Loudspeakers: Know your continuous from your program from your peak.
The power rating of a loudspeaker might be 500 watts; it might be 2000 watts. But how much power can it truly handle?
Here is an interesting question from an Audio Masterclass student:
"I saw something at the back of a Wharfedale PRO EVP-X215 loudspeaker that I don't understand what it means. This is it...
I just want to know what it means."
OK, so we have one loudspeaker with three different power ratings. How does that work?
Well the first thing you have to understand is that loudspeaker manufacturers guess the power ratings of their products. Yes they guess the ratings. Well, they make an informed guess based on testing and a knowledge of their market. But specifying how much power a loudspeaker can take is a lot more difficult than measuring how much power an amplifier can produce.
The first thing to understand is what we mean by power rating. In the most useful sense, it is the amount of power a loudspeaker can handle without blowing. Give it anything up to that amount of power, and it will stay healthy, ready for the next session or gig. Overstep that mark and you have a speaker that rattles or has a drive unit that doesn't work at all.
The ultimate test of any loudspeaker is in the hands of a real-life user. Does the speaker fulfill the user's expectations without blowing? Or does it blow? If the user considers that the speaker has blown prematurely, then they won't buy that model (or from that manufacturer) again.
So the manufacturer has to strike a balance between quoting a high figure that will impress a potential purchaser, and a realistic figure that won't result in too many blowages.
But why three ratings? What does that mean?
The first rating, 'continuous', is where the same amount of power is applied continuously and without respite. It is the most conservative rating and represents a 'worst case' scenario. This is assessed using a test bench signal; in the real world you would get closest to this by playing an Oasis CD that has been mastered to within an inch of its life.
A continuous signal pumps more and more energy into the loudspeaker, most of which turns to heat. Apply too much power for too long and the temperature will rise to a point where the voice coil of one of the drivers literally melts.
The second rating, 'program', represents a more realistic scenario where the input level rises and falls, thus giving the speaker time to rest and disperse some of its waste heat.
The third, 'peak', is the highest momentary level that the speaker can be expected to withstand. Applying levels this high not only implies a lot of heat generation, albeit for short time periods, but also very rapid and extreme movements of the diaphragm of the drive unit. What can happen here is that the turns of the voice coil deform and perhaps separate. So although the drive unit still works, it rattles as the coil scrapes against the magnet.
There are defined specifications by which a manufacturer can make all of these measurements in a controlled and meaningful way. In the end though, it is the responsibility of the person behind the faders to listen to the sound quality being produced, and have a knowledge of how much their speakers can take.
P.S. I was once tasked with assessing how much level I could get out of a speaker on a sound effect at a certain point in a theatre show, without it blowing. I was able to do the test in the auditorium (while the lighting and stage crews were on a break!). After around six or seven blown drive units, I felt I had a pretty good idea!
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