The same aircraft, a different impact

West County makes a claim that's easy to hear as special pleading: the same aircraft, at the same height, is a heavier burden over a quiet rural community than over a neighborhood next to Highway 101. Put that way, it can sound like a request to have the noise graded on a curve. It's worth taking the objection seriously, because the claim is testable and the relevant evidence is already on the record. Acoustic context can make a measurable difference; California's own environmental law recognizes that context can matter; and Sonoma County's 2011 study of this airport measured several of the things that difference turns on.

The rest of this piece shows where each part of that comes from. It also draws two lines the claim does not cross, because they matter as much as the claim itself: it reaches the county's own decisions, not the flight paths the FAA controls, and it puts a fuller question onto the public record without dictating the answer.

A daily average was never built to answer this

Two familiar reference points in airport-noise discussions are CNEL and the 65-decibel compatibility line commonly associated with it. It's worth being clear about what each is, because they're two different things and both get asked to do work neither was built for.

CNEL, Community Noise Equivalent Level, measures cumulative noise over a day, giving extra weight to evening and nighttime noise. (Outside California, the federal government uses a closely related measure called DNL. Definitions of CNEL, DNL, SEL, and other terms used here are in the Sonoma Air Record glossary.) The 65-decibel line is a separate thing built on top of that measurement: a land-use compatibility benchmark, used to judge whether housing belongs in a given noise environment.

Both are useful for what they were made for. Neither, by itself, answers the question West County is actually raising. CNEL rolls many separate noise events into a single average, and checking that average against a compatibility line doesn't put the individual events back. Whether one jet, on one pass, breaks sharply out of the quiet and wakes someone up is not a question a daily average was designed to answer. That doesn't make the cumulative measure wrong. It means the measure can't do this particular job by itself — and California courts have already had to say so.

California law says context matters

The place it said so is Berkeley Keep Jets Over the Bay Committee v. Board of Port Commissioners, a 2001 decision of a California appeals court reviewing the environmental analysis for a major expansion of Oakland International Airport. The port's noise analysis had relied entirely on CNEL, checked against a fixed 65-decibel line — the same pairing that appears in the West County discussion.

Residents outside that 65-decibel line had told the port, when its draft review circulated in 1996, that aircraft noise was already disrupting their lives. Some described being woken repeatedly at night, or being unable to hold a conversation as aircraft passed. They warned that the expansion, with its added nighttime cargo flights, would make it worse. The port's review did not meaningfully analyze any of that. It did not examine how many nighttime flights would be added, how loud a single one would be, or how those events would compare with the quiet that was there before.

The court held that missing analysis inadequate under the California Environmental Quality Act — CEQA, the state law requiring environmental review of projects like this one. Its reasoning (91 Cal.App.4th 1344, 1380–1381, discussing Cal. Code Regs. tit. 14, § 15064(b)(1)) started from an idea already written into CEQA's own guidelines — context matters:

The same change can be minor in a noisy urban setting and much more consequential somewhere quiet, and there is no fixed definition of a significant effect precisely because significance depends on where a project sits.

From that, the court drew the line that matters here: a 65-decibel compatibility standard, set for land-use planning, is not "determinative" of whether a project's noise effects are significant under CEQA (1381). Compatible for zoning and significant for environmental review are not the same question.

The court also pointed to a technical resource state law required the port to use in preparing the review: the State Airport Land Use Planning Handbook (Pub. Resources Code § 21096). The Handbook addresses head-on what a flat 65-decibel line misses — it recognizes that an increase of only a few decibels can still matter where the surrounding noise is already low, specifically below 60 dB DNL (1381–1382).

None of this hands anyone a formula. Berkeley Keep Jets does not replace CNEL with a fixed three-part test that every noise study must run. It establishes something narrower and harder to argue with: where the record shows how quiet a place was, how often aircraft pass, or whether individual flights disturb sleep, those facts cannot simply be disregarded just because the daily average stays under 65. And the principle isn't confined to one airport case. Nearly two decades later, in a case with nothing to do with aircraft, a California appellate court held that an environmental review couldn't lean on a single fixed 65-decibel threshold while ignoring how far the noise rose above the existing background (King & Gardiner Farms, LLC v. County of Kern (2020) 45 Cal.App.5th 814).

Sonoma County already measured these things, at this airport, in 2011

So the setting can matter under CEQA. The next question is whether those effects can be measured in practice. Sonoma County's 2011 runway-extension review shows that they can.

When Sonoma County studied extending the airport's runways in 2011, the environmental review did not stop at an averaged number. Impact 3.10.6 of that study modeled the noise of a single flyover in terms of Sound Exposure Level, or SEL — a measure of one event rather than a full day — for four aircraft representative of what flies out of the airport: a small propeller plane, a turboprop airliner, and two sizes of business jet. SEL is worth a moment on its own. As the study's supporting appendix explains, it takes in both the loudest moment of a flyover and how long the event lasts, describing SEL as the integration of all the sound energy in the event. A single-event level already carries duration, not just peak loudness.

The study went further than mapping those levels. It checked them against a sleep-disturbance curve from a federal interagency aviation-noise committee and built a table of expected awakening rates. For a home of standard construction, it found, an outdoor event of 85 dB SEL could wake almost one person in 25; at 90 dB SEL, more than one in 20. Homes with acoustic treatment did better; homes with open windows did worse.

That table names its sources, and one of them matters. Alongside the federal committee's 1997 report and the county's own 2011 consultant modeling, it credits a 2003 supplemental study by the Port of Oakland. That study wasn't incidental. Oakland released it in September 2003, describing it in its own public announcement as the result of the 2001 court judgment that required the airport to reassess nighttime noise — and its findings on sleep disturbance used the same 85- and 90-dB SEL levels the Sonoma table uses. Neither the Sonoma noise chapter nor its appendix says the county's consultant was consciously applying the Oakland court's reasoning. The record shows something narrower but not merely coincidental: the county's awakening analysis draws on environmental work Oakland itself says was produced in answer to the very case that had required it to examine single-event nighttime noise and sleep disturbance.

The study also measured the quiet that noise intrudes on

A single-event level is only half the picture. The other half is what the event intrudes on — the sound that was there before the aircraft arrived. The 2011 study measured that too. In August 2009, sound-level meters were placed at ten locations around the airport, including two schools and several residential neighborhoods, and left running for several days at the closest sites. The results were broken into several statistical measures of the sound environment, including L90 — the level exceeded 90 percent of the time, a standard stand-in for background quiet. Across all ten locations, the study reported CNEL below 60 decibels. That places every one of them inside the quieter range the Handbook singles out.

That's what lets someone say, with the numbers to back it, how far an aircraft event rises above the quiet that was already there — not just how it compares with a 65-decibel line. And it was being measured around this airport fifteen years ago. It does not have to be invented, or imported from somewhere else.

The same flyover can stay above the background longer

Quiet changes more than the contrast at the loudest part of a flyover. It can also change how long the event stays distinguishable from the background. A jet's sound rises as it approaches and falls as it leaves. The quieter the background, the further that sound has to fall before it merges back into the surrounding quiet — so the event stays audible above the background for longer.

Altitude can lengthen the event too, for a separate reason. For an aircraft in roughly straight flight, the time it spends within a given number of decibels of its own peak grows with the aircraft's height and shrinks with its speed. By the time departures reach West County, they're thousands of feet up, and on a climb an aircraft's height rises much faster than its speed, so the time near the peak stretches out. One effect works on the background, the other on the geometry, and both push the same way.

Put together, that combination can make a distant overflight perceptible for substantially longer than the same event against a louder background. The scale of the effect over West County would need measurement; the illustrative calculation in the note below shows why it can be large. Either way, duration is not a separate thing bolted onto the single-event picture. It's already captured by SEL, the single-event metric the 2011 study used, which takes in how long an event lasts as well as how loud it gets.

A separate piece on the dispersal fallacy takes this further, to what it means for sharing the burden: because the same flight is a different event in a quiet place, spreading flights across more areas is not the same as distributing the harm evenly.

What the argument does not reach

Two limits hold the argument in place, and skipping them would make it weaker, not stronger.

The first is about who is bound. CEQA governs Sonoma County's own discretionary decisions when they're subject to state environmental review. It does not bind the Federal Aviation Administration, which works under federal environmental law, not CEQA. The flight paths aircraft actually fly — including the one that sends departures over West County — are set by the FAA under federal authority. Nothing in Berkeley Keep Jets, or in the county's 2011 review, reaches that decision. What it reaches is narrower: county-controlled airport projects, a runway extension or a terminal expansion, not every action the airport takes.

The second is about what an adequate analysis guarantees, which is nothing about the result. CEQA governs the process, not the outcome. If a fuller analysis turned up a significant noise effect, the county would first have to make findings on feasible mitigation or alternatives. Only where a significant effect can't feasibly be avoided or lessened may it approve the project anyway, on findings that the benefits outweigh the cost. None of that forces a quieter airport, a different flight path, or a court order stopping anything. What CEQA requires, when the issue is material, is that the harder question gets asked in public, on paper, instead of being answered in advance by a single averaged number.

That is a narrower promise than "the noise will be fixed," and a sturdier one. A resident describing one jet breaking sharply out of an otherwise quiet night — not a daily average that happens to stay under 65 — isn't describing the wrong kind of noise. Those are exactly the effects California law recognizes can matter, and that the county's own 2011 study showed can be measured. That isn't special treatment. The narrower conclusion is that acoustic context can be relevant, the effects are measurable, and the county's 2011 review shows those forms of analysis are not foreign to this airport. What follows from that is for the reader to weigh.

What's still unanswered

Whether that kind of analysis carried forward into every noise-related county decision since 2011 hasn't been checked here, and this piece doesn't claim it has. Three questions follow:

1. Does the analysis behind any CEQA-reviewed airport capital project approved since 2011 include the same single-event and background-noise measurement the 2011 runway-extension review did, or does it rely only on an averaged CNEL comparison against 65 decibels?
2. Do the reviews still to come for this airport, including the Airport Area Specific Plan and the General Plan Sonoma update, include the same kinds of analysis of individual flights, sleep disturbance, and existing background noise that the county carried out here in 2011?
3. If a future review uses CNEL without that single-event and background-noise work, what reason does it give for leaving it out?

Note — how long a flyover lasts. For an aircraft in roughly straight flight, the time its noise stays within a given number of decibels of its peak grows with the aircraft's height and shrinks with its speed. With illustrative figures of 1,500 feet at 170 knots near the airport and 5,000 feet at 230 knots over West County, a flyover stays within 10 decibels of its peak for about 30 seconds near the airport and about 75 seconds over West County. The time it stays above the background is longer still where the background sits further below the peak, though the air absorbs some sound over long distances and trims the fading tail, so the exact figure isn't known without measurement. These figures illustrate the effect. They aren't measurements at STS.

Sources

• Charles M. Schulz–Sonoma County Airport, Draft Environmental Impact Report (July 2011), Chapter 3.10 (Noise): significance-threshold discussion at 3.10-1–3.10-2; ambient-noise-measurement discussion and tables at 3.10-5–3.10-9; Impact 3.10.6 and Table 3.10-24 at 3.10-44–3.10-46; SEL definition in Appendix M (Noise).
• Oakland International Airport (Port of Oakland), "Oakland International Airport Releases Supplemental Environmental Impact Report on Airport Development Program," press release, Sept. 29, 2003.
• Berkeley Keep Jets Over the Bay Committee v. Board of Port Commissioners (2001) 91 Cal.App.4th 1344.
• King & Gardiner Farms, LLC v. County of Kern (2020) 45 Cal.App.5th 814.
• California Code of Regulations, title 14 (CEQA Guidelines), § 15064(b)(1) and (b)(2).
• California Public Resources Code § 21096.

Colophon: researched and drafted in collaboration with Claude (Anthropic).

Updated September 2026 to add how long a single flyover stays above the background in a quiet setting, and to lead with the finding.