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)
| dB | Power Ratio (10 log) |
Voltage/Current Ratio (20 log) |
dB | Power Ratio (10 log) |
Voltage Current Ratio (20 log) |
|
|---|---|---|---|---|---|---|
| 0 | 1.0 | 1.0 | 0 | 1.0 | 1.0 | |
| 3 | 2.0 | 1.4 | -3 | 0.50 | 0.71 | |
| 6 | 4.0 | 2.0 | -6 | 0.25 | 0.50 | |
| 10 | 10.0 | 3.2 | -10 | 0.10 | 0.32 | |
| 12 | 16.0 | 4.0 | -12 | 0.05 | 0.25 | |
| 14 | 25.0 | 5.0 | -14 | 0.04 | 0.20 | |
| 20 | 102 | 10 | -20 | 10-2 | 0.10 | |
| 30 | 103 | 32 | -30 | 10-3 | 0.03 | |
| 40 | 104 | 102 | -40 | 10-4 | 10-2 | |
| 60 | 106 | 103 | -60 | 10-6 | 10-3 | |
| 80 | 108 | 104 | -80 | 10-8 | 10-4 | |
| 100 | 1010 | 105 | -100 | 10-10 | 10-5 | |
| 120 | 1012 | 106 | -120 | 10-12 | 10-6 | |
| 140 | 1014 | 107 | -140 | 10-14 | 10-7 |
Simple rule of thumb:
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