The guide on the Kennedy Space Center tour said something about how many dB the water spray reduced the volume by but I can't remember it off hand.
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Charlie from London U.K.
How many gallons of water are sprayed onto the launch pad during the launch of a shuttle?
That's a relatively easy one. There are about 300,000 gallons of water. And if you look at the launch pad, or at some of the pictures we have of the launch pad. Just to the northeast you will see this huge water tower and in that water tower and in the huge piping that comes down from it, we have about 300,000 gallons of water. And at T minus 16 seconds we begin to flow the water underneath the launch pad. That's sound suppression water, in order to stop the echo of the engine ignition and going down and hitting the bottom of the pad then bouncing back and knocking tiles off the orbiter. So about 300,000 gallons of water will flow in 30 seconds. As the orbiter begins to lift off you may have seen in some slow-motion photos right at the pad deck there are these huge stand pipes. We call them rainbirds, and they begin to spray water all over the pad surface to cool and also to deaden the noise there as well.
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Why is there so much smoke during space shuttle and rocket launches?
The majority of the white clouds seen during the first few seconds of a shuttle launch is actually steam, not smoke. To dampen the noise from the powerful rocket motors on the shuttle, approximately 400,000 gallons of water are sprayed underneath the exhaust for six seconds before launch and fifteen seconds afterward. When the hot exhaust hits the water it evaporates, making an enormous cloud which will eventually precipitate and turn into rain.
Without this sound dampening system, the immense blast of noise from the shuttle launching would likely break windows for miles around the launch site.