dB(A) (dB(A))

dB(A) is the A-weighted decibel — a sound pressure level measurement that applies a frequency weighting to mimic the response of the human ear, which is most sensitive at 1-4 kHz and progressively less sensitive at very low and very high frequencies. It is the universal regulatory unit for noise in India under the Noise Pollution (Regulation and Control) Rules, 2000 (notified under the Environment Protection Act 1986) and the standard parameter in every SPCB CTO consent condition.

The A-weighting curve attenuates very low frequencies (-26 dB at 63 Hz, -16 dB at 125 Hz) and very high frequencies (-1 dB at 4 kHz, -7 dB at 16 kHz), so a sound rated 75 dB(A) is heard as equally loud regardless of its frequency mix. The decibel scale is logarithmic — every 3 dB(A) increase doubles sound power; every 10 dB(A) is perceived as roughly twice as loud subjectively. Measurement uses a Type 1 (laboratory grade) or Type 2 (field grade) sound level meter calibrated to IEC 61672.

The Indian zone limits are: industrial 75 dB(A) day / 70 dB(A) night, commercial 65/55, residential 55/45, and silence zone (100 m from hospitals, schools, courts, religious places) 50/40. "Day" is 6 a.m.-10 p.m., "night" 10 p.m.-6 a.m. Limits apply at the boundary of the source premises, not at the source itself — a noisy compressor inside an enclosed shed may run at 95 dB(A) internally but must be inaudible enough at the boundary to be within the zone limit.

For recycling plants, the noise budget is dominated by shredders (95-110 dB(A) at 1 m, dropping to 75-90 dB(A) at 30 m unenclosed), blowers and air classifiers (88-95 dB(A)), compressors (85-92 dB(A)), and diesel engines on gensets and CBG bottling (95-105 dB(A) at 1 m). Mitigation is layered: vibration isolation on shredder bases (rubber pads, springs) reduces structure-borne transmission; acoustic enclosures on blowers and compressors knock 15-25 dB(A); residential-grade barrier walls (4-6 m brick, mass loaded vinyl) at the boundary cut another 10-15 dB(A); silencers on engine exhaust take engine noise from 105 to 85 dB(A). Worker safety under the Factories Act 1948 caps occupational exposure at 90 dB(A) for 8 hours, with mandatory hearing protection above 85 dB(A) — a stricter limit than the zone limit, applicable at workstations.