Thanks, Steve. It worked out. I picked up the general strategy you recommended. Here’s a postmortem:
I did a frequency analysis on the original “reference” soundfile (the one I linked upthread), looking at both linear and log frequency. I took a screenshot of each so I could refer to them easily. Then I did a frequency analysis on the bare voice file (the new one that I wanted to make sound like the old one).
I picked one difference between the frequency analyses of the reference soundfile and the new soundfile, and I made an adjustment to the EQ effect. I executed the EQ and did a frequency analysis on the result. From that I figured out another further change I needed to make to the EQ (normally just focusing on one frequency or small range).
Then I [crtl-Z] (undo) the EQ change I made to the new file (so I’m not doing multiple EQ passes). I made the further changes to the EQ, ran it again, considered the result (so… what else needs changing?), backed out of the change, repeat.
Repeated about 30-50 times, in fact, until I got the EQ graph correct (or close enough, anyway).
Sometimes I had to make a big bend in the EQ graph to make a small change in the frequency profile. Sometimes a much smaller adjustment to the EQ graph made a much larger change to the profile. This depended on the particular frequency in question.
The logarithmic frequency analysis showed a slope on the low end, which suggested that the original sound was subjected to a high-pass filter. So I did one with approximately the same cutoff and dip. Tried this a couple times and compared back-and-forth with the reference profile. (This back-and-forth was basically the same process as getting the EQ correct. Tweak the numbers, execute, compare profile, reverse, tweak the numbers.) The high-pass changed the EQ profile, so towards the end of the effort, you have to consider both changes at the same time.
This process took care of getting the distortion (clipping) more-or-less correct, without any extra effort. Differences from the original are probably a result of different microphone and original recording volume, but this turned out to be only slightly detectable – not really noticeable at all.
Finally, there was a background hum in the original, which I needed to reproduce. It extended a couple tenths of a second beyond the beginning and ending of the actual voice content of the reference file, and it plays clean where the voice pauses for breath in the middle. I picked up some of these pieces and copied them together, removing any slices that showed a pattern to the patchwork. Then once it was a one-second hum with no beats in it, I copied and spliced random pieces of it within itself until it was long enough for my purposes.
I mixed the hum with the voice, and found that I needed to adjust the EQ a bit more. The reference file had the hum, and my new file did not, so that threw off the frequency analysis a bit. But only a bit – it was all sorted in two more minutes.
Of course I saved the EQ profile, in case I need to add any further voice to this project. Now I’ve got a fair emulation of the effect used by the original sound engineer.