Loudspeaker Impedance And Electrical Load
Canonical Definition
Loudspeaker impedance is the frequency-dependent complex ratio of terminal voltage to current. It changes with resistance, inductance, driver motion, enclosure, passive network, wiring, temperature, and frequency.
Z(f) = V(f) / I(f)Four Numbers That Are Not Interchangeable
| Term | Meaning |
|---|---|
| DC resistance, Re | Low-frequency coil resistance measured without normal motional behavior; usually below the nominal label. |
| Nominal impedance | Declared impedance category used for system and amplifier matching under a specified convention. |
| Minimum impedance magnitude | Lowest measured magnitude in the stated band and condition. |
| Impedance phase | Voltage-current timing relationship that affects real power and amplifier stress. |
Typical Curve Features
- Free-air resonance: a large peak near the driver's mechanical resonance.
- Sealed enclosure: resonance shifts and the system peak reflects the combined driver and air spring.
- Vented enclosure: two major peaks with a minimum near the tuning region.
- Inductive rise: impedance generally rises at higher frequency because of voice-coil inductance and other effects.
- Passive crossover: multiple driver and component impedances combine into a system curve that may have new minima.
Temperature Changes The Load
As the voice coil heats, its DC resistance rises. That changes current, electrical damping, sensitivity, current sharing, and the impedance curve. Compare cold and hot results only when temperature state is documented.
Series And Parallel Wiring
Series: Ztotal = Z1 + Z2 + ...
Parallel: 1/Ztotal = 1/Z1 + 1/Z2 + ...Those expressions apply to complex impedance as a function of frequency. Using nominal labels is useful for a first wiring check but does not replace the measured system curve.
Measurement Contract
- State free-air, sealed, vented, installed, or passive-network condition.
- Record fixture, series resistor, cable resistance, calibration, sweep level, and bandwidth.
- Measure magnitude and phase when the result controls amplifier loading or crossover design.
- Record driver temperature and cool-down condition.
- Keep Re, nominal impedance, measured minimum, resonance, and phase labeled separately.
Driver Health Clues
Unexpected resonance shifts, asymmetry, added resistance, missing peaks, unstable curves, or changes after heat can indicate wiring, fixture, suspension, coil, enclosure, or damage problems. Treat the curve as evidence, not a one-step diagnosis.