What slope means
A crossover’s order sets how fast each driver is rolled off past the crossover point. First-order is 6 dB per octave and second-order is 12 dB per octave. Think of it as how hard the network shuts the door on frequencies a driver shouldn’t play. In a passive network, that’s roughly one reactive part per driver per order: a capacitor for the tweeter and an inductor for the woofer in first-order, and both on each side in second-order.
First-order: simple parts, wide overlap
Fewest parts, and the outputs can sum flat on paper. The cost is the slow slope. Each driver keeps playing well outside its band, so the tweeter still gets a lot of energy below the crossover point and the woofer’s breakup region isn’t attenuated much. Rod Elliott (ESP) notes that this suits low-power use, because the tweeter sees substantial out-of-band energy.
Second-order: more parts, more protection
Twice the slope, so the tweeter is better protected and the overlap region is narrower. The catch is phase: the two sections end up 180° apart at the crossover point, so the usual move is to reverse the tweeter’s polarity (How to check speaker polarity). Get it wrong and you get a deep notch right at the crossover point. That’s also why the reverse-null test from the first crossover that actually measures is so useful.
Tweeter excursion and Fs
The tweeter’s own resonance (Fs on the datasheet) is the floor. A crossover point too close to Fs, with only a 6 dB slope, lets more low-frequency energy reach the tweeter right where its excursion and distortion climb. A steeper slope, a higher crossover point, or both buys margin. Read the tweeter’s Fs and the maker’s recommended crossover point first: how to read a driver datasheet. Then place the crossover point itself: how to choose a crossover frequency.
Measure before you swap parts
Electrical order ≠ acoustic slope. The drivers’ own roll-off adds to the network’s, so a “first-order” network on a real tweeter is rarely a clean 6 dB acoustic slope. Sweep each driver through the network (REW basics), then compare. Change one value at a time. Keep the board on screw terminals until the curves agree with the model. Layout mistakes can mimic order problems: crossover layout mistakes.
Starter recommendation
For a first two-way, start from a published, measured second-order design for your exact drivers, built on a test board. It has the tweeter protection and well-documented polarity behavior, and it’s still a small parts count. Build first-order only when the drivers’ datasheets and your own sweeps show they can handle the overlap. This page compares the two. For the build steps, go to the first crossover that actually measures. Woofer inductive rise is handled with a Zobel network. Which kind of capacitor? See crossover capacitors: film vs electrolytic. Which kind of inductor? See crossover inductors: air core vs iron core.