"the new generation of stadiums now incorporate design features that help boost fan support by trapping and amplifying crowd noise. The most important aspects are to keep the size of the stadium as small as possible, and to provide reflecting surfaces that can turn the noise back to the crowd, said Jack Wrightson, a Dallas-based acoustical consultant who has worked on the design of dozens of athletic venues in North America.
"Sound loses energy as it travels, so the key is to keep the venue small and intimate," said Wrightson."
"Using materials like wood and metal increases reverberations and can also boost crowd noise."
"having a roof can help amp up the noise in the stands. European soccer stadiums all have partial roofs, which helps keep noise levels up. At the Seattle Seahawks' CenturyLink Stadium the large overhanging roof reflects noise down towards the stands"
https://www.insidescience.org/content/stadium-acoustics-pump-volume/1609