Subwoofer 101: Single Voice Coil or Dual Voice Coil

Should I get a single voice coil woofer (SVC) or a dual voice coil (DVC) for your car audio system?  Along with “What size woofer do I need,” this is a very common question, and one that is not easily answered. Really, neither an SVC or a DVC woofer is “better” than the other. All things being equal, one will not produce more bass than the other. It is more about producing the right ohm load for the amplifier that you are running.  Ohms are a measurement of electrical resistance. The less resistance, the more power that the amp will put out. Of course, this is limited by the power supply in the amp, and the load that the manufacturer has determined that the amp can run without a high chance for failure. If an amplifier is rated to deliver 500 watts into a 1 ohm load, then you will get the most power out of the amplifier at that load. If you run a 2 ohm load into that amp then with twice the resistance you will get half the power – so it is important to match the amplifiers rated ohm load with the proper woofers to reach that load.

Voice Coil Variables

Here is where the math comes in. The variables that have to be taken into account are mono or bridged stereo amplifier, number of woofers, and series or parallel wiring. We will tackle these one at a time.

Voice Coil Selection and Amplifiers

A dedicated mono subwoofer amplifier makes this easy because the load is what the amplifier is rated for. Low frequency is not as directional as mids or highs, so “stereo” is not important and a mono amplifier works just fine. If you are using a stereo amplifier, most of them can be “bridged” or have the output summed into a mono signal. In this configuration the amplifier “sees” half the impedance (resistance) and that has to be taken into account. If a two ohm load is applied to a bridged amp, it will actually see a one ohm load.

Warning: Math Class Coming Up!

The number of woofers will also factor in because every additional woofer wired into the amplifier will drop the impedance. The same goes for every additional voice coil, which the amp sees as an additional speaker. A single 4 ohm woofer on a mono amp gives you a 4 ohm load. Adding a second one in parallel can be calculated by the formula

Zt = (Za x Zb)/(Za + Zb)

where Zt is the total impedance, Za is the impedance of woofer A and Zb is the impedance of woofer B.

The total impedance in this case is 4 x 4 divided by 4 +4  (or 16 over 8) which is a 2 ohm load. If your amp delivers the best output at a 1 ohm load then you either need to add more woofers, which isn’t always practical, or add more voice coils. If you had two dual voice coil woofers with two 4 ohm coils on each, then each sub could be wired into a net 2 ohm load as above. When combined the load would be 2 x 2 divided by 2+2  giving you 4/4 or a 1 ohm load – which is where that amp needs to be for the most power output and reliability. WHEW!  Schools out!

Final Decision

So, the decision on buying a SVC or a DVC woofer can only be made after you determine how many subs you will be running, and the load that the amplifier can safely handle. Then it is just a matter of doing the math – and you thought you would never use that in real life!