Looking at the WARYN-BRGHT map, some neighborhoods show additional peak-noise burden.
Does that added BRGHT departure burden change their DNL when combined with the noise they
already receive from 33L arrivals?
**Yes, their combined DNL rises — but no home crosses the 65 DNL significance
threshold. A subset (443 homes) does cross a reportable increase threshold, which is a
disclosure-level regulatory event.**
For every residence, we formed the total DNL from the two operations that dominate the
corridor, before and after the change:
Arrivals appear on both sides, so the comparison isolates what the departure change does to
each home's total burden — it does not create false relief by dropping arrivals from one side.
All values use the measured Sound Exposure Level basis and the measured distance attenuation
(K ≈ 1.2), aggregated to DNL with published BWI operation counts.
Decibels are logarithmic, so noise levels cannot be added directly — 51 dB + 50 dB is not
101 dB. Two DNL values combine by converting each back to the sound energy it represents, adding
the energies, and converting back:
DNL_total = 10 · log₁₀ ( 10^(DNL₁/10) + 10^(DNL₂/10) )
Worked example — a home with departure DNL 51 and arrival DNL 50:
So 51 and 50 combine to 53.5, not 101. When two sources are close in level, the total is at
most ~3 dB above the louder one (two equal sources = exactly +3 dB); when one source is much
louder, the quieter one barely moves the total. A home's combined DNL is therefore dominated by
whichever source is loudest.
The before/after change is two such combinations, then a subtraction:
The arrival term is identical on both sides. It does not cancel algebraically (it sits inside the
logarithms), but it damps the change: the louder the shared arrival noise, the less a departure
change moves the combined total. This is why stacking arrivals underneath reduces the apparent
effect of the departure change rather than inflating it — the physically correct behavior.
Where each per-source DNL comes from, one level up, is the same energy-under-a-logarithm
construction applied to the daily flights:
DNL = mean SEL + 10 · log₁₀(N_day + 10 · N_night) − 10 · log₁₀(86,400)
where mean SEL is the average sound-energy-per-overflight, N is the daily count of events (night
events, 10 pm–7 am, weighted ×10 for the penalty), and 86,400 is the number of seconds in a day.
The full chain for a home is: measured single events → SEL per event → per-source DNL → combine
departure and arrival energies → total DNL. Nothing is ever added in raw decibels.
Note on the peak-dBA maps: peak levels are not strictly energy-additive (two 70 dB peaks at
different moments do not stack into one louder instant), so the peak "combination" shown on the
peak maps is an approximation for visualization only. The DNL/SEL combination above — used for the
regulatory answer — is the physically defensible one.
For the ~2,559 homes that see added peak burden under WARYN:
| Measure | Value |
|---|---|
| Combined DNL before (median) | ~51 |
| Combined DNL after (median) | ~54 |
| Median DNL increase | +2.7 dB |
| Largest DNL increase (single home) | +7.6 dB |
| Corridor maximum combined DNL, after | 62.7 |
| Homes tripping a reportable increase | 443 |
| Homes reaching or crossing DNL 65 | 0 |
So the added departure burden is not negligible in DNL terms — it moves the combined
contour up by a median of nearly 3 dB, stacking on the existing arrival noise. But because the
corridor's highest combined DNL peaks near 63, **no residence reaches the 65 dB significance
threshold**, even with arrivals fully included.
Two things are true at once, and they are distinct:
greater increase for an area at or above DNL 65 (or brought to 65 by the action). No home
reaches 65, so this threshold is not met anywhere in the corridor.
receive a 5 dB or greater increase — the reportable-increase trigger for that band. "Reportable"
means the increase is large enough to be disclosed in an environmental analysis; it does not
by itself constitute a significant impact or require mitigation. It is a transparency threshold,
not a blocking one.
The honest one-sentence version: **the added burden raises combined DNL for the
affected neighborhoods and pushes 443 homes across a disclosure threshold, but it does not push
any home across the significance threshold.**
only. A home's true total DNL from all BWI runways and directions would be somewhat higher —
though all measured evidence still places it below 65. A full all-operations figure is AEDT
territory.
arrival distance-falloff borrows the departure-measured slope. Arrival levels far from the
approach are the softer part of the estimate, which propagates into the 443 count.
examples exist.
inform, not to substitute for the FAA's official modeling.
Arrival noise and the added departure burden do
combine, and the combination does move these homes' DNL — by a median of about 3 dB and surfaces 443 homes at a reportable-disclosure level. But even with arrivals stacked
underneath, the combined DNL stays below the 65 dB line the FAA uses to find significance.