And useful hearing range might be viewed in octaves:.
So, the perfect normal Human Sound octaves are approximately 20hz to 40…40 to 80…80 to 160…160 to 320…320 to 640…640 to 1280…1280 t0 2560…2560 to 5120…5120 to 10240…10240 to 20480hz. 10 octaves in all. (Anything below 20hz or so is felt not heard.)
Now if the upper octave is missing, one way to look at it’s loss is to say one tenth of the range is unavailable. And looking at the thing from a power viewpoint most voice power is concentrated well below 5 khz.
I’m not aware of any high grade scientific papers showing that speech comprehension is improved with the most upper octave…maybe out there but as I said, I never ran across them. Ed