Let’s look at what happens when you have your vTuning slightly off from its correct value for your hearing.
The following graph shows the case of some frequency exhibiting a 60 dB threshold elevation. The diagonal black line represents perfect hearing, the red curve represents the recruitment hearing for that elevation. The green curves show the effect of using Crescendo with vTuning running in 1 dB steps from 57 dB to 63 dB.
You see that when vTuning is smaller than the actual threshold of hearing, your perception remains below the ideal hearing line. It does have the effect, however, of moving your apparent hearing threshold to lower presentation levels. That means you can hear fainter levels, below your actual threshold elevation level, but you remain less than ideal.
Conversely, when vTuning is larger than the actual threshold elevation, you achieve something closely resembling NYC Compression at the faint end. And that can sound quite nice, actually…
But note that this family of curves shows the variations with 1 dB increments in vTuning. That isn’t very much. And that’s why I spoke about how touchy it can be to get just the right vTuning level when your hearing needs strong corrections. Furthermore, your adjustments are changing things at the faintest perceptual levels, where it is most challenging for anyone to hear well.
For comparison, here is a graph for the case when your hearing has a milder threshold elevation of only 40 dB, and again, with 1 dB increments in vTuning:
At this threshold elevation, small variations in the vTuning setting are much more forgiving.
For people with more severe hearing impairments, where threshold elevations rise up to and higher than 60 dB, the use of dynamic range reduction compression, used ahead of Crescendo, can help reduce the criticality of vTuning adjustments.