How Many Watts Are Enough for Speakers?

A pair of home theatre speakers
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Amplifier output power is one of the most important considerations in choosing a stereo amplifier or receiver. Power is measured in watts (W) per channel, and the decision about how much power one needs should be based on a few criteria. Consider the selection/types of speakers that you intend to use, the size and acoustic characteristics of the listening room in question, and the desired loudness (and quality) of music that you want to play.

The general rule of thumb is that you should match the power requirements of speakers with the output power of the amplifier/receiver. You'll want to make sure that the power matches the impedance rating for each of the speakers. Keep in mind that some speakers require more or less power than others – loudspeaker sensitivity is expressed in decibels (dB), which is a measure of how much sound output is produced with a specified amount of amplifier power. For example, a speaker with a lower sensitivity (say, 88 to 93 dB) tends to require more amplifier power than a speaker with a higher sensitivity (94 to 100 dB or more) in order to play and sound great at the same volume level.

Measure the Power

Power output and speaker volume is not a linear relationship! Doubling the amplifier/receiver power won't double how loud the music sounds (hint: it's logarithmic). For example, an amplifier/receiver with 100 W per channel will not play twice as loud as an amplifier/receiver with 50 W per channel using the same speakers. In such a situation, the actual difference in maximum loudness would be just slightly louder – the change is only 3 dB. It takes an increase of 10 dB in order to make speakers play twice as loud as before (a 1 dB increase would barely be discernable). Rather, having more amplifier power allows the system to handle musical peaks with greater ease and less strain, which results in better overall sound clarity. There is little point to audio enjoyment if too much power causes the speakers to distort and sound terrible.m

This is why it's good to also know the details of the speakers you plan to use. Some have to work a little harder than others to achieve the desired volume output. Some speaker designs are more effective than others in projecting sound evenly across open spaces. If the listening room is small and/or carries audio well, one may not necessarily need a super-powerful amplifier/receiver, especially with speakers that are more sensitive to power. But bigger rooms and/or greater listening distances and/or less sensitive speakers will certainly demand a lot more power from the source.

When comparing the power output of different amplifiers/receivers, it is important to understand the difference between the types of measure. The most common measure of power is RMS (Root Mean Square), but manufacturers can also provide values for peak power. The former indicates continuous power output over periods of time, while the latter indicates output in short bursts. Speaker specifications can also list nominal power (what it can handle over periods of time) and peak power (what it can handle in short bursts), which should also be carefully considered and matched. You don't want to dial an amplifier/receiver up too high as to damage either itself or any connected equipment, including speakers.

Be sure to compare the same values side by side before making a final decision. Also know that some manufacturers can inflate specifications by measuring power at a single frequency, say 1 kHz, rather than an entire frequency range, such as 20 Hz to 20 kHz. For the most part, you can't go wrong with having more power at your disposal than not, even if you don't plan on blasting music at concert-like levels in rooms. Amplifiers/receivers with higher power ratings can deliver without needing to be pushed to maximum output limits, which will keep distortion down and audio quality up.