County Code 4.88.330 sets exterior noise limits at residences, schools, hospitals, churches and libraries on a sliding scale by how long the noise lasts in an hour. Daytime (7 a.m.-10 p.m.) limits run from 55 dBA (30 min/hr) up to 75 dBA (instantaneous max); nighttime (10 p.m.-7 a.m.) limits are 5 dBA lower, from 50 to 70 dBA.
Section 4.88.330 (Exterior Noise Standards) prohibits creating noise that causes the exterior noise level, measured at any single- or multiple-family residence, school, hospital, church or public library, to exceed the limits in Table I. The limits depend on the cumulative number of minutes the noise occurs within a one-hour period and on whether it is daytime or nighttime. For the daytime period of 7:00 a.m. to 10:00 p.m., the standards are reported as 55 dBA for 30 minutes per hour, 60 dBA for 15 minutes, 65 dBA for 5 minutes, 70 dBA for 1 minute, and 75 dBA as the level not to be exceeded for any period. For the nighttime period of 10:00 p.m. to 7:00 a.m., each level is 5 dBA lower: 50, 55, 60, 65 and 70 dBA respectively. In short, longer-duration noise must be quieter, brief peaks are allowed somewhat higher, and an absolute ceiling applies. These exterior standards apply throughout the unincorporated county; incorporated cities adopt their own decibel tables.
Exceeding the Table I limits is a violation of Chapter 4.88 enforced by the Sheriff and County Code Enforcement (650) 363-4825. Measurements are taken at the affected receiving property (residence, school, hospital, church or library) rather than at the noise source.
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