Loudspeaker polarity reversal is not the same as phase — polarity is frequency-independent

‘Polarity’ and ‘phase’ are frequently confused in loudspeaker work. Polarity refers to which terminal is positive: reversing the + and – connections flips the entire waveform (multiplies signal by –1). This is frequency-independent: every frequency is affected equally. Phase refers to the time or angular offset of a signal at a specific frequency, which is frequency-dependent. When a loudspeaker’s polarity is reversed relative to others in a cluster, the effect resembles being 180° out of phase, but phase cancellation frequency and severity would vary with frequency whereas polarity reversal cancels equally at all frequencies. In a cluster, a reversed-polarity woofer can be physically destroyed at high levels as adjacent in-phase woofers push against its inward-moving cone.

Examples

Two subwoofers stacked together with one reversed in polarity will cancel across all frequencies, halving the effective output and potentially damaging the reversed driver at high power. A 180° phase-shifted signal at 100 Hz cancels only at 100 Hz; adjacent frequencies are progressively less affected.

Assessment

You add a second subwoofer and the bass drops rather than increases. What is the most likely cause and how do you test for it with a 9V battery?