Walls are amplifiers of sound.
Outdoors – when you double the energy with amplifiers or speakers the sound levels increases 3dB.
Indoors – when you double the energy with amplifiers or speakers the sound level of certain frequencies can increase up to 9dB or cancel the sound 100%.
Example Below:
Before acoustical treatment, there would be 1dB loss at 10 ft and 2dB loss at 40 ft. without the sound system.
After acoustical treatment, there is 4dB loss at 10ft and 8dB loss at 40 ft. without the sound system. Also, intelligibility changed from 79% at 40 ft to 92% after acoustical changes. When you add the sound system the coverage with +/-3dB
This was in the middle of the acoustical transformation
Welcome to the world of Church Acoustics & Sound Systems 