What is a decibel?

The decibel is the unit of measurement, one tenth of a Bel (B), used to measure sound intensity. The decibel is logarithmic in scale and thus represents a ratio.

Sound is quite complex and consists of vibrations traveling through a medium, such as air. These vibrations can be sensed by the human ear as sound pressure. 0 dB is used to represent the threshold of human hearing capabilities and is barely audible under extremely quiet conditions. 120 dB is the threshold for discomfort and 140 dB is the threshold for pain. The most common range, however, is anywhere between 30 dB and 100 dB.

Decibels being logarithmic can not simply be added nor subtracted like other linear units. For example, if you took two sound sources that were measured to be 100 dB and operate them simultaneously, the resulting sound level produced would be 103 dB not 200 dB. If you took four of those same 100 dB sound sources and added them together the resulting sound level would simply be 106 dB, and so on. Due to the logarithmic nature of decibels taking one much louder sound source and adding a substantially quieter one on top of it would not move the measurement in most cases, for example an 80 dB sound source coupled with a 100 dB sound source would still measure 100 dB due to how much louder the 100 dB sound source is.

Some common rules of thumb to consider are that most people perceive 6 to 10 dB in sound pressure level to be a doubling of loudness and can not perceive a change in loudness for anything below a change of 3 dB SPL. Another important thing to consider is how far is the listener from the noise source? A gunshot, no matter how loud, 100 miles away will be unnoticeable regardless of the fact that the gunshot itself is quite loud. This brings us to the difference between sound power level and sound pressure level. Sound power level is the actual acoustical energy emitted by a noise source wheras sound pressure level is how loud that noise source is at a given distance.

Under normal atmospheric conditions the maximum sustained sound level achievable on earth is 194 decibels. The reason for this is that the oscillation of the sound wave can not drop below a vacuum created by the sound pressure in air. However, peak sound levels can achieve levels substantially higher than 194 decibels. One incredible example of this would be the volcanic eruption in Tambora, Indonesia in 1815 which ejected 36 cubic miles, equivalent to a 14 gigaton nuclear bomb based on ejected volume. The internal pressure is believed to have been 47 million pounds per square inch resulting in a measured peak decibel level of 347 dB. Click here for a chart of typical noise values.