As an illustration of some of the points I made in the last two posts, here are my own settings for audiology, taken from the Crescendo plugin and drawn in graphical form.
I show in bold colors, the settings I’m currently using after having adjusted the values given me by a professional audiology exam. Those professional measurements are shown in the faint colors beneath my new curves.
The disparity here is caused by the failure of audiological testing to actually test agains ISO threshold levels, using instead, some ad-hoc headphone level that was determined about 50 years ago from a small sample of young Air Force cadets, and using ancient headphone technology no longer available.
You see that almost everywhere above 1 kHz, the professional measurements fall below the actual threshold elevation above the ISO Audible Threshold of Hearing, or 0 Phon. Plugging those professional measurements into Crescendo produces not quite enough correction.
But between my new Left and new Right curves, there seems to have opened a wide disparity in the 1 kHz zone. That’s what I was talking about in the previous post, about no longer being satisfied with using vTuning for myself. vTuning treats both ears as having the same sensitivity across frequencies. In my case, that gulf between left and right is quite noticeable since it occurs at upper mid-range frequencies where music still contains a lot of power. Hence I must use custom audiology settings for best results.
What is also interesting is the graph of my Decruitment / Hyper-recruitment (+D/-H) settings shown here:
In particular, notice how wherever there is hyper-recruitment (negative values), very nearby there is also substantial decruitment (positive values). This might be an interesting area for more research.
And if you ignore for the moment, that rather large hyper-recruitment in the right channel around 1 kHz, looking at the region above 1 kHz, you see very similar curve characteristics. A sharp rise, followed by a sharp dip, moving toward higher frequencies. Both ears are showing that the frequencies just below hyper-recruitment zones exhibit substantial decruitment. This might be expected if hyper-recruitment were the result of outer hair cell damage, which have the job of moderating basilar membrane vibrations.
If there are insufficient hair cells acting to control the basilar membrane then (1) it can’t control the vibration amplitudes in its frequency zone, leading to hyper-recruitment, and (2) possibly because of the asymmetric frequency response of cochlear filtering, they don’t provide sufficient signal levels to the brain at frequency zones just below the damage zone.
These adjustments were obtained empirically, and they might not reflect actual conditions. Just the best I could muster during several listening sessions. But I find it interesting that both ears exhibit that strong decruitment rise, followed immediately by a sharp hyper-recruitment trough.
There are currently no known diagnostic tests for measuring the degree of hyper-recruitment nor decruitment. I’m still thinking about how that might be designed.