Most car subwoofer enthusiasts start shopping for their new system once they have decided on the quantity and size of subwoofers they want to buy. After all, subs are the main part of the system that creates the window rattling, body-panel-flexing bass they crave!
But a common mistake when choosing subwoofers is that the higher the power handling rating, the louder it will sound. This assumption is perfectly logical, but not always true.
To understand this tradeoff, you need to know what makes a subwoofer loud (SPL) and some basic physics. Now, don’t expect this post to be a science lesson with a bunch of technical equations and diagrams. This is a basic explanation of SPL and subwoofer power handling.
First, some definitions you need to know:
- SPL = Sound Pressure Level
- RMS Power Handling = Continuous amount of power your subwoofer can manage without damage
- Peak Power Handling = a measure of power that could potentially be managed for short bursts.
- XMax = The distance a subwoofer voice coil can move in one direction without leaving the magnetic zone a.k.a. how far the woofer can move.
As a disclaimer, remember that a manufacturer can put any incredible number they want for their "Peak" or "Max" power ratings and many manufacturers do this as a marketing tactic to get customers to believe their product will be louder, better, more powerful, etc… You can't cheat physics; anyone afraid to publish their data is probably not being 100% honest about their ratings.
Sound is created by vibration of an object. The vibrating object causes air molecules nearby to move in waves. The more air molecules that are moving, the louder the sound waves will be. As the number of air molecules moving within the interior cabin of your vehicle increases the air pressure increases. This is called sound pressure. The Sound Pressure Level (SPL) is the measurement of this pressure in decibels (dB).
So in the simplest terms, the more air you move, the more SPL will be generated. So go buy a bunch of 18” woofers and you will be loud. Right? Not necessarily. There are other factors to consider. An 18” woofer has a lot of surface area to move air molecules, but can the woofer move a lot of air? This is where Xmax comes in. If the 18” woofer can only move 1/16th of an inch, it’s not going to move much air and therefore will not be very loud.
What dictates a woofer's Xmax you might ask?
Well… A lot of engineering! It takes a good suspension (spider) that can handle the movement while maintaining linear motion. If a subwoofer does not stay linear, it will create distortion, or worse cause a rub or short of the voice coil against the pole piece causing damage to the subwoofer. A quality suspension is very important.
Additionally, you need a strong magnetic field which uses AC current to push the woofer forward (positive) and pull the woofer backward (negative). This magnetic field is created by the interaction of the magnets on the woofer’s motor structure and the electrical current being transferred through the voice coil from your amplifier.
At this point many people get caught, logic would tell you the bigger the magnet, the bigger the magnetic field. The more power through the voice coil, the more Xmax the subwoofer will have. The more Xmax you have, the louder the subwoofer will be. Right? Not necessarily.
You could build a subwoofer with HUGE magnets weighing 200lbs each and a voice coil that is capable of handling 10,000 watts. That’s easy. But designing a suspension that can keep that power under control is the hard part. If the suspension is too tight and rigid to control the power, it will limit the Xmax.
Mass is another factor that must be considered. Subwoofer A has a RMS power rating of 2,000 watts and includes huge magnets with a cone assembly made of aluminum that has a mass twice that of the cone assembly of Subwoofer B which has a RMS power rating of 500 watts with magnets half the size. Which subwoofer has the potential to be louder? This is in fact a trick question.
You can’t determine which subwoofer would be louder simply from this information. Subwoofer A can handle more power but it’s cone assembly is heavier and therefore it requires more power to move it. Xmax could suffer as a result. Subwoofer B can’t handle as much power but it’s cone assembly is lighter and therefore, it’s Xmax could be as good or better than Subwoofer A, thus moving more air. This is called efficiency or sensitivity and is rated in decibels (dB) and should always be measured at (1W/1m). After all, you don’t want to spend the extra money for a HUGE amp or tax your vehicle’s electrical system to power it, right?
As you can see, comparing the performance of car subwoofers purely based on the power rating is not a fair comparison. There are many other factors to consider.
And once you start the installation, consider enclosure design, speaker wire size, and power wire gauge to ensure your subwoofers perform to maximum potential. If you buy a subwoofer with an RMS power handling rating of 2,000 watts, put 2,000 watts to it. Under powering a subwoofer is often times worse for it than over powering. Also make sure your vehicles electrical system can support a 2,000 watt amplifier. And remember a 2,000-watt amplifier (that truly produces 2,000 watts RMS) will generally cost you more than one rated at 500 watts.
So if you are in the market for a subwoofer, don’t be fooled by peak power ratings created by crazy math. These numbers don’t mean much. RMS Power ratings are what you want to notice. This spec doesn’t always reflect performance either, but I will leave this discussion for another article. Call our MTX Support Team if you want to discuss “the loudest subwoofer for your budget" further at 800-225-5689. To read more about choosing speakers, see our MTX Library