San Mateo County's Noise Control ordinance (County Code Chapter 4.88) sets exterior noise limits at affected homes: 60 dBA daytime and 55 dBA nighttime may not be exceeded for more than 15 minutes an hour, with a 5 dBA penalty for tonal or humming sources typical of data center cooling.
Any data center on unincorporated San Mateo County land is bound by the countywide Noise Control ordinance, Chapter 4.88 of the County Ordinance Code. Section 4.88.330 sets a graduated exterior noise standard measured within 50 feet of an affected residence, split into daytime (7 a.m. to 10 p.m.) and nighttime (10 p.m. to 7 a.m.). It is tiered by duration: 60 dBA daytime and 55 dBA nighttime may not be exceeded for more than 15 cumulative minutes in any hour, and 65/60 dBA for more than 5 minutes. The standard drops 5 dBA for simple-tone or recurring impulsive noise, capturing the tonal hum of chillers and cooling towers. Noise at nearby homes is also mitigated through CEQA review.
Cooling or generator noise exceeding the Section 4.88.330 limits at a nearby home violates Chapter 4.88; San Mateo County can require mitigation and abatement, and unmitigated noise can block or add conditions to permit approval under CEQA.
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