Power amplifiers, whether standalone or built into a receiver, integrated, or even our mobile phones, drive our speakers and headphones so we hear sound.

Amplifiers produce Watts and, if you’ve ever wondered why the term Watt is always capitalized, it’s named after a rather famous historical character, James Watt, famous for his invention of the Watt steam engine in 1781 (this was no small invention. It fundamentally changed the world by ushering in the Industrial Revolution, in both his native Great Britain and the rest of the world).

A Watt represents one Joule of energy per second (one amp of electricity passing through one Ohm for one second). But you don’t need to know all that to figure out how much power it takes to make your speakers come alive. It’s actually simpler than all that.

Let’s start with the basics

Most of us have read countless reviews of loudspeakers, which means we’re likely familiar with a common measurement called sensitivity or efficiency. This measurement is typically expressed is dB—and yup, there’s yet another capitalized letter. The dB or Decibel is named after Alexander Graham Bell of “hello Mr. Watson, can you hear me?” fame.

Let’s say your loudspeaker measures 90dB. So, what’s that mean? If you send one Watt into your speaker and place your ear one meter away (3 feet), you will hear 90dB of sound. To put that in perspective, a train whistle produces about 90dB when you’re a few hundred feet away, while a loud rock concert is about 115dB, a jet engine 140dB. Sustained sound pressure levels of 95 dB or higher can produce hearing damage, but not to worry. When’s the last time you were subjected to 24/7 sound pressure?

The thing about using wattage and dB as a guide to choosing proper amplification power is problematic, because the scales are not linear. It takes twice the power to get another 3dB of amplifier power, but even more to get another 3dB louder at the speaker.

Making sense of power

If you double the amplifier’s power, you’ll get an additional 3dB at the output of the power amplifier. But, amplifier power is not sound pressure power.

Amplifier power causes the speaker drivers to move back and forth, creating sound pressure waves that move the air and we hear music. Sound pressure is what we hear and microphones record. Sound pressure is what a sound pressure level (SPL) meter measures (in dBs). While the sound pressure is related to the amplifier’s power, doubling the amplifier’s power does not double the sound pressure. And, at the end of the day, all we’re really interested in is how much power do we need to make our speakers louder.

Right?

If we want to double sound pressure, we need 6dB more amplifier power, which requires four times the number of watts. And, if you’ll recall, the scales we use to convert sound pressure into amplifier power are not a straight line. Instead they follow a progressively larger exponential pattern.

Here’s an easy chart that explains what I am referring to.

Basically, you need one hundred times more power to get another 20dB in sound pressure. That means if it took one Watt to make 90dB, it’ll take one hundred watts to make 110dB (and 115dB is as loud as a rock concert).

Can too much power hurt my speakers?

A common misconception is that having too big of a power amplifier connected to your speakers will damage them. This is like believing a powerful car will always go faster than you want. Neither is true.

Yes, a large amplifier gives you the potential to release the blue smoke hidden in your woofers, just like a big engine in your car has the potential to get you busted for speeding. But neither means any of this will happen. Don’t turn your speakers up loud enough for your ears to bleed, and gentle on the accelerator pedal.

If you need something to worry about, try adding too small of an amplifier to your speakers. Too small of a power amp can clip the amplifier and fry your tweeter.

Bigger is better.

So, how much do I need?

Much depends on your listening habits and your speaker’s efficiency. If you like loud uncompressed music and your speakers are 90dB efficient, 200 Watts is likely plenty of power for you. If you only listen to light classical, jazz and don’t expect them to rock the house, 50 Watts is adequate.

Higher efficiency speakers, like horns, take very few Watts. That is because one Watt can produce 95dB of sound, which is already loud. Another 6dB of sound pressure will cost you four times more Watts—but still, we’re talking fewer than five Watts to produce more than 100dB of sound, and all of sixteen to thirty two Watts for rock concert levels.

In closing, it’s also instructive to recognize that most music isn’t constantly loud. Soft passages fall well below the 90dB levels. This means that for most listening, the average Watts used by your speakers is perhaps ten to twenty max.

If you want the best sound, go for more Watts – but never at the expense of sonic quality.

Not all Watts sound the same.