Power and Decibel

The decibel is one unit that you will hear used (and misused) quite often in electronics. I have two examples here recently that I have seen.

The first one was in a music shop where they were selling speakers. The sign on of these said 600 Watts RMS and 1300 Watts Peak but the box said it was a powered speaker. In the electronics of the amplifier, if the RMS rating is 600 Watts then the Peak Watts is only 1.414 times the RMS value or 848 Watts Peak. It is NOT 2x RMS.

Now one of the other cabinets was a passive speaker (no power) and said 500 Watts RMS and 2000 Watt Peak.

First, there is no RMS in a passive speaker because it is tested for continuous power. 500 Watts with a 3 dB Crest Factor will allow 1000 Watts peaks during a test with pink noise. Now 6 dB above the 500 Watts continuous shall be 4 times the continuous power or 2000 Watts.

Another example is with the continuous decibel specification. The specifications defines this single line array cabinet at 127 dB continuous. In addition, it defines 4 of these cabinets as 139 dB continuous and this is incorrect.

The reason this is incorrect is because 4 times the increase of power will provide you with 6 dB increase for a value of 133 dB not 139 dB.

To summarize, in electronics every time an increase the power by a factor of 2 times, the result will be a 3 dB increase of power. Every time an increase the power by a factor of 4 times, the result will be a 6 dB increase of power. To add to that, an increase the power by 10 times, will provide an increase of 10 dB.

On the passive note as with a raw speaker, a speaker that has been tested and defined as having a power rating of 500 Watts continuous, an increase of 3 dB in the power rating will define the program power rating of 1000 Watts Program. An increase of 6 dB in the continuous power rating will define the peak power rating of 2000 Watts Peak.

And last, an amplifier with a 500 Watt RMS rating will have a peak rating of 1.414 times RMS or 707 Watt peak. Use caution when you are evaluating the specification of an amplifier. To impress you, vendors and manufacturers will provide the peak power not RMS and will also give the rating in 4 ohms and not 8 ohms.

3000 Watts Peak at 4 ohms is only 1500 Watts Peak at 8 ohms. This 1500 Watts Peak at 8 ohms is only 1060 Watts RMS at 8 ohms. The larger numbers looks impressive but is deceiving.

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