Resonance and Impedance
A Sub Has a Note of Its Own
A sub’s cone hangs on a springy surround and spider, and it has weight. Anything with a spring and a weight has one speed it likes to bounce at, like a car on worn shocks. For a sub, that speed is its resonance.
Measured with the sub hanging in open air, it is called the free air resonance, written Fs on spec sheets. Most competition subs sit somewhere in the 20s to 40s of Hz.
A Box Raises the Note
Put the sub in a sealed box and the air inside adds a second spring. Two springs are stiffer than one, so the cone now bounces at a higher note. This is the system resonance. The smaller the box, the stiffer the air spring and the higher it goes.
Below that note, a sealed box gets quieter fast. So a smaller box means bass starts dropping away at a higher note. That is the main reason a bigger box tends to play deeper.
Impedance Is Not One Number
A sub’s impedance is how hard it resists the amp, in ohms. The number printed on it, like 1 or 2 ohms, is a rating, roughly what it shows above its resonance. The real figure changes with every note.
At resonance the cone moves so freely that it pushes back against the amp. The moving coil makes its own voltage that fights the amp’s, so the impedance shoots up into a tall peak. Away from resonance it settles back down near the rating.
On a real sub the curve also starts creeping up again at higher notes, because the voice coil’s own inductance makes it harder for the amp to push current through at higher frequencies. Our simple chart leaves that out, since it matters most above the bass range.
Sealed Has One Peak, Ported Has Two
A sealed box shows one peak, at its system resonance. A ported box shows two humps with a low point between them, and that low point sits very close to the box’s tuning. That makes impedance a handy way to check where a finished box really tunes. See Check Your Tuning With an Impedance Sweep.
Bandpass boxes add more bumps: a 4th order bandpass shows two humps and a 6th order shows three. A vehicle with its openings can behave the same way, so an in-car sweep with an extra bump is not always a fault.
What This Means for Your Amp
An amp makes the most power into the lowest load it is rated for. Because impedance changes with the note, the load your amp actually sees moves around as the music does. Near a peak, the load is higher than the rating and the amp makes less power there. Between the peaks, and in the notes above, it sits near the rating.
This is why two systems wired to the same rated ohms can make different power on the meter at different notes, and why a box tuned right where you burp can change what the amp delivers. A sub’s rating is a starting point, not the whole story.