Leq (equivalent continuous sound level)

Leq stands for equivalent continuous sound level. Real-world noise constantly varies, so a single instantaneous reading is meaningless for regulation. Leq solves this by computing the constant sound level that, over the measurement period, would deliver the same total acoustic energy as the actual fluctuating sound. It is, in effect, an energy-weighted average expressed in dB(A), and it is the metric written into India's ambient noise standards.

Because the decibel scale is logarithmic, Leq is dominated by the loudest portions of the period: brief high peaks contribute disproportionately to the energy total. A plant that is quiet most of the time but has a loud shredder cycle or DG-set surge can still register a high Leq. The measurement period matters too — noise standards are typically assessed as a daytime Leq and a separate, stricter night-time Leq, so a unit must comply with both averages, not just an overall daily figure.

For an industrial operator the practical consequence is that compliance is about the energy-averaged level at the boundary, not the peak. This is helpful — a short, sharp noise does not automatically fail you — but it also means continuous sources like blowers, granulators and DG sets, which run for the whole averaging window, are the real drivers of the Leq. Reducing a continuous source by even a few dB(A) lowers the Leq more effectively than chasing occasional peaks.

For an Indian entrepreneur the guidance is to have boundary noise measured as Leq over representative day and night periods by an SPCB-recognised agency, with the plant in normal operation including the DG set. Focus noise-control spending on the continuous, full-period sources that dominate the Leq, and keep the dated Leq survey on file as evidence of compliance with the zone limit during consent inspections.