Magnetic tape has an inherent noise floor (a tape hiss) deriving from the magnetization of the ferrite particles on the tape emulsion even in the absence of a recorded signal, i.e., if you take a blank cassette tape and play it back at full volume, you will hear some hiss. Dolby noise reduction was developed in the 60’s to try to reduce that inherent magnetic tape noise. Basically, when you record a tape with Dolby noise reduction the high frequencies in the audio you are recording are boosted as they are transferred to the magnetic tape, the higher level (loudest) signals less, the lower level (quietist) signals more. When you play the tape back, you want to use the same Dolby noise reduction circuitry to reduce those boosted high frequencies by the same amounts to restore the original dynamic range i.e., reduce the higher level high frequency signals a little, the lower level high frequency signals a lot. Since the tape hiss comes from the playback tape at a constant low level (as a property of the magnetic tape itself rather than the recorded audio), it gets lowered a lot as it enters the Dolby circuit prior to the playback amplifier (since it is a low level high frequency signal) and drops by as much as 10 dB below the actual audio you recorded (since the low level high frequency audio you recorded was boosted when you recorded it). This reduces the level of tape hiss and its perception by the hearer significantly.
In our test case here, the cassette recording was apparently recorded with Dolby noise reduction, so the high frequencies were boosted. However, when I played them back, I did not have access to Dolby circuitry. When I cut the high frequencies (as I described earlier), I probably reduced the tape hiss a lot, but only reduced the guitar recording high frequencies back to their original levels (since they would have been previously boosted during the Dolby recording process years ago when the tape was dubbed from the reel to reel master). In effect, I manually created a Dolby noise reduction filter.
So, I think you can see some immediate applications for these kinds of techniques in audio recording and mixing. For example, if your acoustic guitar track sounds muddy, look at the frequency plot of the recording and see where the lower frequency levels are, then do some equalization in that area. As a matter of fact, before I began the noise reduction process on this test recording, I did reduce the frequency content below 400 Hz (using the Equalization Effect) in order to clarify the sound of the guitar (and compressed and amplified the track, which was recorded at too low a volume). There are some general rules of thumb for eq of common rock instruments, for example, bass guitar tracks muddy up a multi-track mix unless you eq out the general neighborhood of 250 Hz in the bass guitar track. You can, of course, skip the frequency analysis and go entirely by ear. In that case, just increase the slider levels on particular frequencies of your equalizer until the objectionable sound quality becomes even worse, and then lower those same frequency sliders on the equalizer to eq out the undesired frequencies for that track. Or, conversely, adjust the sliders on the equalizer for a particular track at different frequencies until the track punches through the surrounding mix or acquires the type of sound you are looking for. It is helpful to loop a few seconds of the track (set it up to keep repeating automatically) while you are making this kind of analysis by ear, so you don’t have to keep restarting the track.