This is one of the most commonly asked questions in the world of audio, and subsequently one of the most needlessly confusing topics. Not only do many ostensible sources seem to want to give the reader a degree in electrical engineering, but all the information out there seems to be all over the place. The goal of this article is to concisely explain – without bogging a layperson down with superfluous information – what factors influence how much power a speaker actually needs from an amplifier, and how to pick and match a correct amplifier or receiver to your speakers that will make them sound their best and not blow anything up.
Factor #1 – A speaker’s continuous power handling
Continuous power handling, also often denoted as RMS (root mean square) power handing, is how much power a speaker can safely handle over the long run (as opposed to peak power handling, which is the absolute maximum power a speaker can handle in short infrequent bursts). It’s good to have an amp that can supply at least this much power per channel, i.e. to each speaker, but understand that you’ll rarely actually drive more than a fraction of that amount of power to your speaker. This will make more sense in the next section.
Factor #2 – A speaker’s efficiency rating
The efficiency of a speaker basically means how much actual volume it can produce with a given power input. This is typically measured in dB per 1 watt at 1 meter, meaning how loud in decibels will a speaker be from 1 meter away if you run 1 watt of power into it. Most non-commercial grade speakers have a sensitivity rating around 85 to 90 dB, which is actually quite loud. Comfortable and safe listening is usually around 75 dB. Here’s a chart for reference:
Decibels drop by about 6 for every doubling of distance, but even if your speakers are on the far side in your room, you can see that it does not actually take all that much power to get your speakers’ volume to comfort – 10-20W of power might be all that most people will ever need.
Factor #3 – Extra unused power is better than not enough
One common misconception is that an amp with too high a power rating will inevitably damage speakers. Remember: just because an amp can drive a certain amount of power doesn’t mean that it is. Just be sure not to crank the volume up too high and your speakers (and ears) will be fine. It’s possible that you can damage speakers by driving too much power to them, but chances are, in a near/mid-field setting, your speakers will get way too uncomfortably loud before you come close to exceeding their continuous power handling threshold.
In fact, too little power is a much more common cause of speaker damage than too much power. Why? Well, if an amp can’t actually supply the power that’s demanded of it, what happens is that its signal can “clip.” Basically what this means is that the peaks and valleys of the signal get chopped off, and instead of a nice and smooth wave getting sent to your speakers to replicate, you get a signal that causes the woofers to suddenly jerk to stop and start moving as they attempt to replicate the artificially clipped signal. This can cause the components to overheat and potentially blow out.
Factor #4 – Make sure an amp’s output impedance is (at least) rated for speaker’s input impedance
Impedance is a measure of electrical resistance, denoted in ohms. Most residential grade speakers and receivers are rated at 6-8 ohms, and sometimes you’ll see 4 ohm speakers. Without going on an unnecessarily technical digression, a speaker with a lower impedance of, say, 4 ohms over 8 ohms, will demand and draw significantly more power to function at a normal listening volume. Practically speaking, all you need to make sure is that your receiver or amplifier is rated to at least handle the impedance of your speakers. For example, driving 8 ohm speakers (by far the most common) with an amp rated at 6 – 16 ohms per channel is fine. Driving 4 ohm speakers with an amp that’s not rated for 4 ohm loads is not fine – that can overexert your amp and damage it and/or put your speakers in the bad red-zone seen above.
To make sure you have a correct amp or receiver that will adequately power your speakers:
- If the amp’s watts per channel is at least equal to the speaker’s continuous/RMS power handling rating, you’ll have more than enough power and will likely only ever use a fraction of it.
- More sensitive/efficient speakers (think 90+ rating) will achieve adequate volume with significantly less power.
- It’s much better to have extra unused power than too little power, the latter is what can damage speakers.
- Make sure the receiver/amp’s impedance is rated for at least equal or lower to the speaker’s impedance rating, i.e. don’t ever pair speakers with a lower ohm impedance rating than what the amp is rated to drive.
If you follow those simple guidelines you’ll almost certainly be OK. 100 watts per channel at 8 ohms is a pretty common bench mark for budget receivers, which is enough to drive ~99% of speakers in ~99% of cases.