Two quantities (x, y) are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio (k = y/x). This constant ratio (k) is called the proportionality constant or constant of proportionality of the proportionality relation.

(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 is measured in db/cm and decreases during the propagation.
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 Phon, Pascal, Logarithms, Proportionality Constant, Watt, Diagnostic Imaging.