Environment, Ventilation & Hazards

Noise Exposure & Permissible Time

Permissible daily exposure time from sound level, and the cumulative noise dose when levels vary through the shift.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Hearing damage depends on both level and duration. Standards set a permissible exposure time that halves with every 5 dBA rise above the 90 dBA / 8-hour reference (the 5 dB exchange rate). When a worker meets several levels, the noise dose sums the actual time at each level over the permissible time for that level; a dose above 100 % is over-exposure.

The Core Formula Matrix

Permissible time:

T = \frac{8}{2 ^{(L - 90) /5}} hours

(

L

in dBA).

Noise dose:

D = (\sum \frac{C _{i}}{T _{i}}) \times 100%

C_{i}

= actual time at level

i

T_{i}

= permissible time at that level.

The ‘IIT Traps’

⚠
5 dB exchange rate: every +5 dBA halves the allowed time (some standards use 3 dB).
⚠
Dose > 100 % means over-exposure, regardless of any single level being 'within limit'.
⚠
**Use permissible time $T_{i}$ in the denominator**, not 8 hours, for each level.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

At a steady

95 dBA

, the permissible daily exposure time (90 dBA/8 h reference, 5 dB rule) is:

Q2MEDIUM2 Marks · NAT

The permissible exposure time at

100 dBA

is ______ hours. (Round off to two decimal places.)

hours

Q3HARD2 Marks · NAT

A worker spends

4 h

95 dBA

(

T = 4

) and

2 h

100 dBA

(

T = 2

). The total noise dose is ______ %. (Round off to two decimal places.)