Actually, it's a technique I developed at JBL about 35 or 40 years ago that I'd rather not explain in detail just yet. I will give you some background though. At the time (mid 60's), I was head of Quality Control at JBL.
We had to provide the Army with matched pairs of 075 drivers (the ring radiators - I don't know the current name for them). It would tie up the lab for the entire day, running curves and trying to find the identical units out of a batch of 50 or so units.
While testing them initially to make sure they worked and weren't rubbing or buzzing, I noticed there was a signature sound to each driver when listening to white noise. Just for fun, I separated them into batches, by their sonic signature, and wheeled the cart into the lab. I told the lab technician that these units sounded similar and I pointed out the pairs that I thought sounded identical.
To everybody's amazement, the pairs I thought were identical really were - they matched to within about 1/4 dB. So were all the other pairs that I had matched by ear, but the "similar" units matched to within 1 dB.
The next time we got an order from the Army, I did my "matching trick" again, and after running the charts, we wound up with the same results. I could match a batch of 50 drivers by ear in less than 30 minutes as well as the chart recorder in the lab, which took all day to run and match the drivers, using the paper chart recorder curves.
The Army didn't ask for our curves; we shipped them as "matched units", and they ran their own confirming tests. So, from then on, we supplied the Army with "Harvey matched" drivers, and simply didn't tell them how we matched them. Of course, to make sure, I'd do one "by ear" match from the tray of drivers and have the lab confirm they really were matched.
Over the length of the Army contract, it probably saved us hundreds of man hours and it freed up the lab for other projects.
We'd still leave the tray of drivers in the lab all day, since Ed May (the chief engineer), Bart Locanthi (head of R&D), Gus (the lab technician), and I decided it would probably be best if it remained our little secret.