Feel free to skip this if you already understand them. Decibels, or dB, are used in a variety of scientific and engineering fields and at their most basic describe any phenomenon where there's a logarithmic relationship between signals. The two main fields that we laypeople come across them is in electricity (i.e. remember signal-to-noise ratios in stereo stuff?) and sound, but everything I talk about will be sound-related. The "deci" in decibels indicates the base for the logarithm is 10 - in simpler terms, every increase of 10 db means the signal has increased by a factor of 10. A result is an increase of 3 db means the signal has roughly doubled. A complication is that decibels are by convention generally used to describe relative power levels. Unfortunately the response of humans to sound is more accurately described by pressure level differences, not power level differences. Luckily, the power level ratio can be simply squared-rooted to obtain the pressure level ratio. Translating that back to decibels, it now takes 20 db to get a 10-fold increase in the pressure level, and a 6 db change in levels represents a doubling. When people talk about different db noise levels, they are almost always talking about pressure levels.

In practical terms, here are some everyday samples of how loud things are - these are all sound pressure levels (SPL's).

• Rogers NREL 2006 Paper