COLLEGE PARK, MD (January 25, 2001) - For the biggest football event of the year, much of the experience for fans who are heading to Raymond James Stadium will be how the game sounds. Fans' enjoyment of games can be "about the subjective experience of noise," says Dallas based acoustical consultant David Marsh, who has worked on the acoustical design of a number of stadiums including the soon-to-open NFL Broncos' new stadium, in Denver. "Whenever people go to large assembly spaces, acoustics is something they notice." Marsh says the design of a particular stadium, and how noisy the crowd gets, can make a big difference in how fans in the stands will experience this year's Super Bowl.
Marsh says an advantage of the Raymond James Stadium is that it is open air, which cuts down on reverberations that can interfere with one's ability to understand announcements and other speech produced by the sound system. Reverberation is when sound persists even after the source of the sound has stopped. Reverberation time is a measure of how long it takes a sound to completely die away - technically the time it takes for the sound level to drop by 60 decibels (db).
Marsh and his colleagues use reverberation time as one means of determining acceptable acoustics for a particular space. Unlike open-air stadiums, Marsh says, enclosed stadiums often have reverberation times on the order of 10 seconds, which makes speech difficult to understand. With an open-air stadium, reverberation is nearly non-existent because sound dissipates into the air. However, echoes caused by sound reflected off of distant surfaces must still be dealt with.
Designers like Marsh try to minimize the affects of echoes and reverberation by shaping and treating building surfaces. Sound reflects off of hard, smooth objects like light reflects off a mirror. Designers try to angle echo-producing surfaces so the sound reflects toward surfaces designed to absorb sound. In stadiums, Marsh says, designers often use fiberglass and other porous materials to absorb sound.
Crowd noise is another issue for stadiums. During moments of high excitement at an NFL football game, crowd noise levels typically range between about 95 to 105 dB with occasional peaks approaching 110 dB. This range is equivalent to the loudness of a car horn 10 feet away, on the low end, up to the loudness of an accelerating motorcycle a few feet away. The sound system, Marsh says "must be at least 10 db louder in order to be understood over that noise." This can be a problem when crowds get really loud. Marsh says he's heard of crowds getting as loud as 115 db - a level that can damage a person's hearing if exposed for any length of time. Under these conditions it is not practical for a sound system to produce intelligible sound and any attempt to do so would be a public safety risk.
Marsh says crowd noise will pose a significant challenge to sound system operators come Sunday in Tampa, when crowds are expected to reach record numbers. However, if the stadium acoustics and sound system are designed well, most fans will be relatively unaware of either as they enjoy the game.
The above post is reprinted from materials provided by American Insitute of Physics -- Inside Science News Service. Note: Materials may be edited for content and length.
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