Amplifiers: To Protect Speakers, Account For Peak Power, RMS

Whenever possible, look to follow the manufacturer’s recommendation to yield the best results and minimize the likelihood of damage to your loudspeakers.

Amplifiers: To Protect Speakers, Account For Peak Power, RMS
Ideally, you would purchase amplifiers that the loudspeaker company endorses and use the presets already configured within the amplifier.
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The program content we feed to loudspeakers has both a peak (instantaneous) power level and an RMS (root means square, or average) power level. This is because normal program content (e.g., music and speech) have dynamic range – there is a difference between the average energy in the signal (which is approximately perceived as loudness) and the temporary peaks (transients) that naturally occur, but which don’t have much effect on our perception of loudness. These two different power levels happen to correlate to the two main ways loudspeakers sustain damage.

Loudspeakers have two failure modes. Thermal failure happens when the RMS (average) energy is too high, for too long, and the internal components quite literally melt. Mechanical failure happens when the peak (instantaneous) energy is too high, and that causes the components to attempt to move too far and end up breaking as a result.

The conventional wisdom about matching amplifiers to loudspeakers says that you should have an amplifier capable of delivering twice as much power as the loudspeaker can handle. If you are trying to get the most SPL out of your system, you need to push it to its limits.

If you have an amplifier that isn’t quite as powerful as it should be for that condition, you will clip (distort) the amplifier outputs whenever you attempt to drive the system to its maximum SPL. Whenever you clip a signal, you automatically get a byproduct in the form of unwanted harmonic content. This extra content raises the amount of RMS energy feeding the high-frequency drivers of your PA, and can cause thermal failure. This is where the notion comes from that undersizing your amplifiers is dangerous to your PA.

The reality is that you can damage a PA with almost any amplifier, if you’re careless enough. The solution is to use limiters to protect the loudspeakers from both thermal and mechanical failure. In order to prevent thermal failure, you need an RMS limiter that will stop the average power from exceeding the “continuous” power rating (the thermal limit) of the loudspeaker. In order to prevent mechanical failure, you also need a peak limiter that will stop the instantaneous peaks from exceeding the “peak” power rating of the loudspeaker.

Knowing this, then, the ideal amplifier choice is often one that has a little more power than you need (so there is always a little unused “headroom” to ensure that you won’t clip), and which provides both RMS and peak limiter options as part of its built-in DSP.


More About Brad Duryea
Brad Duryea is an audio engineer based in Houston, Texas, where he is the director of audio technology for Lakewood Church. He can be reached at brad@bradduryea.com or via Twitter: @bradduryea.
Get in Touch: brad.duryea@gmail.com    More by Brad Duryea

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Technology · Audio · Team Management · Budgeting · Volunteers · d&b audiotechnik · JBL · Loudspeakers · Manufacturer's Recommendation · Peak Power · Power Consumption · All Topics

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