To help it survive the kind of power we expect in pro audio, the voice coil is cooled both by sinking heat into the magnet assembly it rides in, as well as by pumping cool air around the magnet structure through the motion of the cone.
Modern voice coils can handle a great deal of heat, but there are still very definite limits. (See Exhibit 3.)
What complicates the amount of heat a driver can take is something called “crest factor.” The crest factor of a waveform is the ratio of its peak amplitude to its RMS value, which can be expressed in dB.
A pure DC signal has 0 dB crest factor. A pure sine wave has 3 dB crest factor. Pink noise has a crest factor of 6 dB. Any two signals that have identical RMS values will create the same amount of heat, but the one with the higher crest factor will have higher peak amplitude, which can make a large difference in the way it sounds.
Most importantly, live music commonly has a crest factor around 20 dB, 100 times more peak power than RMS!
This crest factor is the reason that loudspeakers and components are often given three power ratings, usually described as continuous, program, and peak. In reality these are just different ways of saying the same thing, but with varying crest factors.
Continuous power is defined by the AES standard IEC 268-5 as the amount of power a loudspeaker can handle over a long time period (hours) when driven with pink noise (6 dB crest factor) that is limited to the driver’s operating bandwidth.
This is a pretty good “worst case” scenario, as any music that you are trying to reproduce that has only 6 dB of peak to average ratio is going to sound like noise anyway.
Exhibit 3, provided by Jim Bowersox (click to enlarge)
A program rating for the same loudspeaker is then usually listed, and it is 3 dB higher, or twice the wattage.
At least one major manufacturer doesn’t even list the continuous (AES) power rating for their loudspeakers, because the number that is twice as large looks twice as good to the consumer. They list program and then peak, which is 3 dB more than program and twice the wattage again.
What is important is to realize that all these numbers are simply made up, and the higher numbers simply represent the amount of amplifier power one should apply when reproducing signals with more dynamics, headroom, crest factor, or whatever you’d like to call it.
These numbers assume that the average power of the signal will be the same at each power rating, but as each signal has a progressively higher gap between its peaks and its RMS value, more peak amplifier power output can be applied to reproduce those peaks without overheating the driver.
A more relevant extrapolation of loudspeaker power capability might list even more power levels, but at the lower end of the scale. A driver capable of 100 watts when driven to its limits with the AES specification is potentially only able to handle 50 watts when driven with a sine wave, which only has 3 dB crest factor.
Apply a DC current (0 dB crest factor) and it will likely handle less than 25 watts. No manufacturer in their right mind is going to list a loudspeaker with a power handling of “25 watts DC” when the same driver can be said to handle “400 watts peak,” so we are all left to do a little math on our own.
Fortunately for everyone, the amount of DC or sine wave power a driver can take is pretty meaningless, since music is never made up of waveforms like that. The peak power rating is much more accurate as an indicator of what amplifier power rating to choose in order to get maximum performance from a driver.
Further, there are many more options available to cool a loudspeaker that is being driven with a highly dynamic signal, so gains can be made in terms of power handling there as well.
Let’s try and take a real world example. This short sample (Exhibit 4) from a live board recording of the band All Time Low performing their song “Jasey Ray” shows what a real world waveform looks like. The thick green lines represent the RMS value of the waveform, the voltage going towards driver heating, which is The Enemy.
Exhibit 4, provided by Phillip Graham (click to enlarge)
As you can see, the actual peaks of the signal go much higher than the RMS value—14 dB higher, to be precise. To show how significant this is, an amplifier capable of providing 1,000 watts of power will be at the limits of its rail voltage reproducing this signal, while only putting out 40 watts RMS.
That’s a pretty enormous difference, especially in today’s world of 100-plus watt rated compression drivers and 1,000-plus watt rated cone drivers.
These sorts of differences are exactly why in many pro systems it is not at all uncommon to have an amplifier capable of delivering 500-1000 watts or more to the high frequency section, while that section can only handle a hundred watts AES.
In the case of a compression driver being driven at 100 watts RMS with the waveform in Exhibit 4, the amplifier would have to be capable of providing 2,500 watts to reproduce the peaks! Providing less amplifier power simply clips off the tops of the waveform, creating distortion that is much less pleasing the to ear.
In the real world we never really know what kind of signal we’ll be feeding our loudspeakers. What allows truly pro systems to operate with so much “excess” amplifier power is very carefully set limiters. If the system processing is set so that the limiters keep the long-term power of the signal at or below the RMS capability of the loudspeaker, one can imagine how it would be possible to use an unlimited amount of amplifier power without danger.
Because the limiters are making sure that the heat-generating average power of the waveform doesn’t get to damaging levels, the “excess” amplifier power can safely be used to reproduce peaks and dynamics, ensuring that the snare can still pop without allowing the low frequencies from the kick drum to overwhelm the loudspeaker.
These dynamics can be reproduced up to the excursion limits of the driver, which then becomes the real limiting factor.
Peak Vs RMS
It is important to differentiate between commonly used peak limiters and a proper RMS limiter. A peak limiter may keep an amplifier from clipping, or it may be set to keep signal from exceeding a driver’s peak power rating.