Question

What will be the resultant decibel level when two sources make noise of equal decibels?
1. Decibel level will be the same
2. Decibel level will increase by 3 decibels
3. Decibel level will decrease by 3 decibels
4. Decibel level will be equal to the sum of decibels of the two sources

Answer 1

Correct Answer - Option 2 : Decibel level will increase by 3 decibels

Explanation

Sound pressure level (Lp) uses a logarithmic scale to represent the sound pressure of a sound in relative to reference pressure. It is measured in units of decibels (dB).

Level of Noise

\(L = 20 \times{\log _{10}}\left( {\frac{{P_{rms}}}{{Prm{s_0}}}} \right)dB\)

where

Prms = Pressure level of sound to be measured

Prmso = Standard reference sound pressure

1. The sound pressure of the faintest sound that a normal healthy individual can hear is 20 μPa.
2. It is taken as a reference sound pressure level.
3. A 20 μPa pressure is 0 dB on the sound pressure level scale.

 

Thumb rule to find the resultant sound level due to the sound produced from two sources are as follows:

If L1 > L2

L1 – L2	Resultant ‘L’	Example
0 & 1	L1 + 3	L1 = 50, L2 = 50 → L = 53
2 & 3	L1 + 2	L1 = 50, L2 = 48 → L = 52
4 to 8	L1 + 1	L1 = 50, L2 = 45 → L = 51
≥ 9	L1	L1 = 50, L2 = 40 → L = 50

 

It is given two equal sound level, ∴ Difference is Zero (L1 – L2 = 0 ), so it increases by 3 dB