I am currently editing and adding to a new edition of ‘The Tour Book’. The following information can be found in chapter 7 – At the Show – and I thought it worth repeating here.
“So why does the performance sound so different at sound check than it does at show time? Physics plays a large part. (Bear with me—this is very important!) The speed or velocity of sound in air is approximately 344 meters per second, 1,115 feet per second, or 770 miles per hour at room temperature of 20°C (70°F). The speed varies with the temperature of air, such that sound travels slower at higher altitudes or on cold days. Also, in a cold room, the sound waves will refract (bend) downward as they travel away from their source. Therefore, in a cold, empty room, you will have the sound waves slowing down and dipping in height as they reach into the room. For the high-end frequencies, this causes a lack of clarity and direction. For the low frequencies, a dampening occurs more than usual. The result is a smeary, bass-light mess. Add in the highly reflective walls, floors, and mirrors in a bar or club, and suddenly you can see and hear why sound checks often sound so awful!”