Some of us have serious degrees of hearing loss at the higher frequencies. For example, my own hearing much above 1 kHz tends toward threshold elevations in the range 50-70 dB, and possibly higher above 6 kHz. For listening to music, I would definitely call that a serious degree of impairment.
Crescendo can help here, and you will hear quite faint details in high strings, high piano registers, faint percussion and sizzle, and the details of vocal lyrics. But what often accompanies the recovered hearing is a scintillating quality in the faintest sounds. Why is that? And what can we do about it?
Recall that the higher your threshold elevation at some frequency, the steeper your innate recruitment curve:
This graph shows the ideal hearing as the diagonal green line, where you hear what is presented at all levels. The horizontal axis is the presentation level of the music, and the vertical axis is how loudly you perceive it to be.
There are two recruitment curves shown in bold red, one for a threshold elevation of 40 dB, while the other to the right is for an elevation of 60 dB. That rightmost recruitment curve is in the general range of serious hearing impairment at this frequency.
You see how much steeper the more serious curve is, as it trends downward toward threshold levels? That steepness in the curve is the ultimate source of the scintillating sounds at faint levels. And without correction you see that happening for sounds still as loud as 60-70 dBSPL, in the range of normal adult conversation loudness.
Crescendo corrects this recruitment hearing to become the green line over a significant range of presentation levels. But to do so, it needs to boost the input sound level by the distance between the green line and where the recruitment curve has the same perceived level. (that light blue horizontal line, for example)
When a sound is faint and wavering, the steepness in the recruitment curve causes much larger loudness variations in our perception, compared to what is heard by people with normal hearing. It takes only a slight increase or decrease in the sound to swing more widely in the vertical perception axis. And all sounds waver in loudness to some degree. We hear an exaggeration of that waver.
Crescendo is quite good at correcting your hearing, but it isn’t perfect. If Crescendo did a perfect job of hearing correction then you would experience only the slightest waver that everyone else hears – probably imperceptible to most people. But since it isn’t perfect, there is still some residual faint exaggeration. It is that faint residual imperfection that becomes the scintillation of the background sounds.
So what to do? This is where adding a plugin like CLAS, ahead of Crescendo can actually improve the situation for us. CLAS is a dual NYC Compressor, acting separately in bass and treble regions. We are interested in using it for its treble enhancement. The bass treatment is just ear candy for us, since we most likely have near-normal hearing in the bass region.
NYC Compression boosts the fainter portions of music while leaving the louder portions alone.
This diagram shows the ideal hearing as the green diagonal line, just as before. The orange curve is the underlying linear compressor used in a NYC Compressor combination. When that compression is added back to the dry signal, it produces the bold green curve – boosting at lower sound levels, and leaving the louder portions alone.
The red curve is the original recruitment curve for a 60 dB threshold elevation. But when using CLAS compression your hearing becomes more like that fainter red curve. You see that everywhere, its slope is more gentle than raw recruitment. And that’s a key toward eliminating those scintillating background sounds.
By placing CLAS ahead of Crescendo, you modify the sound field that Crescendo must deal with. In effect, the NYC Compression simply brings up those fainter sounds to more loud presentation levels, and reduces the apparent wavering, making it easier for Crescendo to correct for your hearing. NYC pre-Compression helps by deemphasizing the residual imperfections of Crescendo.
Anything that brings up the faint sounds will help Crescendo in the same manner. But typical range reduction compressors are linear compressors, and will affect both the soft and the loud portions of your music. That might be okay for you. But I prefer to leave the loud portions alone, and help on the bottom end.