A decibel is a unit of measure of the intensity of acoustic pressure. 0 dB is barely perceivable by human beings, while 120 dB is considered the threshold of pain and hearing damage. Decibels are measured on a logarithmic scale, which means that an increase of 10 dB corresponds to an increase in intensity by a factor of 10. The acoustic pressure of 120 dB is thus one million times that of 0 dB.
Measures are often given in dB(A) or Leq when a time period is involved. The Leq is the average of noise exposure, often in dB(A), over a time period. It is often measured for the 6h00-22h00 period, which corresponds to the period of the day when most people are awake. Legal limits usually range between 50 dB and 65 dB, above which some abatement measures must be provided (e.g. a sound barrier) or where residential land uses are restricted. For instance, a jet airplane produces a sound intensity of 120 dB(A) during take-off, a heavily used urban road intersection 80 dB, and 70 dB is a common noise level near a highway.