Ohm's Law and Impedance in Car Audio

Every car audio system is fundamentally an electrical circuit. Whether you're running a simple head unit and speakers or a competition-grade SPL setup with multiple amplifiers, Ohm's Law governs how power flows through your system.

Understanding these basics will help you:

• Choose the right amplifier for your subwoofers
• Wire multiple speakers correctly
• Avoid blowing fuses or damaging equipment
• Maximize your system's output safely

Ohm's Law states that voltage equals current multiplied by resistance:

V = I × R

Where: - V = Voltage (measured in Volts) - I = Current (measured in Amps) - R = Resistance (measured in Ohms, Ω)

The Power Triangle

In car audio, we often work with power (Watts). The power formula is:

P = V × I or P = I² × R or P = V² / R

This is why a 4-ohm subwoofer receives more power than an 8-ohm subwoofer from the same amplifier—lower resistance allows more current to flow.

While often used interchangeably, impedance and resistance are slightly different:

• Resistance (DC) is constant
• Impedance (AC) varies with frequency

Speakers are rated by their nominal impedance—typically 2Ω, 4Ω, or 8Ω. This is an average value; actual impedance changes with frequency.

Why This Matters

When you see a subwoofer rated at "4 ohms," this is its nominal impedance. At certain frequencies (like the resonant frequency), impedance can spike much higher. This is why amplifiers need headroom and shouldn't be pushed to their absolute limits.

When wiring multiple speakers, the configuration changes total impedance:

Series Wiring

Connects positive to negative in a chain. Impedance adds up.

Two 4Ω subwoofers in series = 4 + 4 = 8Ω total

Parallel Wiring

All positives together, all negatives together. Impedance divides.

Two 4Ω subwoofers in parallel = 4 ÷ 2 = 2Ω total

Series-Parallel (For Multiple Subs)

Combine both methods for specific impedance targets.

Four 4Ω DVC (Dual Voice Coil) subwoofers can be wired to achieve 1Ω, 2Ω, 4Ω, or 8Ω depending on configuration.

Critical Rule: Never run an amplifier below its minimum rated impedance.

Most car audio amplifiers are rated as:

• Class D Mono: Stable to 1Ω or 2Ω
• Class A/B 2-Channel: Stable to 2Ω stereo, 4Ω bridged
• Class D 4-Channel: Stable to 2Ω per channel

Power vs Impedance Example

A typical amplifier might produce:

• 1000W @ 1Ω
• 600W @ 2Ω
• 350W @ 4Ω

Lower impedance = more power, but also more heat and stress on the amplifier.

The Safe Approach

1. Check your amplifier's minimum stable impedance
2. Calculate your speaker load
3. Ensure the load meets or exceeds the minimum
4. Leave some headroom for real-world conditions