Subwoofer delay and distance

How receivers use distance

A receiver lines up every speaker and the subwoofer by delaying the ones that are nearer, so their sound arrives at your seat together. Each channel is held back by the difference between its distance and the farthest one, at 343 m/s: about 2.9 ms for every metre (0.89 ms per foot). Turned around, making the subwoofer 1 ms later means setting its distance 0.343 m (about 1.1 ft) shorter, and making it earlier means setting it longer. That direction catches people out, because a later subwoofer sounds like it should be farther away.

The distance a receiver's automatic setup gives the subwoofer can be longer than a tape measure says: its own electronics and low-pass filter take time, which the measurement includes. Keep the measured value unless a measurement says otherwise.

Delay, phase and polarity

Where the speaker and the subwoofer both play, around the crossover, their timing decides whether they add or cancel. At a frequency f a delay of t turns the phase by 360 × f × t degrees: at 80 Hz each millisecond is 29°, and 6.25 ms is half a cycle, the same as flipping the polarity at that one frequency. Flipping the polarity switch turns the phase 180° at every frequency, so the two are not interchangeable across the whole crossover region. Two sources at equal level add 6 dB when in phase, are no louder together than one alone at 120° apart, and cancel at 180°.

Finding the right setting

Measure the subwoofer alone, the speaker alone and both together with Room EQ Wizard and a timing reference (our guide: how to measure your room), then open the files in the REW Analyzer. Its subwoofer view predicts the polarity and delay that sum best, and checks the prediction against the measurement of both together; subwoofer integration, step by step walks through it. Without a microphone, play a tone at the crossover frequency and try the polarity switch and small distance changes, keeping the setting that is loudest at the seat.