Audio Basics: What's a dBWatt?

Recently there seems to have been an increasing number of manufacturers trying to produce ever more powerful amplifiers, with many producing hundreds and in some cases thousands of watts output.

Leaving aside the potential compromises that producing such a design has to sound quality, or even the relevance of these massive power figures when we are seeing average speaker efficiency rising, these figures still need to be put into some kind of context.
Power & Sensitivity

On its own an amplifiers power output will tell us nothing. Please excuse the motoring analogy, but its just like knowing a cars horsepower without knowing its weight, without this vital information you would have no idea how rapidly it will actually go. The audio equivalent of the vehicles weight in this example is the loudspeakers sensitivity (which tells you how loud a speaker will go with a single watt of input, measured in decibels or dB for short), and its this that we need to bring into the equation when trying to make a relevant understanding of amplifier power.

What is then needed is some easy way to see what a certain amount of power would do with a known pair of speakers, so this is where dBWatts (dBW) come in. The chart below shows the equivalent figures for the two measurements.
Watts (RMS) & dBW:
1 Watt = 0 dBW
2.5 Watts = 4 dBW
5 Watts = 7 dbW
10 Watts = 10 dBW
25 Watts = 14 dBW
50 Watts = 17 dBW
75 Watts = 18.8 dBW
100 Watts = 20 dBW
150 Watts = 21.8 dBW
200 Watts = 23 dBW
400 Watts = 26 dBW

The beauty of the dBW is that at a glance it will tell you how loud a particular amplifier will go with a known loudspeaker - all you have to do is add the dBW output of your amplifier by looking at the chart above and add this figure to the speakers sensitivity. The resulting figure will tell you maximum volume level attainable from one speaker at a distance of one metre*

So for example a 100dB speaker with a 5 watt amplifier will give 107 dB output, which is exactly the same output achievable with a 50 watt amp driving a 90dB speaker. Or even more extreme, an 84dB speaker would require 200 Watts to attain the same level (assuming it could handle that sort of power level).

Not only do these figures give real relevance to power figures but they also show that the myth that an amplifier with twice as much power will go twice as loud is completely untrue.

You may be wondering why dBW’s are not common place on amplifier specification sheets, and the only reason I can think is that they simply do not make impressive enough reading!

*for the purposes of this basic article we are looking at the sound output of one speaker measured at 1 metre. Calculating the actual volume level achievable in room from a pair of speakers is a little more complex and is beyond the scope of this basic article.