Measuring & correcting

Omnissiah plays noise through each speaker, records it at the microphone, and compares the result with a target curve. Omnissiah then writes four things into each NEXUS output:

• EQ, which cuts the peaks the room adds, mostly in the low end;
• delay, so sound from every speaker arrives at the listening position together;
• gain, so every speaker plays at the same level there;
• polarity and phase, so speakers that share frequencies (a sub and the main pair) add up instead of cancelling.

The correction does not fill deep dips. A dip caused by sound cancelling at the microphone stays a dip however much power goes in, so Omnissiah leaves it and says so on a Diagnostics card. Acoustic treatment or moving the speakers or the listening position fixes those.

Any measurement microphone works. The microphone connects to an audio interface with 48 V phantom power, never to NEXUS.

Calibration files

A calibration file lists the small errors in one particular microphone's response, so Omnissiah can remove them from the measurement. Omnissiah reads calibration files in the REW text format that most microphone makers supply.

In Settings → Audio, next to Mic Calibration, click Import… and choose the file. The plot shows the correction the file applies. Remove deletes an imported file.

Many microphones come with two files:

The 90° file lets the microphone stay pointing up for every speaker, so nothing moves between the left speaker, the right speaker and the sub.

If your microphone has no calibration file, leave Mic Calibration on None. The measurement then includes the microphone's own small response errors, which in a measurement microphone sit mostly in the high frequencies, away from the low end where rooms cause most problems.

During measurement Omnissiah plays noise out of the Output Device chosen in Settings → Audio and records the microphone on the Mic input. For accurate delays, both should be the same audio interface, so they run from one clock. The quick start does this: interface line outputs into NEXUS Input 1-2, microphone into the same interface.

You can instead choose Tantrum Nexus as the Output Device, so test noise goes to NEXUS over USB. The microphone is then on a different clock. In that case open the Speaker Playback About to Begin dialog, expand Measurement Settings, and set Clock Drift Correction to Enable.

The rules from Chapter 3 — Quick start, step 5, apply to every measurement:

• the microphone goes where your head is when you listen, with the chair moved away;
• the same distance from the left and right speakers;
• the capsule at seated ear height;
• pointing at the ceiling with the 90° file;
• at least 30 cm from the desk, walls and other large surfaces;
• fixed on a stand, and still until the app asks you to move it.

More than one position

One position corrects the sound for one point in space. Moving your head a few centimetres changes the low end a little, so a correction built from several nearby positions holds up better as you move.

In the Speaker Playback About to Begin dialog, expand Measurement Settings and set Quick Positions to 3, 5 or 7. Omnissiah measures every speaker from each position and pauses between positions with Position N of M and a Continue button.

The first position is the centre, placed exactly as above. For the others, the app asks you to move the microphone along an infinity (∞) pattern around the centre: about 5–10 cm from the previous spot, varying the height a little (head height, slightly above, slightly below), and keeping clear of seat backs, headrests and walls.

Click to zoom

Redo Last Position repeats the last position if something disturbed it, such as a noise or a knocked stand.

Complete Measurement, the second card on the Calibrate page, measures a wider listening area, such as a sofa or a control room with a producer's desk and a client couch. A Complete Measurement takes about 20 minutes. The Omnissiah manual covers it in its Room correction & calibration chapter.

Level-setting plays noise through one speaker at a time. The meter turns green when two conditions have held for 5 seconds: the noise is at least 6 dB above the room's background noise, and the microphone level is at least -35 dBFS.

"Low signal detected, please check your interface settings or microphone connection." means the microphone hears almost nothing: check that 48 V phantom power is on and that Mic is set to the right interface channel.

Confirm your speaker setup appears after the last measurement. The list shows each speaker with its measured and corrected frequency ranges, and offers three settings: Crossover (where the correction treats the sub and the main speakers as splitting), Match the target down to, and Below that. The defaults suit most systems. Click Calculate to continue.

Crossover here only shapes the correction. The crossover filters on NEXUS are set on the Output page (Chapter 8 — Outputs & crossovers).

Calibration results shows, for each output:

• Before | After: the measured response without and with the correction, against the target. The switch changes the chart only. To hear the difference, apply the correction and use Bypass all (below).
• Chips Ch 1 Output 1, Ch 2 Output 2 and Ch 3 Output 3 to switch between outputs.
• Diagnostics: plain-language cards on anything the correction found or chose not to fix.
• Calculate Again reruns the calculation on the same measurements.
• Export saves the results to a file. Send to team saves them and opens a pre-filled email to us, for when you want help reading them.

The Omnissiah manual's Understanding & applying results chapter explains the charts and every diagnostic card.

Apply to device opens Apply calibration results, with one row per channel showing which NEXUS output receives it. Click Apply to device in the dialog to write the correction. NEXUS stores it and plays through it from then on, with or without the computer.

The Applied Correction page opens next. Each applied set has a card with an on/off switch, Response, Details and Delete.

• Bypass all switches every correction off, to hear your room without it. Enable all switches them back on.
• The Applied row on the Calibrate page has the same Bypass/Enable button and opens this page.
• Delete removes a correction: those outputs go back to playing uncorrected at 0 dB. The measurement stays in History.

Every measurement is kept under History on the Calibrate page, so you can go back to an earlier result and apply it again.

Measure again after anything that changes the sound at the listening position:

• you move the speakers, the subwoofer or the listening position;
• you add or move acoustic treatment or large furniture;
• you change a crossover on NEXUS;
• you add a subwoofer.

A new measurement does not change what NEXUS plays until you apply it.