Читать книгу The Physics and Technology of Diagnostic Ultrasound: A Practitioner's Guide - Robert Gill - Страница 14

The decibel (dB) unit of measurement is based on logarithms. It is always used to express the ratio of two quantities. For example, if the ultrasound power transmitted into the patient is P1 and the power at a certain depth is P2 then the attenuation in decibels is calculated as follows:

In diagnostic ultrasound, decibels are used as the units for attenuation, gain and dynamic range.

Table 1.4 Examples of power ratios and their value in decibels.

P1/P2	dB
1	0 dB
2	3 dB
10	10 dB
100	20 dB
1000	30 dB
0.1	-10 dB
0.01	-20 dB

Some of the advantages that come from using decibels are:

 multiplication and division of numbers becomes addition and subtraction when the numbers are measured in decibels (since decibels are based on logarithms)

 decibels make very large and very small numbers easier to manage

 the calculation of attenuation becomes straightforward

 the concept of dynamic range would be much more complex if decibels were not used.