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What is a "real SPL measurement?"
The article quoted does a good job. Real meaning "actual" as opposed to Weighted.
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Why wouldn't you want to use an RTA to check spectral balance? What other options are there?
I said I don't use an RTA much to check spectral balance, and that I take exception to putting an A weighting curve head of an RTA used for checking spectral balance.
The concept is, watch an RTA and ideally you see something like an average flat response when its mixed for spectral balance. Sorry, that doesn't work, and anyone mixing music would know that already. An RTA shows constantly changing in response to input, so determining a long term average would take extended time integration (which impedes your ability to respond), or "visual averaging", prone to error. What's really needed is a 20 second time "window" that is constantly averaging the last 20 seconds, and some software can do that, but the resulting display is obviously quite slow. The real goal isn't a flat display anyway, it would be a spectrum that faithfully matches the original music. Now to all of that we add a weighting curve, which does not represent equal loudness at high SPL, further introducing error.
I don't use an RTA as a mixing aid because I mostly don't know what the spectrum should be, but I do know that a long term average of any music is never flat, even without an A weighting curve added. Every piece of music is different, but most music shows lower energy in LF and HF areas, so adjusting for a "flat" display would be artificially pushing up bass and treble, in very simple terms. And, where is that RTA getting its signal? A calibrated mic in the FOH mix position, or console out? Again, two very different samples.
So there's really limited value. I do appreciate being able to see a feedback ring, but hopefully those are mostly subtle and tuned out before a live performance, and frankly, by the time I look at the RTA to find it, I've pretty much already decided what frequency it is and started to take action. There's always someone crowding the stage monitor, though.
There is only one good option for judging spectral balance: your own hearing.