The PA sounded wrong. Not terrible — wrong. The kind of wrong where nothing specific jumps out but your brain keeps insisting something is off. The low-mids were thin. The vocal had an odd hollowness. The subwoofer seemed disconnected from the tops. Three hours of EQ tweaking, system processor adjustments, and increasingly desperate amplifier swaps had produced no improvement.

I measured the transfer function. The magnitude response was ±3 dB — reasonable. But the phase trace told a different story. Below 300 Hz, the phase was inverted. Above 300 Hz, it tracked normally. The sub-to-top transition region had a perfect polarity reversal pattern.

The subwoofer was out of polarity with the tops. Not because of a processor setting (I checked three times). Not because of an amplifier switch (I checked twice). Because of one XLR cable — a single 10-meter cable between the processor output and the subwoofer amplifier — that had pins 2 and 3 reversed at one end.

Someone had soldered it wrong, probably years ago. It had been in the cable trunk since, indistinguishable from the fifty other XLR cables, waiting for the day it would be patched into the one signal path where it would cause maximum confusion. That day was today.

I swapped the cable. Three hours of acoustic mystery solved in four seconds. The low end reappeared. The vocal filled out. The system tech looked at the offending cable and said, 'I'm burning it.'

The Moral: When the sound is wrong but the response looks okay, check phase. SonaVyx's transfer function with phase and coherence display would have revealed the polarity inversion in the first measurement — turning three hours of frustration into a thirty-second cable swap.