In acoustics, the decibel is used as an absolute indicator of sound power per unit area. A decibel is one-tenth of a Bel, a seldom-used unit named for Alexander Graham Bell, inventor of the telephone.
Increasing the power of sound emitted does not increase the level of noise by the same amount, by doubling the output, the level of sound received would only increase by 3db. (e.g. If 2 people both talk at 60 decibels at the same time, the power they both emit would only create 63 decibels of sound, 4 people all talking at 60 decibels would only create 66.)
The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz although the high frequency limit varies. Human hearing can be sensitive with decibel levels of over 120db proven to damage human hearing for the long term.
Sound vibrates waves of pressure through the air, the more frequent the wave the higher pitch of the sound.