Impedance Curves

Illustration in preparation Description: Impedance vs frequency graph with labeled features: Fs peak (high impedance), minimum impedance region, inductive rise at high frequency; separate curves shown for sealed vs ported enclosure

Impedance peaks:

• Unenclosed or sealed: Single peak at Fs — the driver's free-air or box resonance
• Ported enclosure: Double peak, split around port tuning frequency Fb. The valley between the peaks occurs at Fb.

Reading Fs from impedance:

The frequency of the impedance peak in a sealed box = system resonance Fc. In free air = Fs.

Reading Fb from ported impedance:

The frequency at the valley between the two peaks = port tuning frequency Fb. This is a reliable way to confirm box tuning without acoustic measurement.

Minimum impedance:

The lowest point of the impedance curve (usually between resonance and mid-bass) is the true minimum load the amplifier sees. This is what matters for amplifier stability, not the nominal rating.

Inductive rise:

Impedance increases above a few kHz due to voice coil inductance. For a nominal 4Ω speaker, impedance at 10 kHz may be 10–20Ω. This affects power delivery at high frequencies and passive crossover design.