What the crossover frequency is
The crossover frequency (often written Fc) is the point where a crossover hands the sound from one driver to the next. In a two-way speaker, that is the handoff from the woofer (or mid-bass) to the tweeter. Pick it from the drivers you have, not from a round number. Rod Elliott of Elliott Sound Products (ESP) puts it plainly: the mid-bass driver’s upper limit and the tweeter’s lower limit determine the crossover frequency you have to use. An octave is a doubling of frequency. ESP’s example: one octave below 300 Hz is 150 Hz, and one octave above 3 kHz is 6 kHz.
Start with the tweeter’s floor
The tweeter’s own resonance, Fs (free-air resonance, listed on the datasheet), sets the lower limit (how to read a driver datasheet). ESP’s guidance for a 6 dB/octave (first-order) filter is an absolute minimum of about 1.5 octaves between the driver’s resonance and the crossover frequency. His worked example: a tweeter with a 900 Hz resonance should be crossed at a minimum of 2,500 Hz, and preferably higher. At that minimum spacing, ESP expects a small peak at the tweeter’s resonance. Why it matters: a slow rolloff lets more power through below the crossover point. ESP says that can excite the tweeter’s resonance and may add a subtle “honkiness” in the upper midrange. ESP also notes that few tweeters will tolerate frequencies much below 3 kHz. The slope trade-off is covered in first- vs second-order crossovers. ESP’s design tables add one more check. A notch filter (a circuit that flattens the tweeter’s impedance peak at resonance) may not be needed if the resonance is more than two octaves below the crossover frequency, but ESP says to test that before you rely on it.
Then find the woofer’s ceiling
A woofer starts to beam when its cone diameter approaches the wavelength of the sound. Beaming means a strong lobe straight ahead and weaker, changing output off to the sides. ESP’s figure: at 3 kHz, one wavelength is only 115 mm. Many mid-bass drivers are wider than that, so their off-axis response is poor by 3 kHz. That is the core two-way compromise ESP describes: the tweeter wants a high crossover, and the woofer wants a low one. Check the woofer’s datasheet for a polar plot, or for on-axis and off-axis curves (often at 45 degrees). ESP notes that some datasheets include these. For a driver covering a band, ESP’s rule of thumb is clean reproduction for at least one octave, and preferably two, above and below the crossover frequencies. Apply the same check to each driver at the point you are considering.
Put the two together
- Read the tweeter’s Fs. With a first-order filter, stay at least about 1.5 octaves above it (ESP: 900 Hz resonance → 2,500 Hz minimum). ESP gives the 1.5-octave figure only for first-order filters; a higher crossover point adds margin with any slope.
- Read the woofer’s off-axis curves and find where it starts to beam.
- Your crossover goes in the window between those two limits. If there is no window, change a driver. ESP mentions a waveguide as one way to let a tweeter work lower than normal.
- Check impedance through the crossover region. ESP says to include impedance correction unless the driver’s impedance is flat across roughly 2 octaves above and below the crossover frequency, and to design for the actual impedance at the crossover frequency, not the nominal rating (speaker impedance curves, explained).
- Build it, measure it, and adjust (REW basics). Watch the parts layout too (crossover layout mistakes).
Slope changes the answer
ESP prefers a simple first-order filter where it works, because it has the most predictable response and is least affected by impedance changes. But it leaves the drivers producing sound outside their limits. ESP recommends against first-order passive crossovers with amplifiers of more than about 30 W, and suggests a series-connected crossover if you use one: a circuit where the woofer and tweeter sit in series across the amp, each bridged by its own filter part. That’s a crossover design, not the plain series wiring used to combine speakers on one channel. ESP’s view on passive slopes is that anything over 12 dB per octave is a waste of time. A second-order network also flips polarity: in a two-way, the tweeter is usually wired reversed. If you’d rather set the frequency in software, see active or passive crossover.
Before you order parts
For a worked first build, see a first crossover that actually measures.