At half-time in the match the PA announcer advised the crowd that there would be three blasts of noise over the loudspeakers. These would, in some way, aid the designers planning the new stadium. There then followed three loud bursts of what sounded like static over the tannoy. They varied in length from about 10 to 30 seconds. The bursts of noise are being used to make various acoustical measurements inside the stadium.
Matter absorbs sound, and that includes people.
Because the stadium will have a lot more matter in it when it is filled with people than when it is empty, the stadium has to be designed for a good acoustical quality when it is full. The walls and other structures have to be designed in a way that eliminates echo, transmits sound properly, and avoids dead spots. The choice of sound equipment is also a function of these measurements. An easy way to accomplish this design task is, as you have experienced, conducting such testing under real-life operating conditions.