night time (night time)

Under the Noise Pollution (Regulation and Control) Rules, 2000, night time is the defined period from 10 p.m. to 6 a.m., paired with day time from 6 a.m. to 10 p.m. The Rules set a separate, stricter ambient noise limit for night across every zone category, because background noise falls at night and disturbance — particularly to sleep — is far more acute.

The night limits are markedly lower than the day limits: industrial 70 dB(A) (vs 75 day), commercial 55 dB(A) (vs 65), residential 45 dB(A) (vs 55), and silence zone 40 dB(A) (vs 50). The same 5-15 dB(A) tightening runs through the whole schedule. Several other restrictions are also keyed to night time — loudspeaker use is generally prohibited between 10 p.m. and 6 a.m., and late-night fire-cracker bursting is barred.

For a plant that runs around the clock, the night limit is the binding constraint. A continuous operation must meet the night figure at its boundary, and because that figure is the lowest of the day, the plant's noise control must be designed to the night limit, not the day limit. A unit near a residential boundary faces 45 dB(A) at night, which is demanding for any operation with DG sets and machinery running.

For an Indian entrepreneur the guidance is to design and verify boundary noise against the applicable night limit for the relevant zone, since meeting the day limit is no guarantee of night compliance. If continuous night operation is planned, confirm by measurement (night-time Leq) that the plant stays within the night figure with the DG set and all machinery running, and consider restricting the noisiest activities to daytime where night compliance cannot otherwise be achieved.