Some of you who read FRONT of HOUSE may be too young to remember, but the 1970s were a booming time for manufacturers of home hi-fi gear. This was an era when people actually sat and listened to music for entertainment, as opposed to using it as background noise while they chopped vegetables or did their laundry. What a concept!
Anyway, in an effort to enhance their sales numbers against fierce competition, many manufacturers massaged the audio specs of their equipment, particularly the power amplifiers. If an amp had a power output of 25 watts per channel, it wasn’t unusual for the advertising to state that it was a 50-watt amp (because 25 + 25 = 50). And if the peak power was 35 watts per channel, well… that makes it a 70-watt amp, right? You can imagine the numbers that could be fabricated for the same amp if power was measured using an extremely high distortion level, or a lower impedance. It’d be easy to make that 50-watt amp look like a 200-watt monster.
Things got so out of hand that, in 1974, the Federal Trade Commission (FTC) stepped in by enacting the Amplifier Rule, which was designed to help consumers make decisions based upon standardized measurements. That’s when you’d start to see power specs that read “50 watts per channel, both channels driven into an 8-Ohm load at 1% THD, from 20 Hz to 20,000 Hz.”
The Amplifier Rule was amended in February 2001, to “exempt sellers who make power output claims in media advertising from the requirement to disclose total rated harmonic distortion and the associated power bandwidth and impedance ratings; clarify the manner in which the Rule’s testing procedures apply to self-powered subwoofer-satellite combination speaker systems; and reduce the preconditioning power output requirement from one-third of rated power to one-eighth of rated power.”
Why the amendment? I don’t know. I’m an engineer, not a politician. However, if anyone actually wants to read more on the FTC amp ratings, the official government document can be found at plsn.me/FOH-FTC.
Luckily, we live in the pro audio world where there’s more integrity from manufacturers, partly due to the fact that they have to live with us for the long-term and need to protect their reputation, and partly due to the fact that they know we earn a living using their gear. Nonetheless, it seems that an increasing number of manufacturers are listing output power specs for their amps or powered speakers as peak power output in watts, as opposed to RMS power ratings. It would be helpful to understand the power specs before making a purchase, so that you can be clear about what to expect in return for your hard-earned bucks.
Whatts?
One thing that’s very clear when trying to understand power ratings is that “100 watts per channel” is insufficient information. What is the load impedance of that measurement? Was the measurement made using a sine wave or noise? What was the distortion level at that output? Is that power rating continuous or peak? The power specs most often seen include RMS, peak, and dynamic.
RMS
RMS is an abbreviation for “Root Mean Square,” and refers to a formula used to derive the continuous average level of an audio signal. RMS is calculated by squaring the instantaneous voltages along a waveform, averaging those squared values, and then taking the square root of that number. Super audio geeks know that “RMS power” or “RMS watts” really should be termed continuous average power. Nonetheless, we see the term “RMS” and since it’s pretty darn close to average or continuous power, an RMS value is in fact a meaningful specification — it’s the power output that an amp can generate comfortably for an extended period of time. If an amp can deliver 200 watts RMS per channel into 8 Ohms with 0.05% THD, 20 Hz to 20,000 Hz, you have a pretty good idea of what the amp can do — though I’d also like to see “both channels driven” as shown in Fig. 1. When you see an RMS spec for a (passive) loudspeaker, it gives you an idea of how much power the speaker can handle, continuously.
Peak Power
Peak power, dynamic power and burst power output are essentially the same specification, which tells you how much power an amp can deliver for a brief period of time (we’re talking really brief). The term “burst” comes from the manner by which such a measurement is made: the amp is fed with a sine wave and driven to the clipping point. The sine wave is cycled on and off so that the amp has time to recover. Manufacturers have different ways of doing this. One example is 20 milliseconds on, followed by 480 mS off. This cycle is repeated, and the resulting power measurement is usually considerably higher than that achieved when measuring average power output. If the amp has a peak power output that’s double the continuous power, it has 3 dB of headroom (a 3 dB increase in SPL requires doubling the power). That’s why dynamic power might be shown as 3 dB instead of X number of watts.
Here’s an example: let’s say that an amp has a continuous power output of 100 watts (per channel, 8 Ohms, 1% THD, 20 to 20 kHz). If the dynamic power is stated as 3 dB, then the amp can produce 200 watts on peaks. Match that amp with a loudspeaker that can accept an RMS input of 100 watts and you have a pretty good match, because most loudspeakers can handle brief peaks of 3 dB.
Let’s Meditate On This (Ohmmmmm)
You’ll notice that manufacturers provide output ratings for different load impedances such as 8, 4, and 2 Ohms. An amplifier might be rated to produce 100 watts into 8 Ohms and 180 watts into 4 Ohms. One thing that this tells you is that the amp can safely power a 4 Ohm load, which is important if you plan to parallel two 8 Ohm speakers per channel. Before you get carried away with the notion of paralleling a bunch of 8 Ohms speakers in order to get “free” power, remember this: as the load impedance drops, the amplifier must deliver more current. When the amp runs out of current, it may severely distort, or generate excessive heat and shut down. Given two amplifiers with the same power rating, the amp with a higher current capability will more easily handle difficult loads. Just because the data for a power amplifier shows an output rating at 2 Ohms, don’t expect that the amp will work at its best when driving a 2 Ohm load. The 2 Ohm data is more indicative of the fact that the amp will be able to easily handle 4 Ohm loads.
What about powered speakers? You may see any or all of the above-mentioned information, but the good news is that the manufacturer has done some of the grunt work for you by matching the power amplifier(s) to the driver(s).
In Fig. 2, JBL provides peak and continuous power specs for the VRX932LAP, and then breaks down separate data for the LF and HF amps. This is valuable information, as it provides an idea not only of how much power the amplifiers can supply, but also how much power the drivers can handle.
What’s removed from the equation is the need for you to calculate how loud the box can play based upon the sensitivity of the speaker and the power capability of the amp. Loudspeaker sensitivity is defined as the SPL produced by the speaker when powered with one watt, measured at 1 meter. Fig. 3 shows data for a loudspeaker with a sensitivity rating of 92 dB.
When using a powered loudspeaker, you don’t need to worry (much) about the relationship between sensitivity and power — the manufacturer has done it for you. As a result, you can pay more attention to the maximum SPL, which (obviously) tells you how loud the thing will play. Now if only we had specs for maximum continuous SPL, as well as peak SPL…
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
