars operate in almost exactly the same frequency range as vocals. This is both good and bad. Its bad if your guitars are too loud when your vocalist is singing because the guitar will make the words sound unclear.
However, its really good when the vocalist stops singing and its time to rip out a mammoth guitar solo because you can get away with turning the guitar WAY up, and I mean WAY up.

If you are playing lead guitar, make sure you always have a boost footswitch for your solos, and you really can get away with having quite a big difference between normal and solo volumes.
Think of it like this... when you are playing during the verses, keep your guitar level right down and just fill in the gaps around the vocalist, during this part you need to be complementing the vocals not competing with them.
Stick to one string stuff and play in a different register to the vocals. If the vocals are singing high, then play low notes and if the vocals are low play high notes.

Then when the vocals stop and its time to solo, think of the guitar as the vocalist during the solo. Make the guitar sing, play it loud and strong then pull way back again when the vocals kick in.

While we are on the subject of guitars, keep an eye on your bass and treble, make sure the bass notes on your guitar don't interfere with the highest bass guitar notes, and make sure your treble notes stay well clear of the range where humans are most sensitive (2k to 3k). A guitar is a mid range instrument, don't try and generate all the frequencies of the spectrum just because you can.

On to vocals, and its important to remember that humans can generate quite low frequencies even when singing higher notes. Percussive sounds like 'P' and 'T' words can also interfere with the bass guitar and make your whole band sound muddy. Turn down the bass control on the vocal channels to improve the clarity of the whole bands sound.
If your mixing desk has a High Pass Filter (button or a knob on more expensive desks), this is the greatest invention in this history of sound and you should use it. Turn on the high pass filter on every single channel except the bass guitar and kick drum and you will see an amazing improvement in clarity.