Bass Bench: Parsing Power Ratings

The process of choosing a new amp is never as emotional as buying an instrument. Still, it's a big deal, and to compare amps and determine whether they'll fit the bill, we need to understand some specs. When it comes to their basses, players often go into great detail about whether they can hear a tonal difference between various body woods and their “resonances," but it's rare to find players who know the most basic internal specs of their amps. Granted, everyone knows if they own a tube or solid-state amp, and how much power it purportedly delivers. But there's more behind power rating than just a numerical value, and it's important to realize that this output power depends on the external load or impedance of the attached cabinet.

In physics, power is permutated energy over a period of time, while in marketing, power has many more names. For example:total system power, sound power, peak power, or even PMPO, which stands for peak music power output. Manufacturers often act as if these are equivalent terms, but they're far from the technical standard of continuous power or RMS (Root Mean Square) power.

The differences in these values for a given amp can be of several orders of magnitude and were once so shamelessly used in marketing that in 1974 the Federal Trade Commission stepped in with its “Amplifier Rule" to combat unrealistic power claims. Unfortunately, this rule was amended in 1998 and power ratings once again got less precise and comprehensible.

The enemy of any amplifying electronic element is heat, which will destroy the device if you aren't able to dissipate it fast enough. A short slap-style riff will give the amp more time to rest and dissipate heat than a sustaining low B, so it makes sense to use the worst-case scenario of a continuous signal to get comparable power values.

Here, permutated energy over time is the key point, and the differences between peak and RMS power are shown in Fig. 1. RMS is the averaged continuous power over time (shown as the line marked P AVG), while peak power (the line labeled P 0) is a meaningless rating for any real musical scenario, as it just counts the interval of the peaks and totally neglects the amp's idle time.

There's another way to take this unrealistic rating even further: Stress an amp with a high signal of a few nanoseconds, followed by a long, long break to let it cool down. That's PMPO! And marketers aren't shy to advertise a power rating of several thousand watts in the headline before finally admitting a total energy consumption of a few hundred watts in the fine print. The total energy consumption or total system power is measured at the electrical outlet—lights and fans included—and thus says nothing about your amp's output power.

The term RMS stands for a mathematical method, but it's a well-accepted standard that clearly defines how to measure the output power by using a continuous pink noise signal within the audible frequency band of 20 Hz to 20 kHz. Additionally, it mentions the impedance or external load—often in the range of 2 to 8 ohms—at which an amp produces a specific power. Besides that, it's also important to know how much clipping or distortion occurs. Those total harmonic distortion (THD) values are typically in the range of 1 percent or lower. In a musical context, values of 5 or 10 percent represent an unacceptable amount of distortion, but would allow for stating a higher wattage value.

If we measure RMS power using different frequency bands, such as with bi-amped systems or active subwoofers, we generate misleading results, but the deviations are not as large as compared to PMPO or other terms mentioned above. The takeaway? It's more important to know what we are comparing than how it's been measured, and if we stick to RMS power, we're pretty much safe. In the end, it's simply about being an informed shopper who's able to accurately compare the output power of different bass amps.