Most folks shopping for a home theater receiver or amplifier are bound to have their eyes peeled for a single number: the power rating. Ideally, this spec will tell you how much juice a particular amp can deliver to a given set of speakers under normal conditions. But the problem is, multichannel audio systems used for watching movies and listening to surround sound music are often subjected to conditions that go beyond what generally passes as normal. And then there's the issue of variations among manufacturer power ratings; some companies offer a generous amount of information about their products, while others serve up specifications that seem almost deliberately obscure.
If the power ratings for some receivers and multichannel amps lack detail, the situation gets even worse — much worse, in fact — when you check out home-theater-in-a-box systems, which generate the biggest sales of audio gear in the current bigscreen HDTV era. A typical power rating for an HTiB system might read "Total Power: 200 watts." But how does that power get distributed among the various satellite speakers and the subwoofer? More important, what are the measurement conditions? (For example, over what bandwidth and at what distortion level is the specification arrived at?) Obviously, some level of standardization for power specifications among audio products would be a welcome thing.
Power Ratings: The Current State
Most of the measurement techniques used by surround sound receiver and multichannel amplifier manufacturers are detailed in CEA 490-A, the Consumer Electronics Association's Standard Test Methods of Measurement for Audio Amplifiers (shown at right), which was published in 2002. This standard was itself developed in response to the Federal Trade Commission's revamping of its 1974 rule governing Power Output Claims for Amplifiers Utilized in Home Entertainment Products — an effort that also sought to include the crazy new world of multimedia speaker systems for computers. Although the CEA standard doesn't cover those devices, it does seek to standardize the measurement of "output power of multichannel amplifiers used in home theater applications" and to create a "standard language for primary ratings of amplifiers that allows consumers to make an ‘apples-to-apples' comparison between various brands and models of amplifiers." Compared to the relatively threadbare FTC rule, the CEA standard provides a more comprehensive set of recommended measurement practices for rating audio gear. Unfortunately, while the FTC's rule carries some legal weight, the CEA standard is a set of guidelines that depends on voluntary compliance from audio manufacturers.
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CEA 490-A
Consumer Electronics Association Standard Test Methods of Measurements for Audio Amplifiers |
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Primary Rating & Disclosure a) Monaural b) Stereo c) Multichannel In each instance, the following information should be included with the primary rating disclosure: at 8 ohms,1 kHz, and 1% THD. |
In the absence of ironclad laws governing how receiver, amplifier, and HTiB manufacturers define power output capabilities, there are a few telltale signs to look for that can help disclose what's going on with a specific product. One is a power rating for all-channels driven, a spec that lets you know how much power an amp can deliver with all five, six, or seven channels engaged, as opposed to a single channel. This spec, when available, doesn't exactly tell the whole story — in the real world, program material that engages multiple channels at full or near-full power simultaneously is rare. (Surround sound pipe-organ music, anyone?) But with the soundtracks of major movie-studio releases tending more toward the manic with each passing year, it's good to know that your system can handle a full multichannel assault. For that reason, it's also good if manufacturers provide single-channel, stereo, and all-channels-driven power ratings in their specs.
Another thing to look for when comparing amps or receivers is the load used for conducting power measurements. This number, which gets specified in ohms, is used to represent the speaker load presented to the amp's output circuit. For example, a typical power measurement might read "100 watts per channel at < 0.07% THD from 20Hz–20kHz into 8 ohms." What this spec basically tells you is that the receiver can deliver 100 watts of continuous power across the entire audio bandwidth with less than 0.07% total harmonic distortion (THD) into an 8-ohm resistance. (A maximum measured distortion level below 1% across the full frequency range is likely to be inaudible on normal program material, such as movie soundtracks or music.)
Although 8 ohms has long been considered the standard load for measuring amp power (most speakers are spec'd at 8 ohms), an actual speaker usually presents a fairly complex load to an amplifier, with its impedance varying widely over the frequency spectrum. That's why it's helpful to see both 4- and 8-ohm power specs — you'll have increased confidence that the receiver can be mated with a broad range of speakers without overheating and shutting down.
A more rarefied power rating you'll find on some high-end receivers and amps is dynamic headroom. In some ways, this number is more significant than a "continuous" rating because it defines a product's ability to unleash additional power reserves in short bursts. Amp manufacturers issuing dynamic-headroom ratings — NAD, for example — state them in dB, expressing decibels of power available above and beyond the amp or receiver's "continuous power" specification for transient bursts (20 milliseconds). So if an amp's dynamic headroom is rated as 3 dB, that means it can deliver twice its rated power (doubling power delivers just a 3 dB loudness increase) for quick transients in music.
What to Look For: The Bad
As I said earlier, the grayest area by far when it comes to power ratings is that found in HTiB systems. A lump specification such as 200 watts total power (in some cases, you'll be lucky to dig up that much data on the product) doesn't begin to tell you what the system is capable of, or if it will even the exceed the output of your TV's own built-in speakers significantly. Even some of the more expensive all-in-one systems reveal certain eyebrow-raising elements stashed among their specifications. One popular system, for example, boasts of in excess of 1,200 watts total power. But after reading between the lines, it turns out that per-channel measurements are limited to a 1-kHz tone as opposed to the entire audio bandwidth, and with a sizeable 10% THD. Not exactly Hendrix territory, but lots of distortion nonetheless!
Even some well-known makers of mid-priced receivers are known to play the numbers game. One current seven-channel model, for example, has its power spec'd at 150 watts per channel — a sizable amount, given the product's reasonable price. But a second glance at its data sheet reveals the power rating to be for a 6-ohm load, rather than the standard 8-ohm specification. Not surprisingly, its test-bench performance puts it on a more even playing field with its similar-price competition, most of which is spec'd for around 100 watts per channel or less into 8 ohms.
A Better Future?
Will the consumer-electronics industry get its act together and decide on a consistent method of measuring, rating, and specifying power for audio components? In the absence of strictly enforced government regulations, the answer is probably no. Despite this, the Consumer Electronics Association's Audio Systems Committee is currently drafting a new standard for rating home theater receivers and amplifiers. Details on the progress of those discussions — and on the players involved — aren't yet available, but the new standard's intended scope is to tighten and expand recommended measurement procedures in the existing one. Most interesting of all, the new standard will reportedly cover the exploding HTiB category. That might possibly be good news for the average consumer, who would presumably be given a better shot at having a satisfying audio experience when he carts home an all-in-one system to go with his 50-inch flat-panel TV.
Sussing out what's behind the numbers in receiver ratings and specs can be difficult. In most cases, you have to take the manufacturer at his word — although it's always a good idea to check the detailed test-bench results in reviews such as those found here at soundandvisionmag.com for confirmation. Just remember: As with any purchase, proper research can give you a more solid footing as you prepare to make your launch into higher-quality audio. Here's to finding truth in numbers!
For more, read our In-Depth Guide to Power Specifications.
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