Before any successful show there is one element that must be accomplished after the load in process, When all the sound equipment, lighting, and processing gear is taken from the trucks and set up on the stage, it's then you will you will perform "The Soundcheck." it's during the sound check that you do exactly that...check the sound (or acoustics) of the room that you will be playing in, so that you can hear what the room sounds like, (against your type of music) and then dial in the sound that you want using processing equipment. For example, if you are giging in a room with a lot of glass (windows) or "glass fixtures." your amplified sound tends to "bounce" off all that glass and create a "super reverbed" room that can be very unpleasant. On the other end, you might play a venue where there are wood floors, low ceiling, and lots of carpeted areas, and the sound tends to "stay right there" and not bounce but sound "tight." And your first song during your performance is not the time to find out the room sucks. So you run a soundcheck by playing a few songs from your playlist for that particular show while your soundcrew moves sliders, knobs and wheels helping you sound like you know,you're supposed too.Not only that, you get to play through all your other equipment to make sure it's working properly as the last time you used it. When you play big shows with multiple acts like a music festival, it;'s a easier process because the sound crew has "tweaked" the house already during the other bands performance. That is why you ALWAYS do a sound check!