CNEL vs. what you'll actually hear: why quiet-on-paper isn't quiet
A house can sit outside every mapped noise contour and still get a distinct jet overhead every three minutes at breakfast. That is not the map being wrong — it is what happens when a year of sound is compressed into one averaged number. Here is how CNEL's math works, what it hides, and how to hear past it before you buy.
The averaging problem, with real numbers
Decibels are logarithmic and CNEL averages energy, not loudness as you experience it. Compress a day that contains forty 30-second flyovers peaking at 80 dB into a 24-hour energy average alongside a ~45 dB suburban background, and the result sits in the mid-to-high 50s CNEL — comfortably “compatible” on paper. Nothing in that number tells you that conversation on the patio stops forty times a day. The metric is doing exactly what it was designed to do: rank long-term cumulative exposure for land-use planning. It was never designed to describe an afternoon in the backyard.
Three numbers, three questions
| Metric | Answers | Typical use |
|---|---|---|
| Lmax | How loud does one flyover get at its peak? | Sleep disturbance, speech interruption |
| SEL | How much total sound energy did one event deliver? | Comparing aircraft types and procedures |
| CNEL | What is the year-long weighted average exposure? | Contour maps, land-use regulation, disclosure |
CNEL’s evening (+~5 dB, 7–10 pm) and night (+10 dB, 10 pm–7 am) weightings help — a night flight counts as ten times its energy — but weighting an average still produces an average. Two homes at 60 CNEL can live completely different lives: one under a steady stream of moderate overflights, one hit by a handful of very loud night arrivals.
Why the same house varies week to week
The contour also averages away time structure. Airports flip runway configurations with the wind — the Bay Area’s summer sea breeze and winter storm patterns each favor different flows — so a neighborhood can get weeks of quiet followed by weeks under an arrival stream, all consistent with the same annual number. Seasonal fleet changes and nighttime cargo schedules add more texture the average flattens. Residents call these “bad weeks”; the metric calls them nothing at all.
How to hear past the average
- Start with the regulatory floor: is the address inside a mapped contour at all? Our CNEL guide covers what contour membership triggers, and each city page shows how much of that city is inside 65+ dB.
- Look at what actually flies over: the sitelume map’s flight-tracks layer shows observed flight activity over any address, refreshed daily — the event-frequency picture the average hides.
- Visit at the honest hours: a 6 am weekday (first departure banks) and after 10 pm (arrivals and cargo). Windows open.
- Ask about the house itself: homes near contours may have been through a sound-insulation program — upgraded glazing and an avigation easement on title are both worth knowing about before escrow, not after.