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Determining the compatibility between subwoofers and amplifiers is complicated. The more you research, the more questions you have:
- Can my 1000w max amplifier power my 1000w RMS subwoofer?
- Why is that low power amplifier so expensive?
- Can we even believe the numbers on the spec sheet in the box?
Let's demystify car audio power ratings.
TL;DR: You should pay attention to the RMS power ratings. Throw away all the other numbers when looking at power. Well, for the most part. A couple of brands have decided to reject RMS reality and substitute their own. We'll discuss this in a bit.
Subwoofer Power Rating
A subwoofer's power rating represents the limit of power the voice coil can handle over a period of time. The woofer's motor assembly and suspension will determine the power limit. When we want to match amplifiers and subwoofers, we only need to look at RMS power.
We can ignore any power number on the spec sheet that says "max power" or "music power" or "program power". These numbers are meaningless when it comes to matching amplifiers and subwoofers. Your spec sheet may include these numbers for marketing reasons. The woofer cone may also have "1000 WATTS MAX" emblazoned on the dust cap. Again, this number is meaningless.
Amplifier Power Rating
Amplifier power ratings are a different animal altogether. A reputable amplifier manufacturer will include an accurate RMS power rating. This is not always the case. Some discount manufacturers will include a power number the amplifier cannot produce. These amplifier manufacturers are predatory. They are hoping you don't do enough research or read articles like this one. They want you to buy into the lie.
There are ways of protecting yourself. A simple way to determine if a manufacturer is truthful is to check the fuse rating. I kinda got burned in my younger days, buying a "1000w" amplifier that came with a 20A fuse. A vehicle's electrical system will produce 14.4v charging. Given Ohm's Law (current * potential = power), we know that 20x14.4 = 288. The most power the amplifier will be likely to produce is 288 watts. It wouldn't be unfair to say the amplifier could produce 200 watts. A good rule of thumb: multiply the fuse rating by 10, and you'll get close to the RMS power rating.
Some "discount" manufacturers let their marketing team get in the way of a good product. These products have been independently tested. These amplifiers make a lot of clean power for the asking price. They have giant "max power" ratings emblazoned on the heat sink. One such discount manufacturer also includes a "CEA compliant" number. They have a max power number, to compare to other discount amplifiers. The CEA number is to compare power to reputable brands. In the end, they're only serving to confuse the marketplace.
What Happens When I Over-Power a Subwoofer?
Simply put, the voice coil cannot dissipate the excess heat. The insulative varnish on the voice coil strands melts. The voice coil shorts out or the individual strands break. If the coil is shorting out, you run the risk of damaging your amplifier.
Using an amplifier that is too large for a subwoofer could fry voice coils. We can prevent this. We can "nerf" an amplifier through Ohm's law and a multimeter. We can reduce the amplifier's output so we don't damage our woofers. Play a 40hz tone. Disconnect the subwoofer. Probe the outputs of the amplifier with your digital multimeter. Turn down the gain while monitoring the AC voltage until the voltage meets your target.
What Happens When I Under-Power a Subwoofer?
Nothing. Under-powering a subwoofer is not bad. It is not detrimental to the subwoofer. You're under-powering the speaker any time you turn down the volume!
Why did this "rule of thumb" come about? Simple: Manufacturers and equipment dealers want to encourage users to use equipment properly. Inexperienced people would turn the gain up because "the subwoofer can handle it!"
Trying to push the amplifier beyond RMS power will result in clipping.
Clipping is bad. The term "clipping" comes from the visualization in an oscilloscope. The waveform appears to have had the peaks and valleys "clipped off". The clip point is where the signal goes beyond the electrical limits of the amplifier. The subwoofer only moves when the amplifier says to move. The subwoofer moves the way the amplifier says to move. That clip is a hard corner when the woofer was designed for a gentle curve.
Clipping also produces harmonic distortion. This is another way of saying the amplifier is introducing extra tones.
It is okay to "under power" your subwoofer. Don't try to drive more power out of your amplifier. You'll clip the signal and wreak havoc on the voice coil. Push clean power out of the amplifier.
How to Fix Amplifier Clipping
The best way to fix amplifier clipping is to use a tool. Your ears will not detect distortion before a tool will detect distortion. Your ears aren't that sensitive. Sorry. We also don't want to run our systems in our driveway at full tilt trying to find distortion. Do it quietly with a tool. This is a list of three tools we can use to set our gains.
1. The SMD DD-1. Plug it in in place of your speakers. Follow the directions. The device uses flickering lights to alert you to distortion. It is the simplest tool to use, though it is expensive.
2. An inexpensive Piezo tweeter will let you know you're clipping. It works because a Piezo is a tweeter that doesn't play low frequencies. It doesn't need a crossover. We can use a Piezo to listen to the harmonic distortion discussed above. Disconnect your subwoofer. Connect a cheap piezo to the subwoofer outputs. Play a 40hz Tone at 90% volume at the deck. Adjust the gain until the piezo is silent.
3. Use a Digital Multi Meter. We used this method back in the day. We would start off with ohm's law to compute voltage. We would disconnect the woofer and probe the amplifier output with a digital multimeter. Then we would play a test tone and adjust the amplifier gain until we reached the desired voltage. There are edge cases where a digital multimeter isn't "perfect". In the case of discount amplifiers, they don't provide accurate power ratings. You'll be clipping all day long. On the other side of that coin, tuning a "cheater amp" with a DMM means you're leaving performance on the table.
Read more about setting amplifier gains with a digital multimeter here!
Final Thoughts
Car audio is a fun hobby. We need to play by a few rules to keep the hobby fun. One of those rules is not abusing our components to get more performance out of them. Abusing the components may lead to blown amplifiers. It could lead to blown subwoofers. Abusing components may even lead to a car fire.
Let's keep it safe and keep it fun!
