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Wiring Subwoofers What's all this about Ohms?
How wiring affects your sub's output
Confusion concerning watts, amps, and ohms has been around for as long as people have been putting subwoofers in cars. Watts and ohms are entangled in a series of mathematical formulas that link them intimately together such that when one of them undergoes a change in value, the other often does too.
Briefly, here's how it works
Amplifiers provide the electrical pressure in a circuit; ohms measure the resistance, or load, against that pressure; and watts measure how much power is released as work. So, using one of those math formulas, an amplifier that provides 100 watts of power through a 4-ohm speaker, will produce 200 watts through a 2-ohm speaker, because it's easier to push that reduced load.
Voice coil makes sound — amplifier pushes it
At the heart of every speaker and sub is a voice coil. This is the device that puts up the electrical resistance and performs the work. (The amplifier provides the power.) The resisting property of a coil is called its impedance and is measured in ohms. The lower a speaker's impedance, the easier it is for an amp to supply power to it. Problems arise when the amp's output meets very little resistance (low impedance) and it tries to put out more power than it was designed to produce. This leads to the amplifier overheating and then, hopefully, shutting down to protect itself from burning up.
You don't want the amp to over-do it
So the capability of an amplifier has to be considered before applying a load to it (hooking up a speaker). The manufacturer's specifications indicate an amp's minimum impedance requirements. Almost all amps can drive a 4-ohm load. Most amps can work with 2-ohm loads on each channel, but not when the channels are bridged together. Some amps can drive a load as low as one ohm.
The two channels of this MTX Terminator amp can be bridged by wiring the sub as indicated (click to enlarge)
Wiring options change a sub's impedance
To add to the confusion, in multiple-sub systems, the total impedance depends on how the subs and their voice coils are wired together — in parallel or in series. Parallel wiring means that the connection ends of each device are connected to the same things — plus to plus, and minus to minus. Series wiring means that the devices are wired one after the other — a plus of one to a minus of another.
When speakers or voice coils are wired in series, you add their impedances together to find their total impedance. Two 4-ohm speakers wired in series have a total impedance of 8 ohms. When speakers or coils are wired in parallel, however, the formula for their total impedance is more complicated. When the impedances of all the devices are the same, their total impedance, when wired in parallel, is that impedance value divided by the number of devices. For example: four 4-ohm speakers wired in parallel have a total impedance of 1 ohm.
This Rockford Fosgate DVC 2-ohm sub has a built-in jumper so you can choose whether it has a 1-ohm or a 4-ohm total impedance (click to enlarge)
Dual Voice Coils give you even more wiring options
Subwoofer manufacturers make subs with dual voice coils (DVC) to take advantage of this difference in wiring schemes, so the user has more freedom of system design. A DVC 4-ohm sub can be wired into a system as a 2-ohm or as an 8-ohm load. A DVC 2-ohm sub can have a total impedance of 1 ohm or 4 ohms.
An example using 4 subs:
If I wanted 4 subs in a system, I might consider using 4 DVC 2-ohm subs, each with their voice coils wired in parallel to make them each 1 ohm subs, and then wire the 4 of them in series so my amplifier could drive them as a single 4-ohm load. Or, I could wire the voice coils in series, making them all 4-ohm subs, and then wire the subs in parallel so the amp would see the total load as one ohm. It would depend on my particular amplifier — which load it would do the best with.
Match power to power — use RMS only
To make subwoofers sound their best, they should be powered by an amp whose RMS output rating comes close to or even slightly exceeds the total of all the subs' top RMS ratings. If you want to run three subs whose RMS ratings are 300 watts each, you'll need about 900 watts RMS of power to run them.
Underpowered subs won't sound good or play very loud. Then, if you were to turn up the input to compensate, the amp could end up sending out clipped or distorted signals that could damage the subs. It's okay to overpower your subs a little bit, as long as the signal's distortion-free, because they're made to withstand occasional peaks well beyond their normal RMS rating.
Optimize it for your gear
The ultimate goal is to have the amp send its full power to subs that can handle it. The object of using different wiring options to optimize the impedance is to get the most power out of the particular amp and subs you have at hand. This also happens to be the most efficient way of running an amplifier/subwoofer system.