mgb_phys said:
dB is decibel is a measure of the relative power or amplitude between two signals.
It is an exponential unit
i think it's more common to call it a "logarithmic unit".
because signal strength covers such a wide range of values,
there are more intrinsic reasons. it's because, if viewed linearly, no particular value of signal strength (or relative
change in signal strength) deserves to be defined as the unit for which all other signal strengths to be measured against.
for example, regarding the price of stocks: http://www.fool.com/foolfaq/foolfaqcharts.htm
\% ( dummy - the first % symbol is not rendered correctly)
if we
redefined the meaning of "percent change" from the existing:
\% \mathrm{ change} = \frac{ V_{after} - V_{before} } { V_{before} } \ \times \ 100 \%
to
\% \mathrm{change} = \log _e \left(\frac{V_{after}}{V_{before}} \right) \ \times \ 100 \% = \left( \log _e (V_{after}) - \log _e(V_{before}) \right) \ \times \ 100 \%
then we can confidently say that if the stock rose in price exactly 5%, then later fell precisely 6%, then later rose another exact 1%, with the latter definition we could say that the final value of the stock is exactly what we started with. not so with the conventional definition of % change.
dB is similar to that but a dB would be more like 11.5%, the difference being just one of convention. we
can say that a signal that increases exactly 5 dB, then later fell precisely 6 dB, then later rose another exact 1 dB, has the exact same amplitude at the end as it started with.
basically all you need to know is that every 3dB change is a doubling of power and every 6dB is a doubling of amplitude.
it would be good to know more than that.
