The average human ear begins sensing at a pressure of only 20 micropascals and begins to experience pain at approximately 100 pascals. Because the human ear perceives pressure changes logarithmically over a large range, sound pressure level (SPL) is measured in decibels (dB). The decibel is logarithmic in relation to sound pressure (sound pressure is measured in pascals) giving us a more manageable scale.
Because of this logarithmic relationship, one cannot simply arithmetically add, subtract, or average decibel levels. For example if you have two 60 dB noise sources occurring simultaneously they combine to equal 63 dB. While a 3 dB change is generally just perceptible, a ten decibel change, either up or down, is approximately twice or half as loud.