Easy dB Math in 5 Minutes

by | May 6, 2014 | Blog

I’ve seen grown men get quite nervous and trigger their anxiety attack reflexes whenever the Decibel Math slide comes up on the screen during training courses. This makes perfect sense, most of use haven’t needed to use “higher math” functions since college, those with English degrees maybe as far back as college.*

To lower your anxiety a bit, just think of the “dB” as a set of words. Instead of reading it as “decibel” – just replace the dB with the words, “Compared To”. Decibel functions are all comparative, comparing something, to something else.

Most of what we compare in Wi-Fi are decibel to one thousandths of a watt – or mw for short. You’ll see this written as dBm – or “Compared To a Milliwatt”.

Most start teaching dB math with a formula of the actual decibel math, something like this.

 

Screenshot 2014-05-05 20.34.51

This is what brings back all the anxious thoughts – seeing logarithms again. Perhaps the only thing worse would have been if we used Sine and Cosine formulas…

And yet – the actual process of doing decibel math isn’t difficult at all. I’m sure you can all pass the “Pre-Test” below.

  • What is 2 times 10?
  • What is 20 minus 3?
  • What is 2 plus 2 plus 2 plus 2?

If you could do the easy math questions above, you have the math skills necessary to learn decibel math!

First – the Rules

In doing decibel math – all you have to do is remember two rules. These are the fixed, and immovable rules for all dB math functions. And no, we aren’t going to be using any logarithmic function. Just simple math, adding, subtracting, multiplying and dividing – things you can do in your head.

But you have to follow the rules – that’s it, just follow the rules outlined below, and you too can be a whiz at decibel math!

Rule #1

In decibel math, the first rule is:

The left side is the decibel math side,

and the right side is the normal math side. 

That is all, nothing more to rule number one. Just remember decibel on the left, and normal on the right.

Rule #2

Just memorize this – no need to try to understand it – just memorize this. Don’t ask questions… don’t try to understand – just flat out memorize these!

Plus = Times

Minus = Divide

10 = 10

3 = 2

Some try to think here… resist the urge… this is a math trick, just follow the rules and everything will be fine.

That is all there is to doing decibel math. All you have to remember is these two rules above. Its easy! Lets give it a try together.

Application of Rules

Now, all we have to do in order to calculate decibel math functions is to refer to the two rules above.

To apply the first rule, I like to draw a vertical line on the paper. Specifically to give me a left side for decibel math, and a right side for normal math. Just remember. Left side is for Decibels and Right Side is for Normal.

As a reminder, I like to add the memorized details of Rule #2 at the top of my paper – something like this:

Line Down The Middle

Let’s start with a very simple example. The question at hand is what is what is the conversion from 6dB to normal math?

Since the 6dB is in decibels, we’ll have to put it on the left side of the line.

What is 6dB

Then we have to ask ourselves, “How many threes and tens are there?” – In this case, we have Two Threes in Six. So write it down on the left side of your line.

Now that we have the left side, the decibel side completed, we just need to apply Rule #2 on the right side to get our answer.

If on the left we have a “3dB + 3dB” – then on the right side we need to put the equivalents based on Rule #2.  The rule says threes are equal to twos. And the rule says pluses are equal to times. Thus we will write on the right side of the line, “2 times 2”. To the normal math on the right side equation and you’ll have your answer!

4mw

Simple as that!

Perhaps now we can try one a bit more difficult: What is 29dBm?

How many Tens? How many Threes? – the left side should have something like this: “10dB+10dB+3dB+3dB+3dB” – and the right side would then be converted using Rule #2 to: “10 X 10 X 2 X 2 X 2 = 800”

29dBm

That is all there is to it.

Well, you say, but what about when the number doesn’t contain Tens or Threes? My answer: the Rules stay the same. Just use your imagination a bit.

Here’s one to stretch your noggin some: What is 17dBm?

You might try adding up, one ten, plus two threes, plus… one one? The rules don’t pertain to anything except for Tens and Threes. So you can get the answer to this one by saying you have TWO tens, and 17 is 20 MINUS three… so the left side would look like this: “10dB+10dB-3dB” – and the resultant application of Rule #2 would make the right side “10 X 10 / 2”.

17dBm

With the appropriate use of adding and subtracting Tens and Threes, you can get to any number you wish.

Just be sure to follow Rule #1 – and keep decibel numbers always on the left, and then apply the conversions of Rule #2 to get the converted numbers on the right.

That is all there is to decibel math!

Might I suggest you spend just a couple of minutes and try your new -found skill on the following quiz? After just a bit of practice, you’ll have decibel math down cold! 

Quiz

*I am obviously just teasing those of you, like my #1 daughter who can legally join the P.O.E.M. group – Professional Organization of English Majors.