So, one would have to a stand a certain distance, lets say 50 feet, away from the center line and measure the sound level (decible) as the car passess. And if it exceeds zero in this case, you are in violation?

Looking up the Noise Ordinance for San Diego:

Sec. 59.5.0402 Motor Vehicles
A. Off-Highway

1. Except as otherwise provided for in thin article, it shall be unlawful to operate any motor vehicle of any type on any site, other than on a public street or highway as defined in the California Vehicle Code, in any manner so as to cause noise in excess of those noise levels permitted for on-highway motor vehicles as specified in the table for "45 mile-per-hour or less speed limits" contained in Section 23130 of the California Vehicle Code, and as corrected for distances set forth in subsection A.2. below.
2. Corrections
The maximum noise level as the on-highway vehicle passes may be measured at a distance of other than fifty (50) feet from the center line of travel, provided the measurement is further adjusted by adding agebraically the applicable correction as follows:

Distance (feet) Correction (decibels)
25 -6
28 -5
32 -4
35 -3
40 -2
45 -1
50 (preferred distance) 0
56 +1
63 +2
70 +3
80 +4
90 +5
100 +6


3. A measured noise level thus corrected shall be deemed in violation of this section if it exceeds the applicable noise-level limit as specified above.