Speaker Power Ratings Explained

What is a Loudspeaker Power Rating?

A loudspeaker's power rating tells you how much electrical power the speaker can handle from an amplifier without sustaining damage. It's measured in watts (W) and will be presented to you (confusingly!) in several different forms, with the three most common being RMS, Peak, or Max power. These ratings not only inform you about the capabilities and limitations of your speakers but also play a significant role in ensuring compatibility with amplifiers and preventing potential damage.

When using a speaker's wattage rating, RMS is by far the most accurate figure, representing the continuous power level that a speaker can handle safely over time without suffering from distortion or damage. In contrast, Peak Power refers to the maximum power level that a speaker can handle in very short bursts, and Max is the same but for a longer period. As a general rule, RMS is the true rating, with Max being 2x RMS and Peak being 4x RMS, so very exaggerated.

The 3 Most Common Power Ratings

• RMS Power Rating

  This is the most reliable measure of a speaker's power-handling capability. It indicates the maximum continuous power a speaker can handle safely. If a speaker has an RMS rating of 50 watts, it means it can handle 50 watts of power continuously without harm.

• Max Power

  The most common rating to be found on PA speakers and amplifiers, Max is a measurement that is taken with the speaker literally at its maximum output, taken over a period until the speaker fails. It's usually twice the RMS figure, so a 50W RMS speaker will be rated at 100W Max.

• Peak Power

  The maximum power level a speaker can handle in short bursts without getting damaged. It's much higher than the RMS rating and is not a reliable indicator of overall performance. It’s measured in the same way as Max, but driven even harder and the rating is taken in the short period before failure. Its generally 4x the RMS (though sometimes even higher). So a 50W RMS speaker will at best have a Peak rating of 200W.

As an example, a PA speaker might have a Peak power of 400W, which looks impressive, but that's 100W RMS, which suddenly doesn't look quite so amazing.

While there is nothing wrong with Max or Peak, it must be understood that Peak especially is an extreme measurement created for marketing purposes and not a true reflection of the performance level you should expect under normal usage, so you should always convert down to RMS for a real-world number.

The image below shows the standard specification sheet for our PD400 range of active speakers, with both Peak and RMS ratings

Why Speaker Power Ratings Matter

Understanding and respecting these ratings ensure that you:

• Avoid Damage

  Overpowering a speaker can lead to physical damage to the components while underpowering a speaker will lead to distortion and electrical damage.

• Achieve High-Quality Sound

  Properly matched speakers and amplifiers produce clearer, more dynamic audio, and will allow you to get the very best performance from your audio system.

• Maximise Longevity

  Using your equipment within its power limits can significantly extend its life. Many people don't understand that a speaker power rating has been determined at the design stage as the maximum level it can safely operate. It will of course go past this and carry on working (for a while!), but you risk permanent damage and it can happen very quickly. It's far better to get your desired volume running a powerful speaker at 60% as opposed to a less capable speaker at 100%.

• Avoid Disappointment

  It may sound obvious to some, but cost and performance generally go hand-in-hand. A low-cost set of speakers that's rated at 50W Max may look tempting, but will simply not be able to produce either the volume level or sound quality that even the smallest of gatherings would usually demand.

• Amplifier Matching

  An amplifier with a power output that exceeds the speaker's rating can lead to physical damage to the driver if not used carefully, while an underpowered amplifier (much worse) will be working so hard to provide the power and volume required that it will run hot and send distortion to the speakers, causing serious electrical damage their voice coils.

What is a Good Output Power for Speakers?

Determining a "good" output power for speakers depends completely on the intended use and the environment in which they will operate. For instance, a home setup is obviously not going to require as much power as a large outdoor event. Generally, a speaker with an RMS power rating between 15 to 40 watts can provide satisfactory sound quality in a typical home environment when matched to a suitable amplifier.

For larger rooms, speakers with an RMS rating of 50 watts or more are advisable. However, it's not just about wattage. The efficiency of the speaker to convert the electrical power from the amp into a physical movement (also called sensitivity) is of huge importance and is measured in decibels (dB). A speaker with a higher sensitivity rating (90dB and above) requires far less amplifier power to produce the same volume level as a less efficient speaker. Therefore, when considering output power, it's crucial to also take into account the speaker's efficiency figure and the size of the listening environment.

For PA speakers that are being used for different audience sizes, the old 5W per head rule gives you a loose guide - so if your gathering is 150 people, you really should be looking at speakers around the 750W mark (minimum!) to ensure they can cope. They should also be designed for that type of professional use, so no using Hi-Fi speakers trying to fill a hall and getting destroyed in the process.

The image below shows the info label on a passive speaker, showing Impedance, Max Power, Frequency Response, and SPL

How to Use Power Ratings

Knowing and understanding the power capabilities of your speakers is a huge part of getting a high-quality sound. It's an important part when planning and putting together any audio system, and will help with:

• Matching with Amplifiers

  Ensure your amplifier's output power is within the loudspeaker's power rating range. A good rule of thumb is to choose an amplifier that can deliver power equal to or slightly higher than the RMS rating of the speaker. This allows the amplifier to operate unstressed for the best performance.

• Capability

  Larger spaces might require speakers with higher power ratings to fill the room with sound effectively. The speakers needed for a living room or bedroom will be very different to those required for a mobile DJ or live band. A power rating can give you a rough idea of capability, though as we will discuss, its greatly affected by other factors.

While it is important to understand wattage and how it's presented to you (RMS, Max, Peak, etc), this is still purely a measure of a speaker's electrical capability. A 100W speaker can receive and convert from 0-100 Watts of electricity. If you try and feed it say 200W, it will work, get very hot, and then fail. It's worth noting that this is generally not an issue with active PA speakers as their power amp units and speaker drivers are matched correctly, but it's still worth being aware of.

Similarly, if you feed that same 100W speaker with 80W, but it's from an 80W-rated amplifier, you will also cause damage to the speaker even though you aren't near its rated maximum. This is because the amplifier is working so hard that its components are fighting for their life, getting roasting hot doing so. This causes it to clip and distort the output stage, which destroys the speaker's voice coil through the presence of DC in the audio signal. (this can also happen with an over-gained input signal).

So what about Impedance?

We will delve a bit deeper into impedance a bit further on, but for now, it's important to note that the impedance rating of speakers (the resistance it gives the amplifier from delivering its power, given in Ohms) also affects its power handling capacity. Speakers with a lower impedance require less power to reach a given volume level but can be more demanding on the amplifier, causing it to run hot. Certainly, when selecting passive speakers of any type, it's essential to consider both the stated power rating and the impedance to ensure compatibility with your audio system and to achieve the desired sound quality.

Tips for Beginners

• Don't Obsess Over Watts Alone

  Such a huge emphasis has been placed on wattage when it comes to speakers that for many it's all that matters. However, it's only a piece of the puzzle, and often not even the most important one. Wattage is an electrical consumption rating (your kettle is likely 1500W for example), so has no direct rating to sound, only the speaker's electrical capability. The other figures, sensitivity, impedance, and SPL, are crucial factors when determining a loudspeaker's possible performance.

• Head over Heart

  It's easy to get swayed and sidetracked when looking at shiny new gear. The latest designs, the inclusion of fancy features, the lure of a big-name brand. All will be vying for your attention, and it's easy to convince yourself of what you want rather than what you need.

• Read Reviews and Specifications

  Look beyond marketing terms and focus on detailed product specifications and user reviews if available. The information is always there if you know how to decipher it, and there is no reason not to create a list of suitable candidates based on your budget, and filter down your choices logically.

• A Flexible Budget

  As with any purchase, it is usually advisable to go for the best you can afford, but with audio equipment, this is especially so. The difference in performance and overall quality can jump significantly with a difference of just a couple of hundred pounds when it comes to speakers, so try and factor in a little wiggle room with your funds.

• Consult with Professionals

  If you're unsure, or are half sure and just want some confirmation, it's always a good idea to seek advice from audio professionals. Any decent retailer will be more than happy to help you with anything from a simple question to putting together a system based on your needs.