Air & Noise Pollution

Noise Pollution & Decibel Addition

Sound pressure level, logarithmic decibel addition, Leq and noise standards.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Sound level is measured on the logarithmic decibel (dB) scale. Because dB is logarithmic, levels do not add arithmetically: two equal sources combine to give +3 dB, ten equal sources +10 dB. The A-weighted scale dB(A) approximates human hearing, and the **equivalent continuous level

L_{e q}

** represents fluctuating noise as a steady value of equal energy.

The Core Formula Matrix

Sound pressure level:

L_{p} = 20 lo g_{10} \frac{p}{p _{0}}

(

p_{0} = 20 μ Pa

)

Decibel addition:

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

Doubling rule: two equal sources ⇒ +3 dB; ten equal ⇒ +10 dB.

Equivalent level:

L_{e q} = 10 lo g_{10} (\frac{1}{T} \int 1 0^{L (t) /10} d t)

The ‘IIT Traps’

⚠
60 dB + 60 dB = 63 dB, not 120 dB — add on an energy (logarithmic) basis.
⚠
dB(A) ≠ dB. A-weighting de-emphasises low frequencies to match hearing.
⚠
Distance doubling in free field drops a point source by 6 dB.

📚 Standard references

Environmental Pollution Control Engineering — C.S. Rao · Noise Pollution

PART 2

Progressive 3-Tier Question Suite

Q1MEDIUM2 Marks · NAT

Two machines each produce 70 dB at a point. Their combined sound level is _____ dB.

Q2BASIC1 Mark · MCQ

The A-weighted decibel scale dB(A) is used because it:

Q3HARD2 Marks · NAT

Ten identical sources each emit 60 dB at a receiver. The combined level is _____ dB.