When talking about dishwashers, decibels refers to the noise level a dishwasher produces when it is in operation. The quietest dishwashers have decibel ratings around 40 and slightly below, while the noise level in decibels of a normal conversation is about 60.
Since decibels measure sound intensity, a small increase in decibels can mean a large increase in sound. For instance, a one-decibel increase is equivalent to a 30 percent sound increase and a three-decibel increase doubles the sound level. Another way to measure noise levels of dishwashers that manufacturers sometimes use is a sone rating, with quieter dishwashers having lower ratings. The quietest dishwashers have a sone rating of four.