Sep 2, 2009 12:00 PM, By Trevor Boyer
Designing and implementing stadium audio systems for the 2009 MLB season.
Potts cites the audio from the video replay as an example of a difference between the functions of the systems at Busch Stadium and Citi Field. “The difference in this system is that the audio operator controls absolutely every audio that’s heard in the system here,” he says. “At Busch, they have a sub-mixer from the video-replay system that’s fed to the audio system.” (One measure of the degree of the AV systems’ complication might be the miles of cable that snake around both stadiums. At Citi Field, Potts estimates that there’s 200 miles of cable, much of which sits in neat, dense rows in two trays that hang from the ceiling of the service-level underbelly of the stadium.)
For the New York Mets organization, the comparison is not Busch Stadium but Shea Stadium, which was quickly demolished as Citi was being built across the street (relatively quickly, by New York standards, that is). Shea had an analog audio system based on a center cluster of loudspeakers. At Citi, it’s a digital distributed system, with a loudspeaker hanging every 30ft. to 40ft. around the top of its seating bowl. AX series (396, 364, and 344) and MK series loudspeakers from EAW cover the bulk of the seating at Citi. The long-throw EAW AX loudspeakers are custom units designed by the manufacturer and WJHW specifically for Citi Field.
In some areas, especially in seats under a roof, the low frequencies produced by standard long-throw AX loudspeakers would be excessive. “When you’re under deck here, sound has a tendency to billow,” Potts says. “It builds up, and that low frequency makes things unintelligible.”
According to Beaudoin, it’s a simple result of the inverse square law that dictates how volume dissipates over distance. Ideally, you want a 3:1 to 4:1 ratio between the farthest reach of the loudspeaker throw and your closest listener. “Because of the architectural design of that stadium, the ratios were bad, and we were forced to put the loudspeakers on club level,” he says. “If the throw to your furthest seat is 100ft. and the measured SPL at that seat is 100dB, then somebody sitting right underneath that same loudspeaker is hearing about 120dB.”
To alleviate that problem, EAW and WJHW designed “canceling” woofers for the bottoms of the cabinets, which sit at a 90-degree angle to the cabinet front. Each cabinet has dual 10in. woofers, and technically they don’t cancel frequencies so much as reduce them. “The concept is to create a companion source of same signal but different phase so as to provide cancellation of the main signal in areas where the main is excessive in level,” says Kenton Forsythe, EAW co-founder and VP for strategic engineering. “I say ‘phase’ as it is not merely inverted polarity. The adjustment is done with time and level so as to maximize the reduction where it is needed, with minimal reduction of the forward radiation.”
Beaudoin estimates that in Citi Field, the “canceling” woofers effectively reduced the low frequencies by 6dB to 8dB. Higher frequencies (above 500Hz) were not as much of a problem, because they respond much better to the directionality of the loudspeakers.
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