Noise Pollution

Sound and noise, frequency and audible range; sound intensity, pressure and power levels in decibels; addition of sound levels and attenuation with distance; frequency weighting (dB(A)); noise indices — Leq, L10, L50, L90, Ldn, noise pollution level, traffic noise index; sources of noise; effects on health (auditory and non-auditory); ambient noise standards and rules in India; occupational exposure; measurement; control at source, path and receiver — with solved numericals.

Noise Pollution

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (10 sections)

Last reviewed 16 Sept 2026 · 7 min read

Sound and noise

Sound is a pressure wave transmitted through a medium. Noise is unwanted sound — sound that is loud, unpleasant or disturbing.

Frequency (Hz): audible range for healthy young humans is about 20 Hz to 20 000 Hz; below 20 Hz — infrasound; above 20 000 Hz — ultrasound. The ear is most sensitive around 2000–5000 Hz.
Wavelength $λ = c / f$ ( $c$ ≈ 343 m/s in air at 20 °C).
Pitch relates to frequency; loudness relates to intensity (and frequency).

Decibel scale

The ear responds to a very wide range of pressures, so a logarithmic scale is used.

FormulaSound levels

Sound intensity level:

L_{I} = 10 lo g_{10} \frac{I}{I _{0}}, I_{0} = 1 0^{- 12} W/m^{2}

Sound pressure level:

L_{p} = 20 lo g_{10} \frac{p}{p _{0}}, p_{0} = 20 μ Pa = 2 \times 1 0^{- 5} Pa

Sound power level:

L_{W} = 10 lo g_{10} \frac{W}{W _{0}}, W_{0} = 1 0^{- 12} W

0 dB — threshold of hearing; about 120–140 dB — threshold of pain.
Doubling intensity adds about 3 dB; a 10 dB increase is perceived roughly as twice as loud.

Adding sound levels

L_{t o t a l} = 10 lo g_{10} (\sum 1 0^{L_{i} /10})

Two equal sources: $L + 3$ dB.
$n$ equal sources: $L + 10 lo g_{10} n$ .
If two levels differ by more than about 10 dB, the lower one adds almost nothing.

Attenuation with distance (free field)

Point source (spherical spreading): $L_{2} = L_{1} - 20 lo g_{10} \frac{r _{2}}{r _{1}}$ → 6 dB decrease per doubling of distance.
Line source (busy road, cylindrical spreading): $L_{2} = L_{1} - 10 lo g_{10} \frac{r _{2}}{r _{1}}$ → 3 dB per doubling.
Additional attenuation by air absorption (high frequencies), ground, barriers and vegetation.

Frequency weighting

Sound level meters use weighting networks to approximate human hearing: A-weighting — dB(A) (most common for environmental and occupational noise); C-weighting for high-level and low-frequency noise.

Downloaded from civilgyantests.com · Not for resale. Verify every code clause and date against the official source.

This chapter is in the syllabus of

Open an exam to see where this chapter sits in its syllabus, and to practise it.

SSC JE Civil UPSC ESE Civil RRB JE Civil CBT-2 NTPC NGEL Engineer Civil IOCL Graduate Engineer Civil BOI SO Civil Engineer MbPA AEE & EE Civil DSSSB AE Civil DSSSB JE Civil TGPSC Manager (Civil)TNPSC AE Civil RSMSSB JE Civil (Degree)APPSC Assistant Environmental Engineer

✅ Free — no sign-up needed

How ready are you for Environmental Engineering?

Ten questions from the real syllabus, about five minutes. You will see your score and which subject is holding you back — before you create any account.

10 questions · no timer · no payment