11 May 2006

China Basin noise study

Two nights ago I hung out around the perimeter of AT&T Park. While I was disappointed in the numerous empty seats, the noise promised to be especially heavy for Barry Bonds as he attempted to tie the Babe. When Juan Pierre robbed Bonds on his shot to deep center, the circumstances were perfect. More on that in a bit.

Many of AT&T Park's signature elements make it a poor design for placement into a residential neighborhood. Chief among them is the open outfield design. Still, when San Jose looked around for a ballpark study, they went with AT&T Park design firm HOK. The sketches in the EIR had the ballpark oriented east-northeast (similar to the Coliseum's orientation), with large open spaces beyond the outfield walls. The resemblance is rather uncanny:

The grandstand shape (beneath the foul lines) is virtually the same. There's more space in right field for seating since there's no pesky body of water in the way, but otherwise it's a pretty good copy. That's good for skyline views, bad for containing noise. The noise contour sketch shows noise spilling further out past the right field fence as opposed to the left field fence. This may be due to the grandstand's shape. In the left field corner, the grandstand and the roof above the upper deck make a near right angle turn towards the foul pole. In right field, no such grandstand turn exists, allowing the noise to escape unabated. The SJ design has a two levels of seating beyond the right field fence, but it's not nearly as tall as the grandstand and doesn't have a roof. More noise would escape over the top of those seats during games.

To test the assumptions made in drawing up the contour, I monitored noise levels from three points outside AT&T Park. The top picture depicts those three points:
  • 1 - The promenade or knothole area along McCovey Cove. I was at the railing, approximately 200 feet from the center of the stadium.
  • 2 - The other side of McCovey Cove, near Willie Mac's statue. This location was approximately 800 feet from the center of the stadium.
  • 3 - The South Beach Harbor playground, near the funky sculpture that looks like a big red compass. This location is also approximately 800 feet from the center of the stadium.
The distances are important, because sound loss is based on distance. With every doubling of distance from a sound source, there is a loss of 6 dB. In reality the loss may be more like 3-4 dB because sound can bounce off the ground and other surfaces without being absorbed. Accoding to most documentation I've read, a drop of 3-4 dB is barely detectable. Keep increasing the distance and it adds up. A loss of 10 dB means that a sound is half as loud. Drop 20 dB and it's only one-fourth as loud. (This also works in the opposite direction, so a 20 dB gain equates to four times the loudness.)

Baselines were set around the perimeter of the ballpark. At the intersection of King and Third Streets (Willie Mays Plaza), ambient noise was usually 65 dB, though traffic or MUNI Metro trains could cause the meter to spike 10 dB or more. At point #1, the typical ambient noise (uninterrupted by the PA, music, or loud cheering) was 72 dB - the volume of a running vacuum cleaner. Below is a table showing readings taken at various times during the game. Margin of error is plus/minus 3 dB.

Notice how the readings from point #3 are uniformly, consistently lower than the readings from point #2 even though they're the same distance away from the source. The grandstand (and the attached ramp) appears to have really shown its absorption capacity, and that's only partial containment. Additional noise mitigation could be achieved by further enclosing the structure. I'm not talking about building Mt. Davis. It would be more akin to the outfield seats built in San Diego, Philadelphia, and Cincinnati. Sacrifices would have to be made regarding the skyline views. Is such a compromise worth it? Maybe not. Then again, if the final design resembles the 360 architecture concept, those outfield structures (hotel, party suites, condos) will reduce noise leakage just by being there. They'll also restrict the view - just by being there.