Back to Basics: Decibels or “dBs”

Everything with a signal is measured in dB’s, also known as decibels. You might have thought that decibels were a unit of sound but they’re not really, they’re a unit of change. We talked about dB’s in detail a few months ago in a strangely unpopular article, so let’s try it again but make it a little simpler.

Let’s leave out the fancy math and boil it down to two things.

dB’s are a little weird to measure.
The whole point of a dB is it lets you measure very big changes without going to very big numbers. Each 10dB difference increases the signal power by 10x, so if you’re gaining (or losing 40dB) that’s a 1000x difference in signal. That’s a lot, but much of the time that’s not super important. What is important is…

dB’s make it easy to measure the impact of amplifiers and cable runs.
The coolest part of dBs is that you can add and subtract them. If you were talking about volts or watts, it’s a lot harder to measure the differences. With dB’s it’s stone cold simple. If you have a signal that’s -30dB, and you add a 25dB amplifier, the signal becomes -5db. (Add 25 to -30 and you get -5.) If your cable is rated to lose 7dB for every 100 feet, then if the signal level was -30dB and you run 100 feet, it’s now -37dB. (Subtract 7 from -30 and you get -37.)

You can even get fancy: start with a -105dB cell signal, run it through 50′ of cable for 5dB loss, add a 65dB booster, and the output level at the booster is -45dB. (-105, subtract 5, add 65 = -45.)

See, dB’s aren’t that hard to understand, after all.

Most of the time dBs are negative numbers because they measure really small power levels, that’s pretty much what you need to know there.