Complete Guide To Speaker Power Handling & Wattage Ratings

There are many specifications used to define speakers, and power handling or “wattage rating” is one of the most common specs we encounter when reading into using or buying a speaker. The spec will look something like 1000 W; 350 W_rms, or 800 W_peak.

What is speaker power handling (wattage rating)? The speaker power handling specification (aka wattage rating) is the measured or theoretical limit of electric power the speaker is capable of handling before burning out. The spec is given in watts and can be measured/calculated as a continuous, peak, or root mean square (rms) value.

In this article, you'll learn all about the somewhat confusing wattage rating (power handling specification) of speakers and how it may not or may not be useful to you when choosing speakers.

What Is Power?

To understand the power handling ratings of speakers, we should understand what power is in the first place.

Electric power is defined as the rate per unit time at which an electric circuit transfers electrical energy.

It is measured in the SI unit of watts (W). 1 watt is equal to the transfer of 1 joule per second.

The power amplifier in our signal chain provides amplified audio signals that can properly drive our speaker(s) to produce sound. The transfer (strength) of these audio signals is typically rated as an amount of electric power (in watts).

Speakers Are Transducers

Basically, speakers are transducers that convert electrical energy (audio signals) into mechanical energy (sound waves).

In the vast majority of cases, this is done via electromagnetism. The audio signal passes through a conductive voice coil, inducing a magnetic field that reacts with a permanent magnet to cause diaphragm movement.

Essentially, the alternating current of the audio signal causes a varying voltage across the conductive voice coil, which, in turn, causes the diaphragm (attached to the voice coil) to move in accordance with the electrical signal.

In this way, the speaker's sound output waveform mimics that of the audio signal waveform.

The transducer elements of a speaker are called drivers. For more information, check out the following My New Microphone articles:
• What Are Speaker Drivers? (How All Driver Types Work)
• How Do Speakers & Headphones Work As Transducers?

Why Is Power Used To Rate Speakers?

So if it's ultimately the alternating electrical current flowing in the voice coil that causes the speaker to produce sound, why are we concerned with power? Why not be concerned with voltage and current itself?

We'll start with current.

Though ultimately what audio transducers convert, electrical current is not a good measurement for electrical audio devices.

This is because audio devices (like speakers and amplifiers) have impedances. Electrical impedance resists the flow of alternating current.

Since different devices have different impedances and these devices can be mixed and matched within a signal chain, current is rarely used to describe the specs of the devices.

There are just too many variables. Attempting to account for every connected device in an audio chain would be futile.

What about voltage?

Actually, voltage is generally used to measure audio signal levels and is regularly used in audio device specifications.

In audio, crudely speaking, we generate a voltage (by electromechanical means like a microphone or vinyl needle/stylus, or via digital-to-analog converters). We use this voltage to get a current. This current is resisted by the impedance of the audio devices in the signal chain.

So then, voltage is commonly used to measure signal strength. This is true of microphone signals and analog line signals, whether they're measured in:

• Millivolts (mv)
• Volts (V)
• Decibles relative to 1 volt (dBV)
• Decibels relative to 0.775 volts (dBu)

But the fact remains that power is generally used at speaker level to define the “output levels” of power amplifiers and the “input levels” of loudspeakers.

One reason is that, while the voltage of the audio signal and current in the signal chain will oscillate between positive and negative values (current will flow in both directions), power is always positive.

In this way, power is a bit easier to understand as a value. Note that the AC power of an audio signal, like voltage and current, oscillates between a peak and a trough.

It can be snuffed up to old nomenclature as a way to help us select suitable amplifier and speaker matches.

Power ratings, in general, are useful and relate to voltage, current and resistance (which we often replace with impedance) by the following equations:

P=IV

P=\frac{V^2}{R}

P=I^2R

Where:
P = Power
I = Current
V = Voltage
R = Resistance

Of course, these formulae simplify matters of AC audio signals but can be used to effectively understand the way electrical power works between an amplifier and a speaker.

Speakers & Amplifiers

Speakers are required to produce sound waves out of electrical signals. This takes a lot of work, and when we factor in the inefficiency of the typical moving-coil driver, we see that speakers require amplifiers.

To learn more about speaker efficiency (or lack thereof), check out my article Full Guide To Loudspeaker Sensitivity & Efficiency Ratings.

Amplifiers act to increase the signal strength (voltage/power) of the audio signal. They boost line level signals (used in recorded audio, mixing boards, etc.) to speaker level signals.

With the help of external power and gain, amps will take line level signals at their inputs and output speaker level signals at their outputs. The input is connected to a mixing board, playback device, etc., and the output is connected to the speaker(s).

Another way to look at the speaker-amplifier connection is that the speaker draws power from the amplifier.

This is commonly brought up when discussing speakers of different impedances. For example, a 4Ω speaker will draw more current than an 8 Ω speaker using the same amplifier.

Amplifier outputs are rated with power as well. Their power output ratings are generally given at a certain frequency (often 1 kHz) into a common load (speaker impedance) of 4Ω, 8Ω, etc.

It's critical to note that we do not need to match the amplifier's power output rating with the speaker's power handling rating.

In fact, a speaker with low power handling can be connected to an amplifier with a higher output so long as the amp isn't turned up too loud.

Similarly, a speaker with high power handling can be connected to an amplifier with a lower output so long as the speaker isn't trying to draw too much power from the amp (this is generally only an issue with low-impedance speakers).

It's all about keeping the electrical power between the amplifier and speaker below a certain point to avoid overheating either.

The Power Handling Specification

The power handling specification of a speaker is the maximum electric power it is capable of handling from an amplifier before it begins to sustain damage.

There are two main ways in which excess power will damage a speaker.

This results in 2 different types of power handling:

• Thermal power handling
• Mechanical power handling

What Is Thermal Power Handling?

Thermal power handling refers to the limit of power the speaker can handle before its voice coil begins to burn and/or melt.

As discussed earlier, the audio signal (alternating current) flows within the voice coil due to the electrical power supplied by the amp.

Some of this power is used to move the voice coil (and diaphragm) to produce sound. However, most of it is lost as heat due to the remarkable inefficiency of the speaker.

The more power sent to the speaker, the more heat is dissipated.

Typically this heat is dissipated from the surface area of the voice coil as it oscillates back and forth within the magnetic gap. We can think of the speaker's motor as a sort of air-cooled motor.

However, there is a threshold at which the speaker will no longer be capable of dissipating enough heat to keep the voice coil safe. At this point, the voice coil will burn and/or melt, and the speaker will sustain permanent damage.

This type of burn-out happens when the thermal power handling limit of the speaker is exceeded.

For more information on speaker burn-out, check out my article Loudspeaker Blow-Out: Why It Happens & How To Avoid/Fix It.

A melted/burned voice coil is the most common way in which a speaker will burn out. Therefore, a speaker's power handling limit or wattage rating generally refers to the thermal power handling limit.

What Is Mechanical Power Handling?

So overloading a speaker will typically cause the voice coil to burn/melt due to thermal limitations. However, speakers can also be overloaded mechanically.

There are two main thresholds of mechanical movement in a speaker driver:

• Maximum linear movement
• Maximum mechanical movement

The first is the point at which the speaker ceases to perform linearly. That is, it begins to distort.

This maximum linear movement is defined as the point at which the voice coil has moved far enough outside of the magnetic gap that the coil no longer experiences the full magnetic flux density of the motor.

At this point, the electrical audio signal no longer has as much control over the motor movement. This results in non-linearities (aka distortion) in the sound produced by the speaker.

The maximum mechanical movement is past the linear threshold to the point at which the speaker can no longer move.

This happens inwardly when the coil slams against the rear plate of the magnetic structure. It also happens outwardly when the diaphragm moves to the point of stretching its surround.

Exceeding the mechanical limitations of the speaker leads to damage and can be caused in certain instances by supplying too much power to the speaker.

As discussed, however, the thermal limit is much more apt to be exceeded.

Subwoofers are really the only speakers that may possibly reach their mechanical limits before reaching their thermal limits. There are two main reasons for this.

First, the voice coil of a subwoofer is relatively large and, therefore, better at cooling itself.

More importantly, however, is the amount of excursion required of the subwoofer. To produce the lowest frequencies of the audible spectrum (down to 20 Hz), a subwoofer must push a lot of air.

In addition to having a large diaphragm area, a subwoofer must oscillate great distances to produce low-end frequencies with any amount of loudness. When too much power is applied, the subwoofer may indeed be forced into over-excursion where it will become damaged.

Measuring Power Handling: Peak, RMS, Continuous & More

The most confusing part about speaker handling is knowing what the specification is actually referring to.

So far, we've covered that the power rating of a speaker is the maximum power the speaker can handle before burning out or otherwise becoming damaged.

However, that's not all there is to the story. We must understand the time horizon for which the power handling rating holds true.

Will any instant of time above the rating burn out our speaker, or is the limit referring to the safe amount of power that can be sustained for hours at a time?

This is where the numerous variations of power handling specifications come into play. They include:

• Peak
• RMS
• Average
• Continuous
• Program
• Nominal

Of all these specs, the peak value is the one that should never be exceeded.

However, with the other values, we may periodically surpass the threshold without causing speaker damage. The peaks of a dynamic audio signal may very well send spikes in power to the speaker.

So long as we stay, on average, below the “non-peak” points, our speaker should stay safe.

Remember that power handling is mostly about heat dissipation. We can turn up the heat for brief periods of time so long as we bring the heat back down to a certain level for the majority of the time to allow the voice coil to cool off.

Unfortunately, there are quite a few variations of the speaker power handling spec, which causes confusion. Different manufacturers use different terminology and, even worse is, some have different definitions for the same terminology.

It’s always best to find out how the manufacturer comes up with its speakers’ power handling specifications to know exactly what you’re reading when it comes to power handling.

That being said, let’s try to make sense of it all.

In the explanations below, I'll be using a theoretical speaker with the following specifications:

• Nominal impedance : 8 Ω
• Peak power handling: 1000 W

With this theoretical example speaker, we'll figure out the variations in the different power handling ratings.

Peak Power Handling

Peak power handling refers to the maximum power the speaker can handle for any instant in time. If, at any point, the speaker draws more than power the peak power rating, the speaker will sustain damage.

Peak power is often the preferred method for marketers as it gives the highest wattage rating. Bigger numbers typically look better to the consumer.

In the case of the example speaker, the power handling is listed as 1000 W.

Using the power equation P = V² / R and the nominal impedance, we find the peak voltage of the circuit to be 89.44 V_peak.

RMS Power Handling

RMS power handling is actually an erroneous term though it's commonly used in speaker specifications sheets.

To understand this variation of the power handling spec, we must first understand what RMS is.

RMS (root mean square) is technically a measurement of the square root of the mean square (the arithmetic mean of the squares of a set of numbers).

Alternating current (and voltage) goes in both directions, and so will both positive and negative values. This is easily seen in a sine wave. Let's blow up the voltage portion of the above diagram here:

The average amplitude (in volts) of the above sine wave is actually 0 volts because the signal spends equal time and amplitude in the positive as it does in the negative.

However, these signals still produce results and drive speakers. The trick is to calculate the average of the absolute amplitude of the sine wave rather than the actual amplitude.

This is where RMS comes in handy.

Let's look at the calculations for root mean square voltage since we're currently discussing voltage.

For complex waveforms, the RMS calculation is:

V_\text{RMS}=\sqrt{\frac{1}{T_2-T_1}}\int_{T_1}^{T_2} [f(t)]^2 \, dt

For simple sine waves (which have a single frequency and are typically used in speaker specification calculations), the RMS equation can be boiled down to:

V_\text{RMS}=V_\text{Peak}\sin(2πft)=\frac{V_\text{Peak}}{\sqrt{2}}≈0.707 V_\text{Peak}

Using the 89.44 V_peak we calculated from the 1000 W Power_peak of our 8Ω speaker (assuming a sine wave), we can calculate that the V_rms is equal to 63.24 V_rms.

The peak voltage and rms voltage are shown on the diagram below:

As an aside, the RMS value of a DC voltage is simply the amplitude of the DC voltage itself. Of course, audio signals are, by nature, AC, but this may help us to understand.

Power, which we know is always positive, does not have a root mean square value. Rather, we can actually calculate the average amplitude of the power rather than relying on any root mean square formulae.

So then, what does RMS Power (P_rms) mean?

Well, it should mean the average power handling limit according to the maximum rms voltage the speaker can handle. So then:

P_\text{"RMS"}=P_\text{avg}=\frac{V_\text{RMS}^2}{R}

In the case of our example speaker, the average power rating would be equal to 63.24 V_rms² divided by 8 Ω (nominal impedance).

This gives us an “rms” average power of 500 W, assuming a perfect sine wave signal.

This is easy visualized if we just look at the graph showing peak power:

It's easy to visualize the average power by looking at the graph above.

However, to help solidify our knowledge, let's briefly discuss power and voltage in terms of power and root-power quantities. What do I mean by this?

• Power quantities are quantities directly proportional to power.
• Power-root quantities (sometimes referred to as field quantities) are quantities that, when squared, are proportional to power in linear systems.

Electrical power and acoustic power/intensity are power quantities, whereas voltage, current, and sound pressure level (SPL) are power-root quantities.

This helps to explain the equations

P=\frac{V^2}{R}

And

P=I^2R

It also helps to explain (if we oversimplify and make rms and average the same thing) why the average “rms” power is 1/2 peak power while the “average” rms voltage is √(1/2) peak voltage.

Everything would work out if we plug 1/2 P and √(1/2) V into the equation

P=\frac{V^2}{R}

Unfortunately, RMS power handling has come to mean the amount of continuous power the speaker can handle. This is technically incorrect though sometimes a suitable limit for speakers.

The “rms” power, which equals half the peak power, is simply a calculation based on mathematics rather than an actual rating based on speaker testing.

Even still, some manufacturers list “RMS Power” as being the continuous power limit the speaker can handle.

Average Power Handling

As we've discussed, the average power is equal to the RMS voltage squared divided by the resistance (or impedance at the given frequency) of the speaker.

It would also be equal to the RMS current times the RMS voltage (P = I • V).

P_\text{avg} = I_\text{avg} V_\text{avg}

P_\text{avg} = \frac{V_\text{RMS}^2}{R}

However, power handling is rarely, if ever, given as “average.” Rather, it is given as the somewhat confusing RMS value.

Continuous Power Handling

Continuous power handling (often called inaccurately “RMS power”) is the wattage that a speaker can comfortably handle for an extended period of time.

To arrive at this value, manufacturers can actually test the speaker's limits by running pink noise through the speakers for hours on end.

Various tests can be conducted, testing for the power level that will, over time, cause the voice coil to burn out.

The testing methods I've mentioned are vague, and it's ultimately up to the manufacturer to lay out the test procedure for us to understand how they conclude the continuous power handling specification.

In many cases, the pink noise used in the test will have a crest factor between 2 and 2.828 (√8). In other words, the RMS value of the pink noise signal will be between 0.5 to 0.3536 that of the peak.

To understand this difference, let's get into the weeds about crest factor and decibels.

As we've mentioned, pink noise can have various crest factors, but the crest factor in testing is typically 2. I'll use a crest factor of 2 for this explanation.

The crest factor is a waveform parameter that describes the ratio of peak values to the effective rms value. In other words, the crest factor indicates how extreme the peaks are in a waveform.

A crest factor of 1 would indicate no peaks, such as direct current or a square wave. Higher crest factors indicate peaks and are common for audio signals to have.

So then, a pink noise signal with a crest factor of 2 would have an rms voltage 1/2 (0.5) times its peak value.

A pink noise signal with a crest factor of √8 would have an rms voltage of 1/√8 (~0.354) times its peak value.

A sine wave, as another example, has a crest factor of √2 (~1.414) and an rms voltage of 1/√2 (~0.707) times its peak value.

The crest factor can also be expressed as the peak-to-average power ratio (PAPR). The PAPR is given as the peak amplitude squared (giving the peak power) divided by the RMS value squared (giving the average power).

PAPR is simply the square of the crest factor. However, it is normally expressed in decibels (dB).

When measured in dB, the crest factor (C) and the power-to-average power ratio (PAPR) are equal due to the power being a power quantity and the voltage being a root-power quantity. For reference, here are the equations:

C=\frac{|V_\text{Peak}|}{V_\text{RMS}}

C_{dB}=20\log_{10}(\frac{|V_\text{Peak}|}{V_\text{RMS}})

PAPR=\frac{|V_\text{Peak}|^2}{V_\text{RMS}^2}

PAPR_{dB}= 10\log_{10}(\frac{|V_\text{Peak}|^2}{V_\text{RMS}^2})=C_{dB}

Crunching the numbers gives us the following:

• PAPR of sine wave (crest factor 1.414) = 3 dB
• PAPR of pink noise (crest factor 2) = 6 dB

Or, in other words:

• The peaks of a sine wave will be 3 dB higher than the sine wave's rms value.
• The peaks of the pink noise will be 6 dB higher than the pink noise's rms value.

As we can see, the speaker will have an easier time (they'll produce less heat) producing pink noise than simple sine waves at the same rms level.

In general, the continuous wattage rating will often end up being about 25% of the peak power handling.

This is because if the pink noise yields a PAPR of 6 dB, it means it will take 6 dB of power (a quadrupling of power) to reach the real peak level.

Continuous power handling specifications are the most useful because they give us a sense of the average power we can safely supply to the speaker for extended periods of time.

So for our theoretical 1000W 8Ω speaker, the continuous power handling would be around 250W.

Program Power Handling

The program power handling specification can be thought of as a value for the recommended output power of the speaker's power amplifier.

The music/program rating is nearly always twice the continuous rating.  It is a higher rating because music has many peaks and dips and is not as abusive as a continuous signal. 

Each doubling of power yields a 3 dB increase (and each halving of power yields a 3 dB decrease).

So the continuous power handling rating (measured with pink noise that usually has a crest factor of 2) puts us (usually) a healthy 6 dB below the peak. This means the power is about a quarter of the peak power, so we have 6 dB of headroom with the amplifier.

It's typically recommended to have an amplifier capable of keeping up with the speaker's continuous power rating. However, it's also important to have an amp that can properly produce the peaks in the audio signal.

An amplifier with double the speaker's continuous power handling rating can safely drive the speaker and will typically handle the peaks in the audio without distorting.

For this reason, the program spec is often twice (or roughly twice) the continuous power rating. Again, it all depends on how the manufacturer tests for continuous power handling and how confident they are about recommending an amplifier.

Remember that the program/music power rating is less about the speaker's limitations and more about helping the user choose an appropriate amplifier.

For your information:

• Speech, which is commonly reinforced through a microphone and PA speakers, has a typical crest factor and PAPR of around 12 dB.
• Music often has a crest factor and PAPR of about 18 dB and even more for dynamic music.

So for our theoretical 1000W 8Ω speaker, the continuous power handling would be around 250W, and the program power handling would be around 500W.

Nominal Power Handling

Unfortunately, there is great confusion about nominal power handling as well. If a manufacturer uses this variation of power handling on their sheet, please consult with the manufacturer about what they mean.

In some cases, it simply means the same thing as continuous power handling. In others, it’s defined as half the continuous rating.

And even more definitions can be found, including the following:

• This is the maximum power handling of the speaker calculated at its nominal impedance.

• It is the max theoretical electric power that would be transferred from amplifier to speaker if the loudspeaker was actually exhibiting its nominal impedance. The actual electric power may vary from about twice the nominal power down to less than one-tenth.

All this is to say, once again, that it’s best to find out how the manufacturer tests for power handling to understand the specification properly. It’s unfortunate, but power handling is actually a poor specification to use when comparing speakers and/or matching an amplifier to a speaker.

Are Speakers With Higher Wattage Ratings Louder?

When we see a large power rating on a speaker, we generally assume that the speaker will be loud but is a higher power handling rating actually indicative of a louder speaker?

Well, as we've mentioned in the previous section, it depends on what variation of the power handling spec we have.

We can safely assume that all else being equal, a 250W continuous speaker will be louder than a 250W peak speaker.

But there are other factors that affect the loudness of a speaker.

Before we begin, I'd like to mention that loudness is an acoustic and psychoacoustic phenomenon, unlike the electrical factors we've been discussing up to this point.

Two people may experience the exact same sound in a different way depending on their own psychoacoustic (hearing) profile and the acoustics of the space around them.

It's also true that while electrical and acoustic power are power quantities and voltage, current and sound pressure levels are root-power quantities, perceived loudness is strictly psychological.

That being said, decibels can still be used to approximate “loudness” vs power and sound pressure level:

Maybe that's a bit off track, but a fairer question could be:

Do speakers with higher power handling ratings produce more acoustic power than speakers with lower power handling ratings?

The answer to this question, which relates to loudness, is “not necessarily”.

Allow me to explain.

Perhaps the more obvious reason is that the speaker's acoustic power depends on the amplifier's power output.

For example, if an amplifier is outputting 100 W of power to a 1000W speaker and another amp is outputting 500 W of power to an 800W speaker, then, all else being equal, the 800W speaker will be louder.

This may seem obvious, but it is worth mentioning.

Another key factor in determining the acoustic power output of a speaker is the sensitivity rating and, by association, the efficiency rating.

Speaker sensitivity ratings measure the sound pressure level of a speaker at a distance of one meter (on-axis) when the speaker draws 1 watt of power.

Efficiency is the ratio of the acoustic output power of a speaker to the electric power drawn by the speaker.

Let's continue with our examples of an 800W and a 1000W speaker.

Let's say that the 1000W speaker has a sensitivity rating of 84 dB SPL @ 1W/1m, and the 800W speaker has a sensitivity rating of 90 dB SPL @ 1W/1m.

So then, at any given power (within the limits of each speaker), the 800W speaker will produce 6 dB more sound pressure at 1 metre (and any distance for that matter) than the 1000W speaker.

For the 84 dB SPL @ 1W/1m speaker to produce the same SPL at a given distance as the 90 dB SPL @ 1W/1m speaker, it would require 6 dB of extra gain.

This 6 dB gain increase is a 4x increase in amplifier power.

So then, power handling ratings have to do with the maximum power the speaker can handle but do not necessarily mean the speaker will be louder.

That being said, a 1000W speaker, when paired with an appropriate amplifier that is turned up, will certainly sound louder than, say, a 100W speaker with an appropriate amp that is turned up. The speaker with the lower power handling just wouldn't be able to keep up without burning up.

Ultimately, maximum SPL and sensitivity ratings are more conducive to the loudness of a speaker than are power handling ratings.

Engineering Or Marketing?

Oftentimes we'll see peak power handling ratings on speakers. Though this variation is absolute, it doesn't really tell us all that much.

As discussed, the continuous rating and even the program rating are more useful power handling specs to use.

However, bigger is better, so marketers often use peak power handling to make the speaker look like a better choice.

Of course, peak power ratings are important to know to keep the speaker safe in extreme situations. Still, it's arguable that the spec is more of a selling point than a useful piece of info for the user, especially when the peak power rating is the only wattage rating given.

Active Vs. Passive Speakers

Though active speakers sometimes have power handling ratings, it's typically a passive speaker specification.

This is because passive speakers require external amplifiers to drive them properly. We can “mix and match” speakers and amplifiers, so knowing the power handling limitations of the speaker (and of the amplifier) is important when making an amp-speaker matching decision.

Active amplifiers are designed with built-in amplifiers, and so while their drivers certainly do have power handling limitations, the amps and drivers are built to work together. So the power handling rating is less of a concern to the user.

To recap, yes, active (and powered) speakers have power handling limitations, but the power handling spec is generally more useful for passive speakers that require a separate amplifier.

Sometimes we'll see the peak power handling spec for an active amplifier as a marketing spec.

Does Wattage Matter? What Is A Good Power Handling Rating?

While wattage certainly matters, it's not overly important unless we plan to drive the speakers with heavy-duty amplifiers.

More useful specifications, as we've discussed, include maximum SPL, sensitivity and efficiency.

Two speakers with the same wattage (power handling) rating can have differing sensitivity and efficiency ratings. The speaker with a higher sensitivity and efficiency rating will produce more sound at a given wattage.

Since both speakers have the same power handling, the speaker with the higher sensitivity will also be capable of a higher maximum SPL.

A “good” power handling rating is subjective to your listening habits and depends on the speaker's sensitivity and the intended listening position.

Remember that listening safety is critical to the health of our hearing. Here is a table representing the safe listening levels as recommended by the NIOSH (National Institute for Occupational Safety and Health) and OSHA (Occupational Safety and Health Administration):

So let's say we want a speaker that can provide a safe 90 dB SPL (according to OSHA, we can listen to 90 dB for 8 hours safely). We want to listen to the speaker at a distance of 1 metre.

• A speaker with sensitivity 90 dB SPL @ 1W/1m will only require 1 watt of power to make this happen so any continuous power handling rating above 1W would suffice.
• A speaker with sensitivity 84 dB SPL @ 1W/1m will require +6 dB of gain to produce 4 watts of power to make this happen so any continuous power handling rating above 4W would suffice.

Practically every speaker has a power handling above 4 watts.

But now, what if we wanted an SPL of 102 dB at a distance of 8 metres from the speaker (let's say we're at a very loud concert).

Now, we would generally use multiple speakers to achieve this SPL rather than relying on a single speaker. Let's say we use 4 identical speakers. This means that there will be 4 times with acoustic power output (+6 dB), so each speaker could be turned down 6 dB to achieve the same result.

We must account for a drop of 6 dB for every doubling of distance. Therefore, 102 dB at 8 meters would be 120 dB at 1 meter (this helps in calculations that include sensitivity).

So then, each speaker would be required to produce 120 – 6 = 112 dB at a distance of 1 metre.

A speaker with a sensitivity of 90 dB SPL @ 1W/1m will require 22 dB of gain. This means that each speaker would require 159W of average power (peak power of 317W) to reach this level. Any speaker with power handling above these values would work (assuming an appropriate amplifier).

A speaker with a sensitivity of 84 dB SPL @ 1W/1m will require 28 dB of gain. This means each speaker would require 631W of average power (peak power of 1262). These values are incredibly high, and it's unlikely you'll find a speaker with these specs designed for the purpose of live sound. This is just an example.

Again, these are theoretical examples to help illustrate the variety of speaker applications and what constitutes a subjectively “good” power handling rating in a speaker.

Speaker Minimum Power Ratings

If a speaker requires a large amount of power, it may come with a minimum power rating.

As the name suggests, this specification refers to the minimum power level required to drive the speaker to produce any noise at all.

The amplifier, therefore, must be capable of outputting more power than the minimum rating to work with the speaker. This scenario is rare unless we have large speakers and puny amplifiers.

Amplifier Power Rating

Amplifiers also have power ratings that pertain to their outputs.

Generally, these ratings include the following information:

• Measurement variation: like speaker power handling, amplifier power ratings can be measured/calculated as peak/PMPO, RMS/average and others.
• Watts per channel: how many watts can be produced by/drawn from each channel of the amplifier (each channel sends one audio signal to one speaker or to multiple speakers connected in series or parallel).
• Maximum power output per impedance: different speakers present different load impedances to the amplifier. Speaker will generally provide more power to lower load impedances.
• Frequencies & frequency ranges: the specific power ratings will generally be measured at single test frequencies or across specified frequency ranges.
• Distortion: the point at which the amplifier will begin to distort (typically measured as a percentage value of total harmonic distortion).

Related Questions

Do amplifier and speaker power ratings affect sound quality? All else being equal, a difference in power handling or power output limitations will not cause a difference in sound quality unless, of course, these limits are exceeded into the realm of distortion.

What amplifier class is the best? Each amplifier class has its own benefits and drawbacks, and the term “best” is subjective. Objectively, however:

• Class A/B has the best cost-to-performance ratio.
• Class D is the most efficient (coolest).
• Class A has the purest sound.

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