This is a unit of measurement that gives a comparison or the ratio of two powers such as the gain of an amplifier or the S/N ratio of a television cable.

Thus if power P1 is to be compared with power P2, then in terms of decibels we have

For example, if P1 is twice the value of P2, then

Conversely, a ratio of 1:2 is 10 log 0.5 = ?3dB.

Since it is more usual to measure the voltage rather than power, the decibel can be re-defined as

The above is true only if V1 and V2 are measured across the same impedance.

It follows that a 2:1 voltage ratio is 6dB and a 10:1 voltage ratio is 20dB and so on. Conversely, a 1:2 voltage ratio = ?6dB.

The following is a list of some dB values and their corresponding power and voltage ratios:

In many applications, dB is used to indicate the power or voltage level at a test point. To do this, a reference power or voltage level has to be assumed and this is normally 1mW, 1mV or...