What is a speaker?
A speaker is a transducer that converts electrical energy into sound waves. PA speakers turned up to 95dB (reasonable indoor volume) make big waves. You not only have to consider wave direction but also what they bounce off of.
Picture dropping a stone in the middle of a round pool. The waves will be of equal energy in all directions. Now drop the stone near the edge of the pool and watch the waves interact. These peaks and dips are the same as changes in volume of individual frequencies the audience hears. You won’t be pointing your speaker at the wall but you can see how messed up your intended sound wave will get when it bounces off any flat surface.
Then there’s the dreaded resonant frequency between two parallel surfaces. This is when those peak waves keep adding together creating an unwanted bass feedback through the PA.
Use Your Equalizer
Your equalizer is for adjusting a speaker to the environment. Turning down one frequency should fix this problem. To find the frequency take the SPEED of SOUND 1130ft/sec and divide by distance between walls, or floor and ceiling. Example; twelve foot ceiling, 94Hz and around G2 on piano. Faster than doing the math, turn down low frequencies on EQ one at a time to find offensive frequency. You can do reverse math 1130ft/sec divided by frequency to get the distance to help identify offending surfaces, if it’s not obvious. Placing speakers at least 9ft (3 meters) away from a wall on the side and behind if possible, is a good place to start to avoid reflections.
Look up the spec sheet for your speaker for coverage in degrees, horizontal and vertical. The middle of the magnet on the horn driver is the point where these angles originate. Sound is more directional as frequency increases (location is usually the upper back of speaker, exact point not important). Imagine these angles extending from speakers. Outside of this pattern the high end drops off. Hopefully, your speaker can cover your audience, side to side and front to back. A big mistake is placing the speakers too high up when there is a high ceiling or when you are outside. Sound bounces around a big empty space or isn’t being heard by anyone. Aim speakers just over most people’s heads so the crowd receives direct sound.
Before the band sets up, look for the best location for speakers: compromise.