Think about this for a moment, were playing music in a high volume situation were sound quality is important and distortion sounds very bad, so why can DJs keep the levels on the DJ mixer out of the red? This issue is more complicated than you might think. It could be that many DJs are un aware of what the level meters and 0dBfs mean or is it bad monitoring or ear fatigue?
I can tell you form my experience as a resident DJ that also booked the other DJs for the night it was a constant battle to keep the mixer levels in check and it was made clear to me that if the amps or speakers were damaged I would be receiving the rough end of the pineapple, and I can tell it's not nice. As the level creeps up over the night hour by hour until the levels are all maxed out and then the DJ is asking if the system can go any louder? Sound familiar? My response would be to tell the DJ to get out of the box and have a listen on the dance floor if you don't believe me that it's way too loud, you're making everyone deaf and by ignoring the level meters your mixing blind.
Let me share the worst case I've had. I just finished my set and the next DJ started his set. I was continually turning the volume down only for the DJ to creep it back up, then half way through his set the music stopped and there was a shower of sparks and a massive could of smoke and not the good smoke from the smoke machine, the bad type that stank of fried electronics. I knew then the night was over and that I would be getting the rough end of the pineapple.
The music was loud even though the DJ was like sorry man it did not sound that loud it must have been a doggy amp. Well I can tell you he was deaf because of a hearing condition known as temporary threshold shift in which his perception of what is really loud becomes greatly lowered. This will normally go away after several hours of silence but for some people it can become permanent. The trouble is that with continuous exposure to loud music over long periods of time a temporary threshold shift can kick in mid set. This means that your counting on you tired ears to judge where to put the levels and most likely making the wrong decisions. The only logical thing to do is accept that the level meters are correct. On top of this alcohol and drugs can make the situation even worse.
So if we all agree that our ears won't help us keep the levels consistent throughout the night, then you have no choice but to trust the meters on the mixer or even better a decibel meter. It gets more confusing due to the fact that sound is measured in many different ways dependant on whether it's in the analog or digital domain. Some measure peak level were others measure RMS. The human ear hears volume in RMS and RMS is a much better indicator of apparent volume than peak level indicators. The classic VU meter gauge is an RMS measurement device which measures voltage with .775Vac RMS being zero on the gauge. In the digital realm 0dBfs means that you have filled every last digital bit with information and no more can be added. I could bang on about the differences in analog vs. digital but there are lots of good resources on the net and in print.
When the meters on a mixer or software start hitting red or 0dbfs, this is called clipping and distortion begins to occur. It's called clipping because the tops of the sound waves literally get cut off. This sounds terrible. Nothing shits me more than going to a club and the music is distorted so DJs don't red line the mixer and club owners please keep the levels out of the RED.