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Airport noise contours (CNEL) explained

Those nested rings around every airport on planning maps are CNEL contours — the official measure California uses to regulate airport noise. If you are house-hunting anywhere near SFO, OAK, or SJC, understanding what 65 dB CNEL does and does not mean will save you from both false comfort and false alarm.

Why 65 dB is the magic number

California’s airport noise standard sets 65 dB CNEL as the boundary of the “noise impact area” — the level above which residential use is considered incompatible with airport operations unless mitigated. That single number drives most of the machinery around airport noise: sound-insulation grant programs for homes inside the contour, avigation easements recorded on titles, and the shape of what cities may approve for new housing nearby. Contours at 60, 70, and 75 dB bracket it on planning maps; each 10 dB step is roughly a doubling of perceived loudness.

Contours are models, and they move

A CNEL contour is computed from a year of actual operations — fleet mix, runway use, flight paths, day/night split — and redrawn periodically. Quieter aircraft have shrunk contours dramatically over two decades even as flight counts grew; a house that sat inside the 65 contour in 2005 may be well outside it today, and vice versa after a runway-use change. The date on the contour map matters.

The Bay Area picture

Three commercial airports dominate: SFO, whose contours reach into Millbrae, San Bruno and South San Francisco; OAK, affecting Alameda and San Leandro; and SJC, whose contour runs through central San Jose and Santa Clara. General-aviation fields (Palo Alto, Reid-Hillview, Hayward, Livermore, San Carlos) have smaller contours but generate plenty of overflight in their own neighborhoods. Each city page shows the share of that city inside a 65+ dB contour.

Average noise vs. what you hear

The annual-average nature of CNEL is its biggest limitation for a buyer: it smooths away runway-direction days, seasonal patterns, and the difference between one loud flyover and forty moderate ones. Treat the contour as the regulatory floor of your research, then look at actual flight activity over the address and listen in person at the times you care about — a weekday 6 am, a Sunday afternoon.

Check a specific address: the free sitelume interactive map shows which mapped zones any SF Bay Area address falls in — layer by layer, no sign-up.

Frequently asked questions

What does CNEL actually measure?
Community Noise Equivalent Level: a full year of aircraft noise averaged into a single day-night figure, with evening flights (7–10 pm) counted as if they were ~3 dB louder and night flights (10 pm–7 am) as ~10 dB louder, reflecting how much more intrusive noise is when people are home and sleeping. It is an annual average of acoustic energy — not the loudness of any single flyover.
Is a house outside the 65 dB contour quiet?
Not necessarily. 65 CNEL is where California draws the line for land-use compatibility, but research consistently finds annoyance well below it. A home under a departure path at 58 CNEL can still hear a distinct aircraft event every couple of minutes at peak times — the average is compatible with plenty of individually noticeable flights.
What is an 'airport influence area' in my disclosure packet?
A boundary adopted by each county's Airport Land Use Commission around every public airport — generally at least two miles out. Homes inside it get a statutory notice in the sale documents that the property may be subject to airport noise and overflight. It is much larger than the 65 dB contour, so receiving the notice does not mean the home is inside a mapped noise contour.
How can I judge aircraft noise for a specific address?
Three steps: check whether it falls inside a mapped CNEL contour (the sitelume map shows the official contours and the report card gives the band), look at observed flight activity over the neighborhood on the map's flight-tracks layer, and visit at different times — early morning and late evening reveal what the annual average hides.

Related reading

Informational summary of publicly available government designations — not legal, insurance, or engineering advice. Zone boundaries and rules change; verify with the issuing agency before making decisions.