The Ten-Minute Tuning

The Ten-Minute Tuning

Here’s a quick “checkup” tuning on a basic flown L/R point source rig in a small black box theater. I was on the site working on the comms system and we decided to quickly measure the PA before I left. This entire process took ten minutes from top to bottom. The idea here is to move quickly, not getting caught up in the details, and prioritizing the large-scale issues.

We are going to check the aim, make sure both sides match tonally, and make sure both side match in level.

The seating area is narrow enough and far enough from the mains that some semblance of stereo is possible. For this, we depart from the usual half-and-half segmentation approach for mains aim and instead try to cover the entire seating area with each side. 

We start with an ONAX measurement, with the mic placed smack dab in the center of the seating area, which is where both boxes are aimed. 

A lot of ripple in the HF magnitude response, big dips in coherence, and a confused phase trace. All telltale signs of a mistake: we are measuring with both sides on. Let’s try again with just Right:

That looks better, but there’s still a red flag in the phase trace – it’s hanging around 180° above 800 Hz. That’s a polarity inversion. This is why it’s important to pre-verify your rig before measuring stuff – that could be a miswired XLR cable in the measurement rig, not a problem with the system at all. Hold that thought – for now, we note it and move on.

Horizontal Aim

We verify horizontal aim by measuring the response at the outer edges of the midpoint of the seating area. If one edge has more HF rolloff than the other, the aim is off center. If HF rolloff exceeds 6 dB at either edge, we need a wider box. With a helper to move the mic, this takes only a few seconds.

The edges of the seating area have HF that hangs within 2 dB of our ONAX reference (gold trace). These are wide boxes. Horizontal aim is fine.

Vertical Aim

The vertical aim is checked in a similar way, comparing the HF rolloff between the front and back edges and the center ONAX gold reference.

Again, the edge responses are well balanced to each other at HF and within 6 dB of gold so the loudspeaker aim is sufficient to cover the seating area as evenly as we can.

Symmetric Check

The left side is put through the same paces:

Left side ONAX:

Polarity inversion is still there.

Left side horizontal aim check:

Left side vertical aim check:

Truth be told, these are wide boxes and so there is a large margin of error on the aiming.

Finishing touches

After running all the test cables and console together in a loopback, the polarity inversion was revealed to be in the amp/loudspeaker part of the system, so I flipped the polarity on both sides in the DSP. This is probably not the “mistake” that it first appeared to be. Likely, the internal passive crossover design inside these boxes requires a polarity inversion of the HF driver for proper summation (common with crossovers using 2nd order filters on both HF and LF, resulting in 180° phase offset at fc). In other words, it’s a feature, not a bug. This is the type of thing that is easy to overlook when moving quickly.

Since there’s no sub in the space, and there’s no good way to get a clean LF response measurement, it’s a take-your-pick scenario here. I’d rather have the HF be in absolute polarity with the rest of the system, as the LF is going to be quite volatile in such a small room anyway. At any rate, the far more important thing is that all the boxes have matched polarity.

Then I fixed the HF runup around 9 kHz on the left channel with a bell filter (note fixed polarity on phase trace). (Purple = pre-eq, blue = post-EQ)

In a direct comparison, it seems that the HF driver of the right main is 2 dB hotter than the left side.

Since I’m more concerned about HF than LF, and we’re dealing with a passive crossover, I just turned the box down rather than try to place a large shelf filter. Yes, now the LF is 2 dB down, but in my opinion that’s far less important for imaging especially given the modal issues in the room.

And we’re done.

If you’re wondering why I didn’t tamp down that devil-horns bump at 630, note that it doesn’t show up consistently over the coverage of that box, so it’s a localized thing and shouldn’t be fixed with EQ. Overlaying traces from that box taken over the space shows a large amount of variation in the 600 range, and there’s no one-size-fits-all EQ solution here. Compare that against the HF ramp up at 9 kHz. Since that’s consistent in all locations, it’s a good candidate for EQ.

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