HOW LOUD DOES IT NEED TO BE?
Music is quite dynamic in nature. The human voice is capable of a dynamic range (quietest to loudest parts) of over 60 decibels. The dynamic range of human hearing is about 120 decibels (depending on age and environmental factors). Recorded music with a dynamic range of 18dB is considered to be very dynamic in nature. Some modern electronic dance music has a dynamic range of less than 6dB (constant bass heavy thumping). The magic of live performance lies in its dynamic sound, giving it power and emotion that a recording on the radio simply can't recreate, so it's important we don't forget about leaving headroom in our system sufficient enough to recreate all of those short bursts of energy so the mix doesn't begin to sound stale or congested.
So let's use an example: If our speaker can produce 100 decibels continuously at the furthest listener location (as calculated by our efficiency, power handling, and distance formulas above), and the noise floor in the room is 70dB (how loud the room is with the sound system turned off due to HVAC, lighting, cutlery and light conversation) then we have 30dB of audible dynamic range with which to mix our sound before the quietest parts are too quiet, or we reach the upper limit of the reliable operation of the speaker system. Louder rooms (excited audience, appliances like kitchens or air conditioners) will have a higher noise floor, and therefore would leave us less dynamic range for the music to sound natural and may require an audio system with a higher initial volume. Music with very little dynamic range will not sound natural, so it's important to make sure that the speaker system specified for the event gives us enough capacity to be heard "above the noise". High sound pressure levels of high-frequency audio content over time can be fatiguing to listen to without low-frequency support from subwoofers. Adding low frequency support to the system can improve listener attentiveness and reduce hearing pain when higher sound levels are required.