Sound is movement of air particles. This movement is generally expressed in decibel (dB) as sound pressure level, L_p. The sound pressure level is dependent on the distance from the sound source, meaning that the longer away from the sound source you are the lower sound level.

Some basic features of sound pressure levels and dB:

• double the distance from the sound source gives approximately -5 dB (theoretically -6 dB)

• two times stronger sound gives +3 dB

• four times stronger sound gives +6 dB

• eight times stronger sound gives +9 dB

• 0 dB = human hearing threshold for excellent ears at frequency maximum response

• 10 dB = human hearing threshold for good ears at frequency maximum response

• 15 dB = human hearing threshold for normal ears

• 20 dB = leaves rustling

• 30 dB = good recording studio

• 40 dB = quiet bedroom

• 45 dB = noise treated room

• 50 dB = private office

• 60 dB = open office

• 65 dB = normal conversation

• 80 dB = air-cooled electrical motor at 1 meter (3 feet)

• 100 dB = chain saw at 1 meter (3 feet)

• 110 dB = heavy grinding at 1 meter (3 feet)

• 135 dB = threshold of pain (individually)

• 145 dB = close to jet aircraft at take-off

• 194 dB = maximum possible sound pressure level on earth

Note that if a level is expressed in dB(A), db(B), dB(C) or dB(D), a filter has been applied to the linear sound level giving another figure which is dependent on the frequency contents of the signal. There is thus no simple correlation between dB, dB(A) etc for different signals.