The two-way speaker system sounded fine on music. But speech was consistently unintelligible. Audience members complained about clarity, not volume. The SPL was adequate — around 85 dBA at the back row. The STI measurement, however, was 0.38. Poor, bordering on bad.

I measured the transfer function and found the smoking gun: a 14 dB notch centered at 1.8 kHz. The crossover frequency was 1.8 kHz. This was not a coincidence.

At the crossover point, both drivers — the 12-inch woofer and the 1.4-inch compression driver — were supposed to combine at equal levels, producing a seamless transition. Instead, they were arriving at the listening position approximately 160 degrees out of phase at the crossover frequency. Near-perfect cancellation. The woofer's response was rolling off and the HF driver's response was rolling in, and in the overlap region they were subtracting instead of adding.

The cause was a combination of driver mounting depth difference (the HF horn's acoustic center was 8 centimeters behind the woofer's) and a crossover filter topology that added 90 degrees of phase shift on each side. Without delay compensation between the two paths, the acoustic signals arrived out of time.

Adding 0.23 milliseconds of delay to the woofer channel aligned the drivers' acoustic centers. The notch disappeared. The STI jumped from 0.38 to 0.61. Same speakers, same room, same EQ — just proper time alignment at the crossover.

The Moral: Crossover points are where phase alignment matters most. SonaVyx's phase and coherence display reveals crossover cancellations that magnitude-only analysis might miss — and a fraction of a millisecond of delay can be the difference between intelligible and incomprehensible.