The balcony seats at the Paramount Theatre had a coverage problem. The main PA didn't reach the back rows with sufficient level. The solution was obvious: add fill speakers under the balcony. Four compact boxes, properly aimed, properly delayed. Or so they thought.

After the fills were installed, the back rows were quieter than before. Not louder. Quieter. The crew turned up the fills. It got quieter still. They turned them up more. It got even quieter. The production manager called me in what I can only describe as a state of acoustic existential crisis.

The fills were wired in reverse polarity. When their signal combined with the residual energy from the main PA at the listening position, the two sources were 180 degrees out of phase in the critical mid-frequency range. More fill level meant more cancellation. The fills were functioning as acoustic erasers — actively subtracting sound from the balcony.

At the crossover region around 1.2 kHz, where both sources had similar level at the back row, the cancellation was nearly complete. There was a 20 dB notch that carved out the very frequencies human speech occupies. The audience could hear bass and sizzle but no words.

I flipped the polarity on all four fills. The level jumped 6 dB at the listening position. The production manager looked at me as if I had performed sorcery. I had pressed one button, four times.

The Moral: When adding speakers makes things quieter, check polarity immediately. SonaVyx's transfer function measurement shows phase alignment between sources — and a polarity flip is the cheapest, fastest fix in audio.