A venue has two sound problems. One is making an announcer or a band audible across tens of thousands of seats. The other is the sound the crowd makes itself, which a building can help or hurt.
From the megaphone to the PA
A public address system is microphones, amplifiers, loudspeakers and the equipment between them, used wherever a voice has to carry over a large area. Sports stadiums are a typical case. In 1913, multiple units were installed throughout Comiskey Park in Chicago, both for announcements and for music between plays; the owner, Charles Comiskey, is quoted as saying that the day of the megaphone man had passed at his park.
Concert sound
A sound reinforcement system is the PA's bigger relative, used for live music and performance. A stadium rock show can need hundreds of microphones, live mixing and signal processing, amplifiers delivering tens of thousands of watts, and several speaker arrays, operated by a crew of audio engineers.
Line arrays
Most large venues use line arrays. A line array stacks matching loudspeaker elements one above another, driven in phase so that together they behave almost like a single line of sound. The elements are close enough together to interfere constructively, which sends sound farther than traditional horn-loaded loudspeakers and spreads it more evenly.
Hung vertically, an array has a very narrow vertical pattern. That focuses sound at the audience instead of wasting energy on ceilings or empty air, while keeping a wide horizontal pattern to cover the seats. Modern arrays use separate drivers for high, middle and low frequencies, and each enclosure is built to rig closely together with the next. Digital loudspeaker management adds delay so that speakers placed at different distances stay in sync.
The crowd as a sound source
Architectural acoustics is the engineering of getting good sound inside a building. At football grounds, designers often aim to make the crowd sound as loud as possible. Many outdoor football grounds have roofs over the fan sections, which add reverberation and echo and raise the general volume. It is one more reason the choice between an open ground, a fixed roof and a retractable roof changes what a match feels like.