Many people researching car subwoofers and speakers wonder what an ohm is and what it has to do with their driver. The answer is that a driver's impedance, measured in ohms (Ω), has a lot to do with how the driver will interact with an amplifier and the overall system's performance.
What Is an Ohm?
A German physicist named Georg Ohm is best known for his research into the behavior of electrical circuits, particularly the relationship between voltage, current, impedance, and power — today commonly referred to as Ohm's Law. As shown in the Ohm's wheel above, this relationship is fundamental in car audio: an amplifier outputs voltage, a driver's voice coil has a fixed impedance, and together these determine how many watts are created.
Car audio subwoofers and speakers usually have either a single voice coil rated at 8Ω, 4Ω, or 2Ω, or dual voice coils rated at 4Ω or 2Ω. For more on voice coil configurations, see the single vs dual voice coil guide. Understanding a driver's impedance is what allows you to select an amplifier that matches it for optimum performance.
Single Voice Coil Drivers
Drivers with one voice coil are straightforward — there is a single impedance option. If a driver is rated at 50W RMS with a 4Ω impedance, you want an amplifier whose specifications closely match: something between 40–60W RMS at 4Ω.
Making sure both the driver's and amplifier's specs are measured in RMS power is critical to the longevity of both. Peak power and max power ratings are often theoretical and should not be used for matching. For more on this, see The Mystery of Subwoofer Power Handling.
Wiring Multiple Single Voice Coil Drivers
If more than one single voice coil driver is being used, the speakers will have to be connected either in series or in parallel to present a combined impedance to the amplifier. Using two 4Ω drivers as an example:
For wiring diagrams covering every combination, visit the MTX wiring diagrams library.
Dual Voice Coil Drivers
Drivers with dual voice coils present a greater challenge but also more flexibility. Since each driver has two voice coils, you must choose whether to wire them in series or parallel to maximize the output voltage of the amplifier and ensure the right wattage is delivered. The same principle applies as with single voice coil drivers — the combined impedance of your wiring configuration determines how much power the amplifier will produce.
Paying attention to the RMS rating of both the driver and the amplifier provides the guidance needed to make the right match.
Setting Amplifier Gain with Ohm's Law
With a basic understanding of impedance, Ohm's Law can be used to precisely set your amplifier gain. Earlier we stated a driver rated at 50W RMS at 4Ω needs an amplifier putting out around 40–60W at 4Ω. Ohm's Law tells us:
Using a multimeter set to AC voltage connected to the amplifier's output during installation, you can precisely set the amplifier's gain to regulate the correct voltage output. This maximizes both the amplifier's and driver's performance without risking damage. The amplifier's gain is not a volume control — it is a voltage output regulator.
Ohm ratings are critical to the performance of any car audio system. Understanding their relationship to an amplifier makes finding the right speakers or subwoofers and amplifier a straightforward process that will result in a great-sounding system.
