Reading an Impedance Curve: What the Peaks Tell You
The impedance curve plots how a driver's electrical impedance changes with frequency. It looks abstract, but it is the most diagnostic single plot in enclosure design. Once you can read it, you can tell a sealed box from a ported one, find the tuning frequency, and spot a leak, all without touching a measurement microphone.
Impedance is not a constant
A subwoofer is nominally "4 ohms" or "2 ohms," but that nominal rating is just a rough label - the actual DC resistance is lower (a 4 ohm driver often measures around 3.3 ohms). More importantly, the impedance is not a single value at all: it swings widely with frequency because the moving cone generates a back-EMF that opposes the driving current. Where the cone moves most freely, it generates the most back-EMF, and impedance peaks.
The sealed box: one peak
A sealed enclosure shows a single impedance peak at the system resonance frequency (Fc). At that frequency the cone and the air spring of the box resonate together, cone motion is maximal, back-EMF is maximal, and impedance hits its highest value.
The height and sharpness of that peak relate to the system's total Q. A tall, narrow peak means a high-Q, underdamped box; a lower, broader peak means a well-damped one. One peak, and its position is Fc.
The ported box: two peaks with a dip
A vented enclosure shows two peaks with a valley between them. This is the signature of two coupled resonators - the driver and the port - exchanging energy.
The valley between the two peaks sits at the port tuning frequency (Fb). At Fb the port is doing the work: the air in the port resonates and moves the output while the cone motion drops to a minimum. Less cone motion means less back-EMF, so impedance dips. That dip is the most useful single feature on the plot - it is your tuning frequency, read directly.
Using the dip to verify tuning
This is the practical payoff. You designed a box for, say, 32 Hz. Build it, measure the impedance, and look at where the valley sits. If the dip is at 32 Hz, your build matches the design. If it landed at 28 Hz, the tuning came out lower than intended - the port is effectively longer than planned, or the net volume is larger; if it is at 36 Hz, the tuning came out higher - a shorter port, or a smaller net volume. The impedance dip tells you the truth about what you built, not what you drew.
Spotting a leak
A sealed box with an air leak stops behaving like a sealed box. The single clean peak becomes lower and broader, or develops a hint of the two-peak shape, because the leak acts like a badly tuned port. If a sealed enclosure's impedance peak looks wrong, suspect a leak at the driver gasket, terminal cup, or a seam before you suspect the driver.
What the peak height near resonance tells you
A very tall impedance peak means the system is lightly damped at resonance - it will have a pronounced response bump and can sound boomy. A modest peak suggests better damping. You can read the character of the alignment from the shape before you ever look at the frequency response.
Where RokketBox fits
RokketBox plots the predicted impedance curve alongside SPL, group delay, excursion, and port velocity from the same circuit model. Comparing the predicted impedance dip against a real measurement of the finished box is the fastest way to confirm your build tuned where you intended - and to catch a leak or a volume error before you blame the driver.