How can noise levels be measured?

The unit used in acoustics to define and measure sound intensity is Decibel (dB). Decibel uses a logarithmic scale which works very differently from a linear scale.

Why is noise measured with a logarithmic scale? 

Human perception of loudness is not linear, so a logarithmic scale is well suited to express our experience of sound. In fact, when the human ear receives a sound stimulus it tries to amplify very weak sounds and to attenuate very strong sounds. So calculating the Decibel is based on the comparison between the input signal and the output signal.
A linear scale as used on a ruler, where a distance of 30 inches is three times as long as a distance of 10 inches, couldn't adequately express a decrease or increase in sound intensity.
When using a logarithmic scale to measure noise levels, adding 3 dB means multiplying the loudness by two. If we divide the intensity of a sound by 2, the sound level only drops by 3 dB.
EXAMPLE : Using 2 vacuum cleaners simultaneously in one room doesn't double the decibel level, but only increases it by 3 dB. To put it simple: 70 dB plus 70 dB doesn't equal 140 dB, but only 73 dB. Following this rule, 4 vacuum cleaners will create a sound level of 76 dB.
If we measure noise on a scale from 0 to 130 Decibels: 0 dB represents the threshold of audibility, 130 dB represents the threshold of pain. Most of the sounds of everyday life are between 30 and 90 dB. 
Decibel levels for everyday noises:
Sound                                              Decibel level 
Breathing 10 dB
Rustling leaves 20 dB
Whisper 30 dB
Quiet conversation 40 dB
Light trafic at close range 50 dB
Normal conversation 60 dB
Vacuum cleaner 70 dB
Loud traffic noise at close range 80 dB
Headphones at full volume 90 dB
Club 100 dB
Car speakers at full volume 110 dB
Air plane take-off, jackhammer 120 dB
Rock concert 130 dB


What is the increase in sound intensity between two different Decibel levels ?   

Sound intensity is the energy needed to produce a given level of sound and not to be confused with sound volume which is the level at which we perceive the resulting sound.

The mathematical relationship between Decibel (dB) and sound intensity works as follows: each 10 dB increase results in a 10-fold increase in sound intensity which we perceive as a 2-fold increase in sound volume. Thus, from 0 dB to 10 dB there is a 10-fold increase in sound intensity, just as there is from 10 dB to 20 dB or from 34 dB to 44 dB.

Decibel levels for everyday noises VS. increase in sound intensity and perceived volume:

Sound                                           Decibel level  Increase in sound intensity                  Increase in perceived volume 
  0 dB    
Breathing 10 dB 10 times the sound intensity 2 times as loud
Rustling leaves 20 dB 100 times the sound intensity 4 times as loud
Whisper 30 dB 1,000 times the sound intensity 8 times as loud
Quiet conversation 40 dB 10,000 times the sound intensity 16 times as loud
Light trafic at close range 50 dB 100,000 times the sound intensity 32 times as loud
Normal conversation 60 dB 1,000,000 times the sound intensity 64 times as loud
Vacuum cleaner 70 dB 10,000,000 times the sound intensity 128 times as loud
Loud traffic noise at close range 80 dB 100,000,000 times the sound intensity 256 times as loud
Headphones at full volume 90 dB 1,000,000,000 times the sound intensity 512 times as loud
Club 100 dB 10,000,000,000 times the sound intensity 1024 times as loud
Car speakers at full volume 110 dB 100,000,000,000 times the sound intensity 2048 times as loud
Air plane take-off, jackhammer 120 dB 1,000,000,000,000 times the sound intensity 4096 times as loud
Rock concert 130 dB 10,000,000,000,000 times the sound intensity 8192 times as loud



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