Measure Sound/Noise Level in dB with Microphone and Arduino

Noise pollution has really started to gain importance due to high population density. A normal human ear could hear sound levels from 0dB to 140dB in which sound levels from 120dB to 140dB are considered to be noise. Loudness or sound levels are commonly measured in decibel(dB), we have some instruments which could measure the sound signals in dB but these meters are slightly expensive and sadly we do not have an out of box sensor module to measure sound levels in decibels. And it is not economical to purchase expensive microphones for a small Arduino project which should measure the sound level in a small classroom or living room.