Control Booth Monitor Calibration

Control Booth Monitor Calibration

I don’t like mixing from within a booth any more than the next person, and for any serious audio work, I will fight hard for a FOH mix position. However, for “talking head” events or playback-based dance shows, sometimes a booth mix position is a reality. (In the case of the dance shows, it’s actually preferred because there’s a lot of comm activity that would be distracting to nearby audience members.)

During some downtime in a recent rehearsal for a playback-only show, I decided to give some love to the pair of nearfield monitors hanging in the theater’s booth. They’re currently driven off the main system DSP and fed from the console’s L and R signals (although I may repatch them to the console’s solo bus in the future). Although sitting in a tiny, acoustically absorbent booth is nothing like sitting in the middle of a reflective 700 seat concert hall, perhaps there is an improvement to be made: what if we could tune the booth monitors so that the person sitting at the mixing console would experience the same SPL and tonal response as a person sitting in the center of the venue?

I started by taking a couple of traces in the middle of the venue and averaging them together. This will be our target curve for the booth system.

Here we see that the coverage of the main system is quite consistent over the space (another article, another time) and the orange trace represents an “average” system response with the subwoofer muted. That’s going to serve as our target curve for the booth monitor system.

The trick, however, is that we can’t simply just match the booth system to the target trace and call it a day. The booth has a 2′ by 3′ sliding glass window that is open during shows, so we have to treat the booth system more like we would an underbalcony fill. We’re going to measure the output of the mains from inside the booth which will tell us what’s coming through the window, then use the booth system to bring us back to the target. So the booth system will piggyback the output from the mains.

We start by measuring the left side of the mains from mix position inside the booth.

We’re coming in lower in level and slightly smiley’d. To find the dB offset needed to bring us back up to level, you can use any of the following methods to overlay the two traces:

  1. Use [Ctrl + Up Arrow] to shift the trace up
  2. Click “dB +” in the control bar
  3. Actually click and drag the trace data upwards

You could also shift-click to normalize the two traces, but I don’t want to move the reference trace at all. I just want to overlay the booth trace and let Smaart tell me how many dB I need.

Answer? 10 dB, as indicated in the upper right hand corner of the magnitude pane, below the trace name. (You can always clear this magnitude offset by pressing [Y] or Clear dB Offset from the Command menu.)

The brown trace shows the result of knocking down some of the worst offenders in the magnitude response with some EQ in the system DSP. Now a much better match for the target.

But remember, we’re still viewing solo response. We still need to integrate that with the mains coming in through the window.

I grabbed a new target trace: center of the house, with the subs on (orange) and did a quick delay alignment on the booth monitors, timing them to the midrange arrival through the window. The data below shows how the booth mix position (olive) now compares to the reference out in the house. 

We’re getting closer, but not there yet. The HF in particular has a bit of a ramp up (to be expected…the large-format mains are firing through 50 feet of air to reach their listeners, and my little booth monitors are going about 4 feet).

Here we see the results of a hi-shelf filter and a tweak to the delay time.

 HF coherence stinks but we’re trying to align a tiny, very close, passive bookshelf monitor with a large, distant active multi-element array, so a dose of realism is in order here. 

After repeating the procedure with the right side, I ended up adding a few more milliseconds of delay until the alignment sounded as natural as possible from the mix position.

Conclusions: It worked. I still would hate to mix a show from inside there, but it’s certainly helpful having an accurate impression of general tonality and level.

Leave a Reply

Your email address will not be published. Required fields are marked *