When I watched that movie of Crescendo in Action, I never saw any boost being applied up at the highest frequencies. And that’s just where you’d expect your hearing to be at its worst. What gives there?…
Well Crescendo operates in the space of Bark frequencies and Loudness, or dBHL. When the source material is beneath the unimpaired hearing threshold at 0 dBHL, it is not possible for anyone to hear it anyway, so we don’t bother boosting that nonsense. In actual fact, we don’t boost anything below whisper level either.
This is done intentionally to avoid unnecessary clipping in the output waveforms, and to prevent just making a lot of hissy noise. You won’t be able to hear that hiss, but your significant other in the adjacent room might start complaining about it.
When Crescendo avoids gratuitous amplification it keeps everyone happy. People without hearing impairment can actually listen to your own gross corrections and not become grossed out. They might think it sounds a bit too bright for their personal taste but it shouldn’t act as a repellant to them.
When people listen through Crescendo processing, and when they use too much vTuning for their own hearing, it just sounds like someone turned up the treble, but in a way that acts more like conventional loudness controls on 1980’s era amplifiers. The boost is only applied at low sound levels, never at higher levels. So it just sounds a little bright in the reverb tails and background.
And that explains the mystery, in my mind for so long, about why everyone likes Crescendo, even those without hearing impairment. It turns out that this kind of “loudness control” is very appealing to nearly everyone. They like the slight exaggeration of reverb tails and background sounds.
We once went to a recording studio near Palo Alto, just south of San Francisco, to demonstrate the Crescendo system. They had a young guy, a 20-something “Golden Ear” mixing engineer, who listened to Crescendo through the big studio monitors in the control room. And he actually chose a non-zero vTuning level for his best preferred level. That has to tell you something…
[I have never found anyone who didn’t like some amount of Crescendo, hearing impaired or not. They might like it, although not enough to strongly desire it. But we need it.]
[ For the technically inclined, there are actually two things going on at the very highest frequencies. First is what was described above about not amplifying the noise floor unnecessarily. Second is the fact that we operate in Loudness space.
In Loudness space, it takes only a small amount of gain, at the highest treble and deepest bass frequencies, to produce a large amount of Loudness boost. That’s why the high end boost is generally much lower than for the rest of the audible range, whenever it isn’t actually zero.
You can see this effect arising from the crowding of iso-loudness contours in the Fletcher-Munson Loudness map.]