Hi-Fi: How Much Power Is Enough?

Ever since, as a penny-pinching student, I went without lunches to save up for my first hi-fi system, I have been keenly aware that lifelike sound is not merely a technical challenge but also a financial one. A happy few meet this challenge with a casual wave of the checkbook, but most of us have to reconcile our desires with their cost.

Since the price of sound gear goes up sharply with a system’s power rating, the question arises: how much is enough? For example, a receiver rated at 20 watts per channel typically lists for about $250, while receivers rated about 60 watts per channel have tags twice as high. And from that point on the price of power gets even steeper.

So here’s your dilemma: no point in paying for wattage you’re not likely to use. Yet if your amplifier or receiver is too puny to satisfy your musical demands, you’re obviously not getting what you want. Solution: formulating a reasonable “power budget” will save you disappointment and possibly a chunk of money.

First off, let’s clear the air of a popular misconception. A lot of watts doesn’t necessarily mean a lot of sound. A 100-watt amplifier, for instance, doesn’t play 10 times as loud as a 10-watt amplifier, because the human ear doesn’t translate power output directly into loudness. What you gain from the added wattage is not so much extra volume as greater clarity of sound during the loud sections.

Amplifier power is like horsepower in cars – handy in the tight spots. You don’t always drive with the gas pedal jammed to the floor, pulling every bit of available power from the engine. Similarly, an amplifier (or receiver) rarely operates at full output. But there are moments in music – just as there are moments on the road – when extra power helps you over the hurdles. On the road, you may need that power surge to pull ahead of a trailer truck on a hill. The musical equivalent may be a big orchestral climax, a heavy chord struck full force on a piano or the deep, punchy thrust of an electric bass. If you don’t have the needed power margin at such crucial moments, the sound breaks up. The music is loud, all right, but what should have been a thrilling musical climax gets churned into sonic hash.

This is called “peak clipping” and may last for only a fraction of a second – just the moment of the heavy bass beat or the instant when the drumstick hits the skin. Even so, the overall effect is spoiled. The human ear (or maybe the mind) rejects the distortion. It builds up a psychological defense known as “listener fatigue.” After a while, you just want to turn the damn thing off. By contrast, an amplifier with sufficient power glides smoothly over those loudness peaks and lets you listen for hours without getting edgy.

Higher power also helps pump out better bass. To get an idea of the energy contained in the deepest musical sounds, visualize such instruments as the kettledrum, the tuba or the string bass. Think of the sheer physical force required to play them. To reproduce corresponding energy levels in your living room takes a lot of watts pushing the speakers; large amounts of air must be set in motion to replicate those shuddering, deep bass thrusts that are felt in the stomach as much as heard by the ear.

Power also helps to get to the top. If an amplifier is driven to the brink, the highs get harsh and the overall sound texture – instead of remaining transparently clear – gets fuzzy. Lots of listeners are so accustomed to flabby amplifiers that they take the fuzz for granted. A friend of mine, listening to my strong-muscled rig, exclaimed, “It’s like washing a dirty window. Suddenly you get a clear look at the world.”

It will probably surprise you that on the average, at normal listening volume, an amplifier rarely puts out more than 3 or 4 watts. (You can verify this by watching the power-output meters now provided on many amplifiers and some receivers.) Why then, you’ll ask, all this fuss about power margins?

The answer is that at those peak moments we were talking about, the sound level suddenly rises by about 15 decibels (db). To get just a 3-db increase, you have to double the amplifier power. And to get an extra 10 db, you have to multiply the power by 10. Mathematically, it’s called a logarithmic progression, and you can expect to run into hefty figures rather quickly.

But enough theory. The amount of power you actually need depends on three factors: your choice of speakers, the size and furnishings of your room and your musical taste.

As for the first factor, some speakers are more efficient than others; they’ll give you more sound per watt. It’s got nothing to do with the way they sound; that is, an inefficient speaker isn’t necessarily bad. In fact, some of the best speakers on the market – especially the so-called acoustic suspension designs – are highly inefficient; they gobble up a lot of watts to produce room-filling loudness levels. But lately some very good speakers employ so-called ported enclosures or passive radiators, which allow them to gain efficiency without sacrificing tonal quality.

To find out how efficient a speaker is, take a look at the suggested “minimum power requirement” in the speaker specs. A high-efficiency speaker may require less than 10 watts per channel to drive it adequately. A low-efficiency speaker usually takes at least twice as much power. And since power is the most expensive item in the average system, you can save money – without compromising performance – by choosing a high-efficiency speaker and combining it with a relatively low-power receiver. Anyway, take the minimum wattage required for the speaker of your choice and make it the starting point for figuring your power budget. If each of your speakers requires 15 watts to drive it, then your receiver must deliver at least 15 watts per channel. Simple, so far.

Which brings us to the second factor. Those “minimum power requirements” are just rough approximations that presuppose a “normal room.”

Let’s say normal is a room anywhere from 2,000 to 3,000 cubic feet in volume. For 4,000 cubic feet, you’d better step up your amplifier power by 30 to 50 percent, and for 6,000 cubic feet you’d better double it. If you live in a barn or a baronial mansion, nothing less than 70 or 80 watts per channel will do, and more may be handy.

But that’s only the beginning of the calculation. Next consider the furniture. Suppose the surfaces of your habitat reflect sound rather than absorb it – with smooth plaster walls, just a few rugs scattered on hard floors and no heavy drapes. This makes a “live” environment in which echoes reinforce the output of your sound system. As a result, you can cut your power budget by about 30 percent. But if the environment is acoustically “dead,” with wall-to-wall carpeting, draperies and sofa pillows that soak up sound, a lot of sound energy gets converted into frictional heat. The amplifier has to make up this loss of audible sound energy, so you have to increase the power rating by about 30 percent over the “normal” figure we’ve calculated above.

Hang on if you’re already confused. Things will clear up considerably the second time you read this. Besides, we haven’t yet gotten to the third factor: your musical taste.

What’s that got to do with watts? Simply this: some kinds of music are more power hungry than others. For example, richly scored orchestral music – symphonies and such – require more power than even the heaviest rock. That’s because the loudest orchestral passages are often sustained in long, swelling crescendos, while rock comes in short, thumping beats that give the amplifier a chance to “catch its breath” in between. If the amplifier has to deliver maximum power only in intermittent bursts, it can usually put forth more undistorted watts than its nominal rating, which is measured for continuous output. An amplifier’s cruelest task is presented by long-held, ultrapowerful bass notes, such as the pedal tones of a great pipe organ or the massive array of bass fiddles and cellos in an orchestra piling up a thundering bottom line in octaves. If that sort of thing is your musical dish (Mahler and Wagner, et al), add an extra 30 percent to your power budget. And, if you like to play your music loud enough to lift the house from its foundations and register 7 on the Richter scale, you’d better make this 50 percent.

I hope you – and your pocket calculator – are still with me. Just take each of the factors mentioned here one at a time, add them up, and you’ll arrive at your power requirement. By realistic I mean all the power you need, without paying for something you don’t need.

When comparing the power ratings of amplifiers and receivers, look out for possible pitfalls. For one thing, the nominal power rating is measured at midrange frequencies, which are far easier to reproduce than either the low bass or the top highs, which is where you really need the power. That’s why you should take a critical look at what is called the “power bandwidth.” This is a statement of the frequency range over which the amplifier or receiver will actually produce its rated power. Preferably, it should extend over the whole musical sound spectrum from 30 to 20,000 Hz. To keep advertisers from fudging their figures and making their wares seem better than they are, the Federal Trade Commission requires that all power ratings must be stated together with the amount of distortion generated at that wattage. If fidelity is your objective, total harmonic distortion (THD) should never exceed 1 percent. In most of the better units, THD is a mere fraction of this value, ranging from about .5 percent in budget equipment all the way down to less than .1 percent in the fancy rigs. Finally, the power rating must take into account the so-called impedance of the loudspeakers. Since most speakers operate at 8-ohms impedance, the power rating of amplifiers and receivers should be specified for 8 ohms. Any rating for values other than 8 ohms may be misleading.

Armed with these facts and figures, you are now a match for any salesman who might try to pressure you into buying a more powerful unit than you really need. You now have an idea how much power is enough, and there is no sense in buying more.