The term decibel is often used but not often fully understood. If a level is stated in decibels, then it is comparing a current signal level to a previous level or preset standard level. A value given as decibels always has a reference, otherwise it is meaningless. If you are talking about sound pressure levels (SPL), the reference is considered to be the lowest level that can be heard by human beings. A one decibel change is considered to be the smallest change that can be percieved by the human ear. If a value is given as negative decibels (-dB), it means that the value is below the reference. You will notice that the "B" is capitalized in dB. This is because it refers to the last name of Alexander Graham Bell.

There are some common reference values in electronics for dealing with decibels. A few are listed below. Pay special attention to the upper and lower case designations of each term.

The following calculator will convert a known voltage level into dBf, dBu, dBm and dBV. You can change the load for dBm if you like but 600 ohms is fairly standard. DBf is used in tuner specs. It is 1 femtowatt (0.000000000000001 watts or 0.00000027386 volts) into 75 ohms (generally).

The two formulas below are helpful when comparing voltage or power levels.

10*(log(power level # 2/power level # 1))

20*(log(voltage level # 2/voltage level # 1))

Example:
First power measurement: 100 watts
Second power measurement: 200 watts
Difference between measurements, in decibels=10*(log₁₀(200/100))
Difference between measurements, in decibels=3dB

In this case it was a positive number because it was an increase in power. If you reverse the first and second values, the answer would have come out as -3dB. Just realize that, if you have an increase, the result will be a positive number. If you have a decrease, the result will be a negative number.

As you may have heard, doubling the power output level of a source (such as an amplifier) is a 3dB increase in its output.

This calculator may help you to better understand how the difference between two values translates into decibels. You'll notice how doubling the voltage results in a 6dB increase and doubling power will result in a 3dB increase.