As time went on, their props and stage equipment became increasingly sophisticated. When the Stones began to fill stadium-sized venues, or larger, they ran into the problem of the audience no longer being able to see them. This was particularly the case when they performed a free concert for an estimated 1.5 million people in Rio de Janeiro on the "A Bigger Bang" tour in 2006. The show required over 500 lights, hundreds of speakers, and a video screen almost in length. Due to the length of the beach on which the Stones performed, sound systems had to be set up in a relay pattern down the length of the beach, to keep the sound in sync with the music from the stage; for every of beach, the sound had to be delayed an additional second.