The dB is the unit which is normally used to describe the relative amplitude of echoes in ultrasound systems. In practice, the absolute amplitude of an echo signal (expressed in volts) is rarely of interest. It is more useful to know how echoes compare with one another. The ratio of the two amplitudes can be expressed in decibels (dB). As the dB is used only for ratios, there are no other units involved (e.g. mW, MPa). The dB is a logarithmic scale, so, in very simple terms, if the ratio is 1000:1 or 1 000 000:1, rather than writing out lots of zeros, the dB scale effectively counts the zeros, rather like expressing these numbers as 103 or 106. The Bel is simply log R, where R is the ratio. Hence a ratio of 1000:1 is 3 Bel, and a ratio of 1 000 000:1 is 6 Bel. In practice, the unit of 1 Bel (ratio of 10) is often too large and it is more useful to use the decibel, which is one-tenth of a Bel. Hence, the ratio R in dB is given by R (dB) = 10log10R dB. The dB, as defined above, is used to express only ratios of power or intensity. For example, as an ultrasound pulse propagates through tissue and is attenuated, the ratio of the intensities within the pulse at two different depths can be expressed in dB.