Day-Night Average Sound Level (Ldn and CNEL)

The day-night average sound level a community noise ordinance is written against, and the two things about it that decide outcomes. Ldn is the 24-hour energy average with a 10 dB penalty on every hour between 10 p.m. and 7 a.m.; CNEL adds about 4.8 dB more between 7 p.m. and 10 p.m. Both average on a 3 dB energy basis, which is a different law from OSHA occupational noise dose -- that uses a 5 dB exchange rate and answers a hearing-conservation question rather than a land-use one, and the two must not be interchanged. The night penalty is the first decisive thing: ten decibels is a factor of ten in energy, so an hour of work at night enters the average as ten hours of the same work by day. Moving two hours of a 78 dB activity from the afternoon to 11 p.m. raises the Ldn from about 67 to about 77 with no change in equipment, level, or duration, taking a marginal operation to a clear violation against a 65 Ldn limit. No practical noise control recovers that. The second is that energy averaging lets the loud activity dominate: twenty-two quiet hours contribute almost nothing to a day containing two loud ones, so reducing the background is close to wasted effort while shortening the loud activity is the lever, and halving its duration takes 3 dB off -- more than most equipment treatments deliver. Both moves are reported side by side because the intuition usually points at the wrong one. Levels are entered and are the hard part, since an ordinance is written at a property line or a receptor and a level measured elsewhere is not that level. This does not measure anything, propagate sound from a source to a receptor, apply distance, ground, barrier, or shielding attenuation, add the tonal, impulsive, or low-frequency character adjustments many ordinances carry, address vibration, or determine which ordinance applies. The governing noise ordinance, the applicable measurement standard, and an acoustical consultant govern.

Ldn is the 24-hour energy (3 dB) average of the hourly levels with 10 dB added to every hour from 10 p.m. to 7 a.m.; CNEL adds a further 4.77 dB to the 7 p.m. to 10 p.m. evening. Each period contributes its activity hours at the activity level and its remaining hours at the background level.

Levels are ENTERED, and that is the hard part: an ordinance is written at a property line or a receptor, and a level measured or predicted somewhere else is not that level. This does not measure anything, propagate sound from a source to a receptor, apply distance, ground, barrier or shielding attenuation, add the tonal, impulsive or low-frequency character adjustments many ordinances carry, address vibration, or determine which ordinance applies or how it defines its measurement position and averaging period.

An energy average and two published penalties; no ordinance text is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: activity_level_db, activity_hours_day, activity_hours_evening, activity_hours_night, background_level_db, limit_ldn_db, ldn_db, leq24_db, night_moved_ldn_db

• Levels are at the receptor a level measured elsewhere is not the ordinance's levelthe governing noise ordinance
• Energy (3 dB) averaging not the OSHA 5 dB exchange ratethe applicable measurement standard
• No character adjustments tonal, impulsive and low-frequency penalties are the ordinance'sthe governing noise ordinance