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The decibel (dB) represents a logrithmic ratio between two quantities and is unitless. If the ratio is refered to a specific quantity this is indicated by a suffix (dBm is referenced against 1 mW and dBV is 1 Volt).
Originally the dB was for power ratios, given by dB=10 log (P1/P2)
Power is proportional to voltage squared, hence the ratio of voltages or currents across a constant impedence is given by dB=20 log(V1/V2) or 20 log(I1/I2)
When working with power, 3 dB is twice, 10 dB is 10 times; When working with voltage or current, 6 dB is twice, 20 dB is 10 times