How Much Power Do I Need?
In November 2023 by Angela-Gilbert Yeung
Edited by George Taylor
How much power is enough?
There are many misconceptions about how much power is required in home audio applications. The common questions are these – How much power is enough? How much power is too much? Can an amplifier with a relatively low output power rating blow up a speaker with a high power handling rating? Conversely, will an amplifier with a high output power rating automatically damage a speaker with a low power handling rating? These questions – and many more like them - show a fundamental lack of understanding of the relationship between amplifier wattage ratings and speaker power handling ratings.
In real world application, any speaker can be driven by any amplifier. The key is to use common sense and restraint. Clearly, if you have an amp/speaker combination that is not playing loudly enough for your taste, then some questions need to be asked about the amount of power that your amplifier can deliver, and the size and sensitivity of your speakers. Conversely, if you have much more volume than you need then you either need to be a little careful with the volume control, or possibly consider a pair of lower sensitivity speakers. Having said that, power handling ratings for speakers are nearly meaningless.
So…How much power do you really need? Life - and audio – have very few "one size fits all” solutions, but let’s look at what might be considered a “typical” situation. For our purposes, a medium sized room with dimensions of 20ft L x 16ft W x 8ft H with the usual amount of furniture, no “audiophile-style” acoustic treatment, and a pair of speakers with a 90db efficiency rating with a 10 to 12 inch woofer (or the equivalent size of multiple smaller woofer). In this scenario 50 watts is plenty for jazz, symphonic, classic rock and big band. For heavier genres such as hard rock and dance music, you can increase this to 100 watts to be on the safe side. That 90db speaker driven with 32 watts will produce 105db at 1 meter. 105 dB is loud – in fact, hearing damage loud. Typical listening levels usually vary between 70dB to 90dB with peaks into the high 90s. Again, this doesn’t cover every situation, but it is a reasonable guideline for most people in most rooms.
Do we need to match the amplifier output wattage to speaker power handling wattage?
The short answer is “No”. Amplifier output wattage indicates the available wattage of a particular amplifier. The amplifier will not output its full rated power unless there is an input signal required to do so. The input signal level is controlled by the volume attenuator setting, which is set by the user - who is you. Simply put, if you don't turn up the volume, there will be no wattage putout to the speaker, therefore, no sound. It is the end user that matches the output wattage of the amplifier to the capabilities of the speaker. Common sense goes a long way here…
What does the speaker power handling rating tell us?
Let’s get one thing of the way immediately – the speaker power handling rating DOES NOT indicate how loudly the speaker will play music. That is defined by the sensitivity of the speaker, and the output capability of the amplifier that you use. What the speaker power handling rating does tell us is what the maximum input wattage is for the speaker before it will be damaged. This max power figure is usually a rating that the speaker can accept for a very short amount of time – usually a millisecond to a few seconds.
For example, if the speaker is rated at 100 watts maximum, putting a 100 watt signal into it for more than 1 second will risk damage. At 50% of the max power rating (or 50 watts input in this example) this speaker can operate for about 10 to 20 mins before causing damage. At 25% (or 25 watts), this speaker would operate without risk of damage free for a few hours. With a signal input of 10% of its power rating, this speaker can theoretically run forever.
Please keep in mind that power handling ratings are measured using a pure sine wave. A music signal is more complex than a sine wave and you would be wise to derate any speakers power handling rating by 15% to20%.
Another thing to consider – the power handling ability of a speaker can actually decrease while it is playing music. When music is continuously playing the voice coil of the speaker driver will heat up. If the voice coil is not properly cooled, heat continues to buildup and the speaker will reach its power handling limit at a lower power level. Using the previous example of a speaker rated for power handling of 100 watts, if this speaker has been playing at a few hours at a relatively high volume its power handling capacity will drop by 10-15%. The maximum power rating of the speaker would then be 85-90 watts, not 100 watts.
Ultimately, amplifier output power and speaker handling power have very little to do with each other. When pairing speakers and amplifiers it is more important to consider the style of music, the size of room, the sensitivity of the speakers, and the amplifier power output.
What is amplifier clipping, and can it damage speakers?
Clipping in solid state amplifiers generally happens under two conditions and is related to the power supply of the amplifier not being able to keep up with the demands of the music. When an amplifier runs out of voltage into an impedance over 8 ohms, or, when an amplifier runs out of current into an impedance lower than 4 ohms, it will produce a flat edged wave form similar to a square wave. Clipping is the elimination of the outer edge of the wave form both on the positive and negative side. The resulting flat topped wave form can cause damage to a speaker. A clipped signal of 5 watts or more is sufficient to cause damage.
Amplifier power output ratings
Amplifier power is usually rated at RMS output. Unfortunately, some manufacturers will fudge this number, using so called peak power or music power. Neither of these is a true measure of actual output power. The RMS power rating gives the most accurate indication of an amplifier’s power output.
These output power ratings are generally accepted as power into an 8 ohm load in home audio. Pro audio equipment most often rates power output into 4 ohm load. Rating RMS power output at lower impedance will yield a higher power rating. Some home audio manufacturers take advantage of this by rating the power output of their amplifiers into 6 ohms to mislead potential customers into thinking their amplifier has more power than an amplifier from other manufacturers.
A reminder - no matter what an amplifier’s rated output is, the amplifier cannot output any signal into a speaker until the user turns up the volume setting. It is the responsibility of the user to exercise their own judgement NOT to turn up the volume high enough to damage their speakers. Almost all speaker damage is a result of speakers being played too loudly for too long without being given a chance for a proper cool down period for the speaker drivers. And believe it or not, most speaker damage occurs when listening under the influence of alcohol. If you like to drink when you listen, you might want to consider getting a designated volume control operator.