“8 ohm” is a label

Speakers are sold as 8 ohm or 4 ohm, but the real impedance changes with frequency. Rod Elliott (ESP) calls many of the claimed figures very misleading for that reason. The rating is a rough single number. The impedance curve is the real load your amplifier sees.

Peaks: resonance

Near the bottom of the plot you’ll see a peak where the driver and box resonate. A sealed box shows one peak. A ported box shows two, with the vent and the box resonance making the lower one (ESP). The driver’s free-air resonance, Fs, is where the peak sits on a driver measured on its own, which is why an impedance sweep is how you measure it (how to read a driver datasheet).

Dips: usually the crossover

Past the peak the curve falls to a minimum, then climbs again as the voice coil’s inductance takes over. REW reports that minimum as Z min, and says a speaker’s Re is generally a little higher than the voice coil’s DC resistance. In a finished speaker, ESP says dips can fall to half the rated impedance or less, and that they’re nearly always caused by the crossover network. He calls a dip to around half the rating the extreme, and he says a deeper one points to a design problem.

Why the amp cares

A lower impedance draws more current at the same voltage, and every amplifier has a limit. ESP adds a second point: a speaker is partly reactive, and a reactive load makes the output transistors dissipate far more than the plain resistor used in most bench tests. If you can read the curve, you can compare the minimum against the amp’s stated minimum load. That’s the check behind the 4 Ω line in matching amp power to DIY speakers. If several drivers share one amp output, work out the combined load first: how to wire speakers in series and parallel.

How to see your own curve

Room EQ Wizard can measure impedance with a sense resistor and both channels of a sound card. REW’s help page covers the wiring and the three calibration steps. A few of its tips are worth knowing:

  • Use sweeps of 10 seconds or longer for a cleaner result.
  • REW says loudspeaker impedance is typically best measured at roughly 100 to 200 mV at the terminals, so keep the level low. If you drive the load with a power amplifier, REW warns that the levels can easily damage sound card inputs.
  • Speakers act as microphones, so a quiet room and a steady, vibration-free setup give a cleaner curve.

New to REW? Start with REW basics.

What to do with the curve

  1. Save the plot and note the lowest point and the frequency where it happens.
  2. Compare that minimum with your amp’s stated minimum load.
  3. If a dip sits right at the crossover frequency, look at the crossover before you look at the amp.
  4. Re-measure after every crossover change and date the file.

Back to the crossover build notes and the box checklist in enclosure mistakes that kill DIY speakers. To flatten the woofer’s inductive rise for a passive crossover, see what a Zobel network does.