Which Watts?

By Daniel Kumin

More amplifier power will reproduce music louder (and more importantly, cleaner-at-the-limit) than less power; everybody knows that. But how much do you really need?

I can’t answer that---not without knowing what speakers in what room; how placed; how furnished, and what sort of music you want to play and how loudly. But I can give you a few tools to consider power claims more clearly.

First let’s parse a typical well-written power spec:

“75 watts continuous/R.M.S. power per channel, at 8Ω, both channels driven, from 20 Hz-20 kHz, with less than 0.1% T.H.D.”

This tells us that the amp or receiver will deliver at least 75 steady-state watts on pure, single-frequency tones while exercising both channels into idealized loudspeaker “loads” (8 ohms is the de facto standard), at any musical frequency from the deepest bass to the highest harmonic, without significantly distorting. (T.H.D. stands for “total harmonic distortion,” and 0.1% is well below audibility)

Now let’s look at another amp spec:

“250 honkin’ watts!!!” 

Note that no speaker-load is specified, nor frequency range or distortion percentage. Amp B may or may not be more powerful than amp A in the real world of playing music into actual loudspeakers: we have no way of knowing. But both these specs could describe the exact same amp or receiver.

Fortunately, most quality electronics are spec’d like amp A. But this still doesn’t tell the whole story. Watt statements tell us only how much electrical current is dissipated into the loudspeaker’s voice-coils: it says nothing about how much of that energy ultimately becomes sound pressure (volume). And it only tells us how the amp behaves under steady-state, single-tone stimulus; music is a decidedly not-steady, but a complex of many tones and harmonics, constantly shifting in frequency (pitch) and amplitude (loudness). Really, all that even properly written power specs are good for is comparisons, and then only assuming truly apples-to-apples ones.

So here are some key facts:

Take two amplifiers, similar except for properly stated power ratings of 25 and 50 watts per channel. With the same loudspeakers playing the same music in the same room, the latter amp will deliver just 3 dB greater net loudness – and 3 dB is the smallest volume change most listeners will even notice. Were Amp B instead ten times as powerful (250 watts per channel), you’d get a 10 dB volume increase—a modest, but at least easily discernible boost, one most listeners will characterize as about “twice as loud” (whatever that means). You have to go to 100x, or 2,500, watts to net a 20 dB volume change, which is much, much louder: about like moving, at a rock concert, from back-of-house to mosh-pit front.

Fortunately, typical consumer speakers in typical listening rooms only require a few watts of steady-state power to deliver satisfying listening levels. With typical, 50-100 watts-per-channel gear this leaves 10 to 20 dB “headroom” for music’s dynamic changes. Which, as it turns out, is just about what most music recordings require.

That said, extra-dynamic music such as scrupulously recorded classical or jazz can demand still more headroom; this means you need an amp or receiver with even more power: or one with some “dynamic headroom,” the ability to deliver more watts, for a fraction of a second at a time, than its steady-state rating. Alas, there’s rarely any way to tell on paper. In the 1980’s the industry established a dynamic headroom spec and test procedure, they never caught on. Any amplifier will have at least some dynamic headroom, but most are limited to 1 dB or less, not a very useful quantity.

The important things to remember?

  • More power is always better than less power. Clean watts rarely damage loudspeakers; it’s the distorted output of an over-driven amplifiers that usually do the job.
  • Steady-state “RMS” or “continuous” power doesn’t necessarily tell the whole story: dynamic headroom matters, too, even though it’s rarely specified or even noted.
  • Speaker sensitivity (sometimes referred to as "efficiency," a related measurement) is as important as amplifier power in determining “how loud?” (and by implication “how clean?”); so are room acoustics, furnishings, and listening-distance.
  • When up-rating power, merely doubling rated watts is not as productive as it looks, yielding a paltry 3 dB increase in potential loudness. Power increases of 3, 4, or 5 times will have a more musically important impact.