How Much Power Do My Speakers Need?

A 3-Step System for Figuring Out the Right Amount of Power for Your System

Back of a speaker
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One of the most confusing topics in audio is figuring out what size amplifier your speakers need. Usually, you're making the decision based on simplistic and sometimes meaningless amp and speaker specifications. Many people decide based on misconceptions about how amps and speakers work. In this article, I'll share the knowledge I've developed in years of testing and measuring speakers -- plus the behind-the-scenes insights I've gained from talking with literally thousands of engineers and marketing pros in the audio business.

The Truth About Speaker Power Handling Specs

First, it's important to understand that speaker power handling specs are usually meaningless. Usually, you just see a "maximum power" rating with no explanation of how the spec was derived. Is it the maximum continuous level, average level or peak level? For how long? With what type of material?

Unfortunately, there have been numerous and conflicting standards for measuring speaker power handling, published by the Audio Engineering Society (AES), the Electronics Industries Association (EIA) and the International Electrotechnical Commission (IEC).

Most of the manufacturers I've talked with don't follow these standards, they just make an educated guess. Often it's based on the power handling of the woofer. (Power handling specs on raw speaker drivers, such as woofers and tweeters, are more standardized and meaningful than specs for complete speakers.) Sometimes a speaker power handling spec is based on marketing.

You may even see a manufacturer give a more expensive speaker a higher power handling rating than a lower-priced speaker even though they use the same woofer.

Volume Settings vs. Amplifier Power

Next, it's important to understand that in most situations, a 200-watt amp puts out exactly the same power as a 10-watt amp.

This is because most listening occurs at average levels of less than 1 watt. Into a given speaker load, at a given volume setting, all amplifiers deliver exactly the same amount of power, as long as they're capable of delivering that much power.

It's the volume setting that matters, not theĀ amplifier power. If you never crank up your system to the level where the volume is uncomfortable, your amp may never actually put out more than 10 or 20 watts.

Thus, you can safely plug a 1,000-watt amp into a little 2-inch speaker -- just don't turn the volume up louder than the speaker can handle.

What you shouldn't do is plug a low-powered amp -- say, a 10- or 20-watt model -- into a typical speaker and crank it way loud. The low-powered amp may clip (distort), and amplifier clipping is the most common cause of speaker failure. When your amp's clipping, it's putting out a high-level DC voltage straight into the speaker, which can burn out the drivers' voice coils almost instantly.

How to Calculate What Size Amp You Need

Confusing as all this may seem, it's easy to calculate what size amp you need, and you can do it in your head. It's not perfect because you'll be relying on the specs from the speaker and amplifiers, which are often vague and sometimes exaggerated.

But it'll get you close enough. Here's how to do it:

1. Take the sensitivity rating of the speaker, which is expressed in decibels (dB) at 1 watt/1 meter. If it's listed as an in-room or half-space spec, use that number. If it's an anechoic spec (like the ones I give in my speaker measurements) add +3 dB. The number you have now will tell you roughly how loud the speaker will play in your listening chair with a 1-watt audio signal.

2. What we want to get to is the amount of power you need to hit at least 102 dB, which is about as loud as most people ever want to get. How loud is that? Ever been in a really loud movie theater? A properly calibrated theater running at reference level will give you 105 dB per channel. That's very loud, louder than most people want to listen, which is why movie theaters rarely play that loud. So 102 dB is a good target.

3. Here's the key fact you need to know: To get an extra +3 dB of volume, you need to double the amp power. So if you have a speaker with an in-room sensitivity of 88 dB at 1 watt, then 2 watts will get you 91 dB, 4 watts will get you 94 dB, and so on. Then just count up from there: 8 watts gets you 97 dB, 16 watts gets you 100 dB, and 32 watts gets you 103 dB.

Thus, you need 32 watts. Of course, no one makes a 32-watt amp, but a 40- or 50-watt amp should do fine. If the amp or receiver you want puts out, say, 100 watts, don't worry about it. Remember, at average listening levels with typical speakers, any amp is putting out only about 1 watt, anyway.

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