Crossover & protection

A horn driver needs a high-pass filter for two reasons. It divides the spectrum with the woofer, and it protects the diaphragm. Below the horn's cut-off the air load disappears, excursion rises steeply and a compression driver can be destroyed by a few watts. The filter must have removed most of the drive by then.

Choosing the frequency

A common rule is to cross over at least an octave above the flare cut-off, $f_x \ge 2 f_c$, and higher still with shallow slopes. The read-outs show the margin in octaves and how far the filter has already attenuated the signal at $f_c$. Aim for 12 dB or more.

Component values

Every passive alignment scales the same way with load impedance $Z$ and crossover frequency $f_x$:

with one coefficient per part. For the high-pass, listed from amplifier to driver:

Butterworth filters are 3 dB down at $f_x$ and sum to flat power; Linkwitz–Riley filters are 6 dB down and sum to flat amplitude on axis with the drivers in phase (second order needs one driver's polarity reversed). Fig. 1 is computed from the actual ladder, so it shows exactly what the listed parts do into a resistor.

The caveat

A loudspeaker is not a resistor. The impedance of a compression driver on a horn swings widely around resonance, and textbook values then give a different response. Two habits keep theory and practice close: put the L-pad between the filter and the driver so the filter sees a nearly constant load, and take $Z_H$ from a measured impedance curve at the crossover frequency.