So I’ve gone down the vinyl record cleaning rabbit hole and have ended up at the point where I am spinning records in an ultrasonic cleaner as a means to remove as much grime and debris as possible. One idea that I had is that I would like to make recordings both before and after a cleaning so that I can compare side-by-side how well my cleaning efforts have paid off. So far I have had success with connecting one of my turntables to a spare preamp which in turn is fed into my sound card for recording via audacity. So far this all works fine and well and I can certainly tell a difference in reduced surface noise. While my ears are ultimately the judge for how well a record has cleaned up, such a test is largely anecdotal. I am wondering if there might be a way of processing the data that might generate some sort of metric so that I might further refine my cleaning techniques. If anyone has any ideas I’d be open to trying out whatever you might happen to dream up.
You can TRY subtracting the tracks to get the difference but that won’t work perfectly and I don’t know if it will be useful at all…
Of course except for the cleaning, the recordings should be as identical as possible… The same recording level and before any editing.
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Open one of the recordings (it doesn’t matter if it’s “before” or “after”).
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Select the whole recording and apply the Invert effect.
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Open the other recording with File → Import → Audio.
The second track will show-up below the 1st and when played or exported they will be mixed = summed (or in this case effectively subtracted). -
Zoom way-way-in to the point where you can see the “waves”, or maybe to the point to where you can see the actual samples.
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Click on the track name above one of the waveforms to slide it left or right to align both waveforms. (Remember the top one is inverted so the waves should be out-of-phase when aligned properly.)
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Play and/or export to see what you get!
…The problem will be timing - If you record (digitize) the same thing twice you won’t get the exact-same result because you are sampling at “random” places along the waveform. For that reason you’ll always get some difference when you subtract. (There will also be differences in the preamp noise.) Plus, there will be some analog speed variation (including some wow & flutter) from the turntable. At 44.100 samples per second (or higher) it doesn’t take much to become “imperfect” and there will be some variation in the digital clock too.
Your records should be cleaned before playing or digitizing but in my experience cleaning doesn’t help that much and most of the “snap”, “crackle”,
and “pop”, remains. 
Spectrograms are going to be different: dirty version, with more hiss & crackle, will have more high-frequency content
