The first thing you need to find out is what standard they base their testing proceudre on? Up until recently most states based their testing on SAE proceudres J1970 or J34. The interesting requirements of these are as follows:
The wind has to be less then 13 mph to do the test.
The meter must be held at arms length
No one besides the operator can be within 15 feet of the meter.
3 db must be subtracted to compensate for ambient noise.
Recently though, I've heard that there is a new 2000 standard that just requires an idle measurement at 3 feet. I'm not sure of the details, but I'll try and get them. There typically are certain baseline situations that must be met as indicated on the other tests.
One of the best defenses though is a video camera. It confirms how things were tested and no officer wants to end up on a training video of "How not to do a noise test"
The video erases any doubts about who's word the judge believes, since typically you, the defendant, will loose, regardless of the truth. (personal experience )
[ 07-03-2001: Message edited by: mltdwn12 ]