A logarithm (log) of a number x is defined by the following equations. If x = 10y then log(x) = y.
A useful application of base ten logarithms is the concept of a decibel. A decibel is a logarithmic representation of a ratio of two quantities.

(dB) A customary logarithmic measure most commonly used (in various ways) for measuring sound. Decibel is a way to express the ratio of two sound intensities: dB=10log10I1/I2 being I1 the reference. If one sound is 1 bel (10 decibel) 'louder' than another, this means the louder sound is 10 times louder than the fainter one. A difference of 20 decibel corresponds to an increase of 10 x 10 or 100 times in intensity.
The intensity of ultrasound decreases during the propagation and is measured in db/cm.
For sound pressure (the pressure exerted by the sound waves) 0 decibel equals 20 microPascal (μPa), and for ultrasonic power 0 decibel sometimes equals 1 picoWatt.

See also dB/dt, Phon, and Logarithms.