Impedance in loudspeakers — what it is and why it varies

What impedance is

Impedance (Z) is the generalisation of resistance to AC circuits. Like resistance, it is measured in ohms and describes the opposition to current flow for a given applied voltage. Unlike DC resistance, impedance has both magnitude and phase, and both vary with frequency.

For a loudspeaker, the total impedance at any frequency is the combined effect of:

• DC resistance (Re) — the ohmic resistance of the voice coil wire, measured with a DC or low-frequency signal. This is the minimum value the impedance can reach.
• Voice coil inductance (Le) — the inductive reactance of the voice coil, which rises linearly with frequency.
• Mechanical resonance — the back EMF generated by the moving voice coil at the mechanical resonant frequency produces a large peak in impedance.
• Enclosure loading — the acoustic load presented by the enclosure modifies the impedance, adding further features (including a double peak in ported designs).

The impedance curve

A typical impedance magnitude plot for a driver in free air shows:

At DC (0 Hz): impedance equals Re. For most drivers, Re is 5–7 Ω for an "8 Ω nominal" driver and 3–4 Ω for a "4 Ω nominal" driver.

Resonance peak: At the driver's resonant frequency (fs), the mechanical system resonates and the back EMF is at its maximum. The impedance rises sharply, typically to 3–10× the nominal value. A driver with Re = 6 Ω and nominal impedance of 8 Ω might show an impedance peak of 50–80 Ω at fs.

Between resonance and the inductive rise: Impedance falls back toward a broad minimum, settling close to Re plus a small motional contribution. This is the driver's normal operating region.

Inductive rise: Above a few hundred hertz to a few kilohertz (depending on the driver), the voice coil inductance Le causes impedance to rise again, approximately proportionally to frequency. A driver with Le = 1 mH has an inductive reactance of 6.3 Ω at 1 kHz, rising to 63 Ω at 10 kHz.

Nominal impedance

The nominal impedance (often stated as 4 Ω, 8 Ω, or 16 Ω) is a simplified characterisation intended to indicate approximate impedance in the normal operating band. It is not the minimum impedance, the maximum impedance, or the impedance at any specific frequency. IEC 60268-5 defines it as the minimum impedance in the frequency range above resonance, rounded up to the nearest value in the standard series.

The minimum impedance can be substantially lower than the nominal value. A nominally 8 Ω driver may dip to 5–6 Ω in its operating band. For amplifiers with limited current delivery, this dip — which is where maximum power is transferred — matters more than the nominal rating.

Why the impedance curve matters

Power delivery from the amplifier. An ideal voltage-source amplifier delivers power P = V²/Z to the load. As impedance varies, delivered power varies inversely. At the impedance minimum, the amplifier delivers maximum power — and maximum current. An amplifier rated for 8 Ω loads may clip or engage protection circuitry when driving a nominally 8 Ω speaker whose impedance dips to 4 Ω at certain frequencies.

Passive crossover design. Passive crossover filters are designed for a specific load impedance. If the driver's impedance deviates from that assumed value — which it always does, across frequency — the actual filter behaviour differs from the designed behaviour. A first-order low-pass filter designed for a flat 8 Ω load will have its -3 dB frequency shifted if the driver presents 5 Ω at crossover. Accurate passive crossover design requires working with the measured driver impedance curve, not a nominal value. An introduction to loudspeaker crossover design covers this in detail.

Zobel networks. The rising inductive impedance at high frequencies can be compensated with a Zobel network — a series RC circuit connected in parallel with the driver, chosen to flatten the impedance above the inductive rise. Typical values: R = Re, C = Le/Re². Flattening the impedance simplifies passive crossover design and improves the load presented to the amplifier at high frequencies.

Multi-driver parallel wiring. Two 8 Ω drivers wired in parallel present a nominal 4 Ω load. In practice, the combined impedance minimum — which may occur at a frequency where both drivers' impedances are simultaneously low — can fall well below 4 Ω. Verify the combined impedance curve, not just the theoretical nominal.

Measuring impedance

Impedance can be measured by applying a known voltage across a series resistor and the driver, and computing the driver impedance from the voltage ratio. Software such as REW (Room EQ Wizard) or DATS (Dayton Audio Test System) automates this. The result is the complex impedance as a function of frequency, from which resonant frequency, Q factors, and voice coil inductance can be extracted and used to derive Thiele-Small parameters.