Matching Subwoofers and Amplifiers

Matching Subwoofers and Amplifiers

Car audio is an intimidating arena for newbies to enter. It starts out harmless but quickly becomes ego driven, hearing loss filled world full of experts and amateurs, and amateurs who think their experts, and experts who really are amateurs. But there’s no better feeling than building your own system, the way you want it. It’s what I imagine having a child is like to a new father. Yup, I made that. Or more accurately, wtf, I’m broke how am I paying for this. And as exciting and flat out fun car audio is, it requires some knowledge, and anyone who is new can get confused easily.

Sometimes the basics are the hardest things to wrap your head around. One of the most common concepts people get confused about is matching subwoofers and amplifiers. I get asked about it every day.

Hey what’s a good amp for this sub?

What’s a good subwoofer for this amp?

Can I use two subwoofers for this amp?

Do I need an amp for each subwoofer?

Does this amp have enough power for this sub?

Is that too powerful for this?

Why are you always yelling?

Hopefully this helps you guys understand the process of matching subs and amps. This is going to be fairly technical, so pay attention, take notes, do whatever you gotta do. You’re gonna learn today.

When matching subwoofer and amplifiers, there are 4 main points to consider.

  1. The rated RMS Power input of the subwoofer, or subwoofers.
  2. The Final Impedance, resistance in OHMS of a single sub or multiple sub wired together at the amplifier.
  3. The type of amplifier you are using, monoblock, 2 channel, 4 channel.
  4. The type of enclosure that you will be using.

RMS ratings are the measure of continuous power that the subwoofer can accept. You may see it sometimes listed as thermal power handling, RMS power is code for the maximum amount power in watts that the speaker or subwoofer can dissipate as heat and sound without damaging the voice coil. Plenty of manufacturers over-rate their max power input and broadcast it everywhere, but it’s much more accurate to use the RMS rating to determine the right amplifier. And although MAX power is a sexy number, it really only represents quick bursts of power, not sustained power.

For example, the DB Drive K9-12D4 is a powerful 4 ohm subwoofer and has a MAX POWER OUTPUT of 2,000 WATTS!!! But realistically, that is only sustainable for 1 or 2 seconds. Looking at its RMS input rating you’ll see it’s 1,000 Watts, which is much more accurate display of its power. Manufacturers throw the max in your face, ignore it.

For your system, you’ll want to match the RMS input rating of the subwoofer, or subwoofers, to that of the RMS amplifier output that you have chosen. You’ll also need to make sure that your amplifier will output this at the impedance that you have wired your subwoofers as. When dealing with multiple subwoofers, combine the RMS input rating of the subwoofers. (2) 350 watt 10 inch subwoofers will need 700 watts combined. A rule of thumb that we use, is to over-rate your amplifier by 10-20%. It’s ok to run a little bit more or a little bit less power but there are tradeoffs to each.

An easy way to match a subwoofer with an amp is to use a little math. Multiply the sub’s rating by 120% and 90% to get the power range of the amp that is needed. Two 350 watt RMS subs together need a total of 700 watts RMS, an amp putting out between 630 watts to 840 watts RMS will do. Now let’s talk about those tradeoffs.

A slightly underpowered amp when nearing its maximum output will begin to output excessive distortion, this distortion causes a large increase in heat at the voicecoil, eventually causing damage. So if you are underpowering your enclosure understand what distortion sounds like and back the volume down when you hear it to prevent speaker damage.

The reason we recommend a more powerful amplifier is for headroom. Having a slightly overpowered amp also allows the amp to run cooler and with more reserve power, and as we stated, heat is a killer. Sustained periods of input power exceeding the thermal limit of the subwoofer will cause the voice coil to fail.

Still with me?

Now let’s move on to Impedance. Impedance is the electrical characteristic of a speaker that restricts, or impedes, the flow of current and is measured in ohms. If you have no idea what the fuck I just said, don’t worry, just pay attention to the numbers, will become clear in a minute.
The problem most people have with subwoofer and speaker impedances is how to combine them when using more than one driver and or if the driver is equipped with dual voice-coils. The impedance directly affects the load of the amplifier not the power requirement of the driver.
Amplifiers have output ratings, those ratings will show different power outputs at different output loads. Llets say we have an amplifier with max ratings of 350 watts x 1 at 4 ohm and 700 watts x 1 at 2 ohms. This means the the largest load you can place on that amp is 2 ohms. So wiring your subwoofers to 1 ohm will either cause the amp to immediately go into protect or will cause the amp to get excessively hot and cook itself.
Larger resistance in Ohms, equals a smaller load at the amplifier, or less heat generated by the amp. The tradeoff of the smaller load is less power output from the amp. For example, an amplifier will usually double its output at 2 ohms compared to 4 ohms but the amp will run much hotter as it’s being asked to work harder. So why not wire my subs to 1 ohm then and get insane bass!!?? Well if this amp was rated for 1 ohm loads, we would do exactly that but it’s not.
Depending on the amount of subs and how you wire them, you’ll need to match the final wired impedance of the subs to that of the power output rating of the amp at your wired impedance.
For example, the Kicker CXA1200.1 monoblock amplifier has an RMS rating of 1200 watts at 2 ohms and 600 watts at 4 ohms. When combining more than one subwoofer, you’ll need to figure out their impedance wired in series or parallel to match your amp.
(Video plug to our wiring video)
In this example let’s use two 2 ohm single voice coil subwoofers with an RMS of 500 watts each. Wiring these in parallel will give you a 1 ohm load, wired in series they’ll give you a 4 ohm load. So based on this, we know that this amplifier wouldn’t be the greatest match for the 500 watt subwoofers, we need 1000 watts. Now, what subs should you get instead?
Ya’ll took calculus right? So ideally here is what we would do: 1200 watts times .8 or 80% for headroom, this leaves us with 960 watts. So if we purchased two subwoofers that had a rating of 450-500 watts each and were single voice-coil 4 ohm, not 2ohm, wired in parallel they would match perfectly to our amp.
But what if the subwoofers you want are only offered in dual voice coil, how do you figure out what subwoofers we need, dual 2 ohm or dual 4 ohm?
Two 2 ohm dual voice coil subwoofers can be setup to have each of their coils wired in series making each woofer 4 ohms, then each woofer can be paralleled equaling a 2 ohm load. They can also be wired in 4-way parallel for a 0.5ohm load.
Still with me? Ok good, if not I don’t care I’m moving on. What do you want me to do? Come hold you hand and walk you through it with drawings?
So how do you select an amplifier? Amps range from 1 channel monoblocks, all the way up to 5-6 channels. A monoblock is generally the best option when powering multiple subwoofers, they’re designed to output tons of power at low impedances.
A 2 or 4 channel channel amplifier can also be used but they are usually limited in the their power output and their impedance capabilities. By bridging channels you effectively combine the output of two channels, so a 4 channel becomes a 2 channel and a 2 channel becomes a mono amp. But again there are tradeoffs. Most 2 and 4 channel amps in bridged mode cannot accept a load lower than 4 ohms. Some full range class D amp allow for lower loads but Class AB are the most common. The reason for this is in bridged mode, each output channel of the amplifier is seeing half the load, so with a 4 ohm load, ½ of the bridged amplifier is seeing 2 ohms.
A monoblock can push any number of subs as long as the minimum impedance is not exceeded and the output power matches up to the subwoofers or speakers you are powering.
For example, you can wire three dual voice coil 2 ohm subwoofers to a final impedance of 1.34 ohms or 3 ohms. Then you just calculate the RMS input power required for the subs (all added together) match the the output of the amplifier.
When trying to make a buying decision, choose an amplifier that is CEA-2006 compliant, this certifies that your amplifier’s output power ratings are REAL POWER numbers, not inflated marketing ratings. It’s also wise to find subwoofers and speakers that use the CEA-2031 compliance standard and this ensures that the subwoofer and speakers are rated with real world power as well not 95 billions watts on a good day, downhill, with a tail wind.
Let’s build a system from scratch. Remember, you want to match the RMS and Impedance. We’ll start with one of my favorite amps, Rockford Fosgate’s P1000X1bd monoblock, which will output 1,000 Watts RMS at 1ohm, 600 Watts RMS at 2 ohm, and 300 watts at 4 ohm. So, what size woofers do we want? Do we want some 15’s, some 12’s, or some 10’s, they bang though. (Friday reference)
How about we get crazy and do four 12’s. Our amplifier can output 1000 watts RMS at 1 ohm, this means we want to look for 250 watt RMS subwoofers. In this system we will use the P1S4-12’s. We’ll wire these in parallel giving our amplifier a 1 ohm load for our 1000 watt output. Boom, perfect match, these are a match made in car audio heaven.
What about if you want three subs with this amp? The Rockford P2 10” subwoofer is a 300 watt RMS subwoofer. So three of these will need 900 watts RMS. But do we need the dual 2 ohm or the dual 4 ohm? We want our amplifier to run at 1 ohm because at that load the amp will put out 1000 watts. Which subs will get us the 1 ohm load. If we choose the dual 4 ohm model the P2D4-10 we can only load the amplifier at 6 ohms or 2.67 ohms, that isn’t going to work. If we choose the P2D2-10’s we can load the amp at 1.34 ohms or 3 ohms. Again, a perfect RMS match with the P1000X1BD. See guys, this isn’t that hard.
So how do you know what kind of system is right for you? The sub and amp you need depends on what you listen to and the amount of bass you want. It’s all personal preference, there are no rules. If you plan on listening to music at moderate volume levels or have a smaller car, a subwoofer with a lower power handling, say 100 to 250 watts, and a matching amp is a good combo.
Subwoofer enclosures also play a huge part in the amount of output a system will have. There are some other important things to keep in mind, like ported boxes require subsonic filters, sealed enclosures benefit from more robust amps, but we will leave that for another video.
Let’s go over some more common questions. “Do I need to get a new car battery if I get a powerful amp?” Generally no, but this depends on the input current requirements of the amp or amps you choose and if you plan on max volume levels all the time. If your charging system has an alternator that is only 80 amps and you went crazy and have two 10,000 watt monoblocks that require 300 amps of input current each, you will need a multiple new batteries, new high current alternator and many other upgrades.
But you can run multiple amps and be fine with the battery and charging system you have, it all comes down to power requirements and the output capability of your charging system.
“I have two subs, do I need two amps to power them?” No, not necessarily, you can choose to run two amps if your pocketbook lets you but you can also choose any amp you want as long as you load it properly.
Don’t forget to tune your system right; crossovers, subsonic filters, input gain all play an important role in what your system will sound like. Make sure to check out our video on setting input gain. Okay, whose brain is fried? If anyone has any questions or comments, or needs a suggestion of gear, whatever, just leave anything below.
Hopefully this helped, let us know what you guys think. Check out QualityMobileVideo.com for all of your car audio gear, subscribe to our channel and thanks for watching! Class dismissed!

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