What's a Decibel (dB)?
dBs measure sound. Every day the sounds we hear usually fall between 0dB (the faintest detectable noise) and 120dB (a thunderclap). Although 0 to 120 may not seem like a big difference, it actually represents an increase of sound energy 1 trillion fold! This is because dBs are special units used to represent significant differences in energy with smaller, more manageable numbers.
All you need to remember is one simple rule: an increase of 10dB means 10 times more energy. So an increase of 20dB means 10*10=100 times more energy. Going from 10dB breathing to a 40dB library means 10*10*10=1000 times more sound energy!
Energy isn't the whole story, though. As humans, we perceive sound energy from higher-pitched noises like sirens and chainsaws as louder than lower-pitched noises like trucks and lawn mowers. To take that into account, we adjust dB to dBA, giving more weight to higher sounds and less weight to lower sounds. A 50dB suburban neighborhood might actually be 48dBA if there are a lot of low frequency sounds present. Because they correlate with our natural perception, dBA numbers are more meaningful quantities than dB.
dBA can be taken one step further to get something called LAeq, or the sound energy over 24 hours. Consider the sound in an urban park. A 130dBA airplane could fly overhead at 11AM and a 100dBA parade pass through from 2-3PM, but most of the time the sound could be ambient city noise at 50dBA. An LAeq value of 60dBA would reflect the more prevalent ambient sounds and occasional periods of louder noise. LAeq values are useful for comparing noise in different locations, taking into account the full variety of sound levels that occur in each location over a full day.