Time-Aligning Left and Right: When It Helps

Two speakers of the same model have the same excess phase, within manufacturing tolerance. The box roll-off and the crossover add the same delay to the same frequencies on both sides.

The difference at the seat therefore comes from outside the speakers. Distance is one part, and the table below gives its size. The room is the rest: a wall nearer one speaker, a window on one side, a desk edge or a rack beside one of them. Each gives one channel reflections that the other does not get.

Imaging is the sense that a sound sits at a place between the speakers, and it depends on how the two channels arrive at your ears. When the two arrive with different timing or level, the image moves. Left and right alignment therefore corrects the difference in timing and level between the two paths.

Sound travels about 343 m/s, so each extra metre of path is 2.9 ms and each 10 cm is 0.29 ms. Level falls with distance as well. In free field it drops 6 dB per doubling, so with the nearer speaker 2 m away, a speaker 30 cm further back is 1.2 dB quieter. A small room follows this law less cleanly, especially in the bass.

The image moves towards the nearer speaker. A centre image starts to move once the arrival difference reaches a few tenths of a millisecond, and sits fully on the earlier speaker near 1 ms. A level difference of a few dB moves the image too.

The last column is a centre-panned signal, which plays from both speakers at once. The two copies add at the seat, and a delay of Δt between them first cancels at 1 / (2 × Δt). The 30 cm row lands at 570 Hz, in the range where both speakers carry vocals, snare and the body of most instruments. The dip is partial, because the level difference stops the cancellation being complete.

A measurement taken at one microphone position gives the difference at that point. Whether it still applies when you lean to one side depends on frequency. Below about 500 Hz the wavelength is 69 cm or more, so the difference between two speakers at the seat survives head movements of a few centimetres.

Above 1 to 2 kHz the wavelength is 17 to 34 cm and the same movement turns the phase by a large fraction of a cycle. What remains once the delay is removed is mostly reflections from the desk, walls and ceiling, and it changes with every centimetre of head position. A filter fitted to those reflections is right at the microphone and wrong close by.

A delay on the nearer speaker helps when the distances differ, and the amount does not depend on where your head is within a few centimetres, since a 5 cm sideways move changes the arrival difference by only 0.15 ms. Phase work at high frequency does not hold across a head's movement. The fuller argument for why is in the article on absolute phase.

A speaker wired or filtered with reversed polarity turns every frequency by 180 degrees on its side. In the bass, where both speakers play the same signal, the two cancel at the centre. With 10 cm of mismatch at 2 m, the reversed pair sums 26 dB below the in-phase pair at 50 Hz and about 20 dB below at 100 Hz. A polarity switch fixes the cancellation.

Check polarity before touching delay, because a delay chosen on a polarity-reversed pair compensates for a polarity error instead of a distance. On an impulse response measured from each speaker, the first large peak should point the same way on both. Polarity, Phase and Delay Are Not the Same Knob covers what each control changes.

1. 1Check polarity on each speaker with the same signal.
2. 2Measure the arrival time from each speaker at the listening position. Use the impulse peak rather than a tape measure, because a speaker's acoustic centre is inside the box.
3. 3Delay the earlier speaker by the difference. 0.87 ms is 30 cm of path.
4. 4Match the level separately. A delay does nothing for a level difference.
5. 5Measure again, then repeat with your head moved a few centimetres each way. The measurements below 500 Hz should agree.

The delay goes to the nearer speaker, because a speaker cannot be made to arrive sooner. Delaying both by the same amount is only latency. If the two are within a few centimetres of equal distance, the table says the gain is small: 10 cm is 0.29 ms and 0.4 dB at a listening distance of 2 m.

In Omnissiah

In Omnissiah, the NEXUS calibration app, the setting is Time-Align Tops. The default is Subwoofer only, which aligns the subwoofer to the tops and leaves left and right polarity-checked but at 0 ms. Switch it to All speakers when left and right sit at noticeably different distances, and the earlier speaker receives the delay. Phase Correction is a separate pass, off by default, that fits all-pass filters with corners at or below 1.5 kHz. It makes calibration longer and helps only where the remaining phase error is large enough to hear.