The first case is a non-issue: Clipping does not typically damage amplifiers. The second case is a red herring; as discussed throughout this article, good drivers can and do take short bursts of signal that are well in excess of their continuous power rating, enabling them to maintain the integrity of signal dynamics.
All a peak limiter then does is limit the very fidelity we are trying to recreate in the first place. Much worse, however, is that limiters can create a false sense of security… An engineer, seeing that a system is indicating that it has passed the limiter threshold, will assume they can continue to drive it into the improperly applied peak limiters and that they will protect it.
Instead, as the signal gets more and more limited, its RMS value increases, but peak values remain the same. In other words, the limiters are not preventing the destructive heating power of the signal from increasing, even though it may appear that they are.
As the engineer continues to apply “gas” the signal will become less and less dynamic, and each increase in signal level will have a smaller corresponding increase in perceived loudness as peaks are squashed further.
Eventually the engineer, fighting for output that is simply not available, will have increased the RMS level of the signal beyond the level that the loudspeaker system can handle and there will be a catastrophic failure due to over-heating.
The above case shows why peak limiters have no place in driver protection, except to limit against driver over-excursion. Given only one limiter in the signal path, it should be set solely to keep drivers from overheating. If the amplifier is driven to clip, but the loudspeaker has not reached either XMax or its thermal limits, then the clipping is of little concern.
Clipping in this case only serves to indicate that there is not enough additional amplifier output to take the system to the limits of its performance capability.
If no limiters are available, then the operator must be very careful to ensure that they do not push the system beyond its abilities. In this case amplifiers sized to the “program” or “peak” rating of the driver or enclosure they power may be appropriate, so that the engineer can use their clip or limit indicator lights to determine that the loudspeaker system is nearing its breaking point.
With highly compressed music (like much dance music), these may not be a very good indicator at all, as the already highly compressed peaks of the signal may not set off the clip/limit lights on an amplifier until the signal’s RMS value is already unsafe.
Attempting to size an amplifier to a loudspeaker in such a way that the amplifier does not have enough power to destroy drivers is an exercise in futility. Most power amplifiers can be driven hard enough that they temporarily output twice their rated power, although what they are putting out will sound horrible due to extreme limiting or clipping.
To adequately protect against this kind of mistreatment one would have to use a power amplifier that is sized at less than half the driver’s AES power rating. The loss in system performance that would result from this sort of compromise is not permissible in any system I’ve ever worked with.
We’re talking about using a 50-watt amplifier on a 100-watt rated driver that might be more appropriately powered by a 400-watt amp. It is much more practical to either trust that your system will be used by competent technicians, or utilize a properly configured limiter.
Exhibit 5, provided by Trace Knight (click to enlarge)
At the bleeding edge of audio it is possible to extract performance from loudspeakers well in excess of what would normally be indicated by simple power ratings. With the aid of robust drivers and large amplifiers, good limiters keep a system safe while allowing enough headroom to cleanly and realistically reproduce the dynamics of nearly any style of music.
By better understanding the realities of driver failure and loudspeaker power ratings you can more realistically select amplifiers to extract the maximum performance from any professional audio system.
One More Thing: The Importance Of Watts
It is important to understand the relationship between amplifier wattage, as the most commonly reported performance specification, and sound output.
Additional sound power of 3 dB from the same loudspeaker requires double the number of amplifier watts. It is commonly accepted that 3 dB is the smallest practical sound pressure difference detectable by the human ear.
If you want your sound system to be noticeable louder, you will need at least twice as much amplifier power or loudspeakers at least 3 dB more efficient. That means if you have a 1,500-watt amplifier and are considering replacing it with a 2,000-watt amplifier you should save your money, as that is only a 1.2 dB difference.
At least 3,000 watts is necessary for a significant improvement in performance… if your current loudspeakers cannot handle that kind of power, you need loudspeakers with either higher power handling or higher efficiency.
Bennett Prescott represents ADRaudio powered loudspeakers in North America.
The author offers his thanks to the many members of the Live Audio Board—including Geoff Dane, Kent Clasen, Trace Knight, and Jim Bowersox—whose excellent photographs of gruesome driver death made it into this article, and many others who submitted equally good shots that he didn’t have space for. Extra special thanks to Phillip Graham who provided much needed technical advice and editorial prowess, as well as the image in Exhibit 4. Evan Kirkendall was kind enough to give me permission to use the segment taken from “Jasey Ray” which he recorded live in his current gig as Band Engineer for All Time Low.