At Queen’s Live Aid performance in 1985, you can actually see the sound waves propagating through the audience.
When Queen performed in front of 72,000 people at London’s Wembley Stadium on July 13, 1985, the band around Freddie Mercury not only delivered what is probably now the best-known performance of their career with the Live Aid gig, but also gave a small physics lesson at the same time: In the Twenty minutes into Queen’s on stage, they got the crowd moving, right down to the back row, so you can actually see the sound waves spreading through the crowds.
And the speed at which sound travels has to be taken into account by the sound technician with an audience of this size: With a sound speed of 343 meters per second and a depth of the audience area of Wembley Stadium of 105 meters, there are delays of fractions of a second, which must be taken into account when using the rear loudspeakers so that the music from the loudspeakers and from the stage does not reach the audience asynchronously.
what that means concretely can be seen impressively in this gif of Queen’s performance: The entire audience is in time – but the sound arrives slightly delayed in the back rows and so the music spreads through the audience like a wave.
In case you got the gif now: Here is the complete Live Aid performance of Queens, which was watched by an estimated two billion people in 60 countries worldwide in 1985: