Speaker power ratings are among the most confusing and misleading specifications in the car audio industry. Once you understand the process used to test speaker power handling, you’ll quickly realize that the information doesn’t always translate to information that’s helpful in choosing an amplifier. Let’s dive into the science behind choosing the best amplifier for your car audio speakers.
Speaker Power Ratings
In short, how much power a speaker can handle depends primarily on the voice coil’s diameter, length and number of windings. Secondary considerations include the gap size between the voice coil assembly, the T-yoke and the top plate of the speaker. Closer proximity helps to improve heat transfer away from the voice coil. A smaller gap also increases efficiency. However, if the gap is too tight, the voice coil or former might rub, which can cause damage and distortion. Cooling technologies like a vented pole piece, vents under the spider mounting plateau and vents in the former also help by allowing air to flow around the voice coil.
The type of enclosure used also plays a role in power handling. A sealed enclosure will trap heat around the motor assembly. A vented enclosure will allow heat to escape as air is exchanged through the vent resonance.
Audio Content and Power Handling
A concept that many audio enthusiasts don’t fully understand is how power is distributed based on audio frequencies. Looking at frequency response measurements on a real-time audio analyzer can exacerbate this misunderstanding.
When we look at acoustic audio measurements of pink noise on an RTA, we expect to see a flat line. This indicates that the amplitude of each frequency is equal. While we might want to bump up the bass to make the system fun or attenuate the high-frequency content by a few decibels, we don’t want peaks or dips anywhere in the graph.
Where the confusion lies is that the technician working on calibrating your car audio system is using pink noise with the RTA. Pink noise contains random frequencies with the same energy per octave. Put another way, dividing power by the range of frequencies in a given octave will give you the same energy per hertz.
Between 10 and 100 hertz, there are 90 1-hertz frequency bands. We have 900 bands between 100 and 1,000, and 9,000 between 1,000 and 10,000. Let’s say we have 10 watts of power to distribute among those frequencies. In our lowest octave, each one of the hertz bands gets an average of 111.11 milliwatts. In the band between 100 and 1000 hertz, each hertz band receives an average of 11.11 milliwatts. In the top octave between 1,000 and 10,000, each hertz band receives an average of 1.11 milliwatts. Once again, this power distribution produces a flat line on an RTA graph.
The bottom line is that pink noise matches how humans perceive sound. Our ears perceive pink as having the same volume level at all frequencies. Pink noise is occasionally called an equal intensity curve.
What Does Frequency Have To Do with Speaker Power Ratings?
Imagine, if you will, a typical mid-level car audio subwoofer. It might have a 2-inch diameter voice coil former with a winding that’s 1.5 inches tall with a two-layer winding. Rated power handling might be around 400 watts. Now, let’s consider the voice coil in a typical 6×9 speaker. The diameter might be 1 inch, and the winding might be 0.75 inch tall. The speaker might have a continuous power rating of 100 watts. All of this makes sense so far. Less mass in the winding means it can handle less heat.
Now, let’s think about a tweeter. It likely has a voice coil diameter of 1 inch, but the winding might be 0.2 inch tall, and it will surely have no more than one layer. Worse, the wire will be tiny in diameter. Even then, many stand-alone tweeters have a power rating of 100 watts. How can this small voice coil dissipate 100 watts of heat? What about P.A.-style speakers? They often have extremely short voice coils. Yet some claim to have 200-, 300- and even 500-watt power handling ratings. How is this possible?
If the power handling test uses filtered pink noise, then the speaker is tested with less power. A tweeter can’t reproduce bass or midrange frequencies. So, to test their power handling, the noise waveform would be filtered at something like 3,000 or 4,000 hertz. Filtering the bass information removes significant energy from the signal.
If we apply a 2-kHz high-pass filter to a 100-watt equivalent pink noise signal, the result would be only 1 watt of power going to the speaker. Midrange and high-frequency speaker power ratings are almost always quantified this way. So, your 100-watt tweeter can only handle 1 watt of power. Your P.A.-style midrange likely can’t play much below 300 Hz. It might only get 3 or 4 watts of power if appropriately filtered from our 100-watt example.
Matching Amplifiers to Speakers
Now that we’ve set the stage for understanding power ratings, we can finally talk about matching amplifiers to speakers. How powerful of an amplifier do you need for your speakers? The answer starts with the frequency range in which you’ll operate the speakers. With subwoofers, you’ll be playing bass frequencies, so almost all the energy in the music will arrive at the speaker. You’re in the same boat if you have a system with 6.5-inch or 6×9-inch speakers and no subwoofer. You’ll be sending bass information to the speakers. If your system has a subwoofer, you’ll likely only send frequencies at 80 hertz and above to the speakers. That’s about 1/10 of the maximum power compared with a full-range signal. In theory, you only need 1/10 the power to your mids as your subwoofers need. So, 500 watts to a sub and 50 watts to the mids. If you have actively filtered tweeters, they likely only need a few watts.
A lot of this is theoretical rather than practical. So, let’s look at midrange speaker power handling another way. Let’s say you have a set of mid-quality component 6.5-inch speakers. They have a 100-watt power rating, and the woofer might have an Xmax specification of 4 mm. Let’s examine how much the woofer cone moves with 100 watts of power.
The graph above shows us that the driver reaches its 4 mm excursion limit at 110 hertz. If you play music with content lower than 110 hertz, the driver might bottom out or, at the very least, sound terrible. If you want your audio system to sound terrible, driving midrange speakers beyond their excursion capabilities is a great way to do it. With an 80 hertz crossover point, 100 watts is too much power. As it turns out, 50 watts at 80 hertz results in a cone excursion of 4 mm.
What About Time?
If you look at speaker specifications, you’ll see both continuous and peak power ratings. Some companies incorrectly refer to the continuous rating as an RMS rating. Using the term RMS implies that the power measurement was done with an RMS current or voltage measurement, not the waveform’s peak values. RMS refers to the amplitude in an AC waveform that can do the same work as an equivalent DC voltage.
Companies with genuine engineering specifications for their speakers will test them at their continuously rated power level for eight to sometimes over 100 hours. The speaker needs to continue to function after the test, and the Thiele/Small parameters should typically remain within 10% of where they were when the trial started. In other words, the voice coil can’t overheat or fail, and the suspension can’t stretch significantly.
With all that said, speakers can handle momentary bursts of additional power beyond their ratings. The problem is that how long these bursts can last without causing damage is difficult to quantify. Let’s say you’re listening to a song with a vocalist and someone playing a guitar. In the middle, there’s a drum break like Phil Collins’ solo from “In the Air Tonight.” If you cranked up the volume during that solo, even at twice a speaker’s continuous rated power level, that’s not enough energy to overheat the voice coil. So as long as the speaker isn’t physically damaged, everything should be fine.
Picking the Best Amplifier for Your Car Audio Speakers
So, after all this science and confusion, how do you pick the best amplifier for your car audio speakers? You choose the amp that sounds the best. Whether an amp makes 45, 50 or 60 watts doesn’t matter, as that’s only a difference of 0.5 or 0.8 dB in maximum output. Is a 100-watt amplifier “better” than a 50-watt amplifier? It is if it adds less noise and distortion to the signal that passes through it.
Think about how reputable companies group the amplifier series they offer. ARC Audio has the ARC Series. Rockford Fosgate has the Power Series. Kicker has the IQ Series. Audison has the Thesis Series. Hertz has Mille. Sony has the Mobile ES line. Aside from some additional features, these higher-end amplifiers sound better than the lower models. They have better signal-to-noise ratio specifications and lower total harmonic distortion numbers. When playing the same music through the same speakers at the same volume, the sound produced by higher-quality amplifiers is more precise and accurate.
How Much Power Do My Speakers Need?
We can’t count the number of times we’ve seen posts on social media asking, “What amp is best for my speakers?” The poster will then list the speaker’s continuous power-handling capabilities, even if the number is irrelevant. You already know your tweeters will never need more than a few watts, so why would you use a 100-watt amp to drive them? Well, if the amp you have in mind has excellent distortion and noise measurements at those low levels, your tweeters will sound better.
What about everyday systems? How much power does a set of coaxial speakers need? If they are an entry-level speaker, 45 to 65 watts is likely more than enough power to drive them to their limits at lower frequencies. If you have a set of mid-priced component speakers, say in the $200 to $400 range, 75 to 100 watts is adequate. If you’ve purchased high-end speakers like the ARC Audio RS, Audison Thesis, Rockford T3 or T4, Hertz Mille, Kicker QS or Sony Mobile ES, an amplifier that produces 100 to 150 watts is a good power range.
Now, is buying an expensive, high-power amplifier a waste of money when using inexpensive speakers? Maybe. Speakers are almost always the weakest link in any audio system when it comes to how much distortion they add to the audio signal. Look at our articles on Understanding Speaker Quality, and you’ll see what we mean. You are better off buying speakers with distortion-reducing technologies like shorting rings, copper T-yoke caps or even more excursion capability and pairing them with a less exotic amplifier. The net result will be much better sound. If you have a great-sounding amplifier already, use it. Just ensure that the technician configuring and calibrating the system confirms that you can crank the volume without worrying about damaging anything. This doesn’t mean setting gains with a scope or distortion tester.
If you need help picking the best amplifier or speakers to upgrade your car audio system, drop by a local specialty mobile enhancement retailer. Please bring your favorite music and listen to it on their display at the same volume levels you would in your car or truck. This demonstration will give the product specialist an idea of the performance level you need regarding speakers and how powerful the amplifiers should be. Don’t hold back. If you like it loud, crank it. Hoping inadequate products will play louder only leads to disappointment, frustration and damaged equipment.