Hi,
my Bernafon aids go up to 10 kHz. But when I look at the insertion gain curve, I see that there is no gain at 10 kHz. So I tried to increase gain in that area a bit and found out, that
a) you get feedback pretty early at that frequency, and it sounds really really annoying.
b) with slightly raised gain at 10 kHz (without feedback), I found the hearing experience quite tiresome.
So my personal opinion is: Get the 10 kHz aid to be sure it really works at 8 kHz.
Now to the question “why”. First of all, you need a sampling frequency of at least two times the maximum frequency you want to sample. Higher sampling frequency means higher battery drain.
Second, even if there are sounds at, say, 12 kHz, they usually are soft. I found this graph
http://www.nagyvaryviolins.com/graph1.jpg
where you can see clearly that the harmonics of the violin at 6 kHz are about 40 dB softer than the base frequency. Let´s assume that the harmonics at 12 kHz (who are surely there, no denying that) are 60 dB softer than the base frequency. If the violin plays at 80 dB, the harmonics at 12 kHz can still be heard by people with good ears.
I have a hearing loss of about 60 dB at 10 kHz, I don´t know about 12 kHz. Let´s asssume the same. For me to hear the harmonics at 12 kHz, I´d need more than 40 dB gain at that frequency. This would be more gain than is used at any other frequency! In other words: This would be ridicoulously much gain!
And I´m talking about a live violin here. If you listen to your cd, at a nice level of, say, 60 dB. Then you´d even need more gain to hear something at 12 kHz.
So, what I´m trying to say: Even with a mild loss, you´d need very strong amplification to hear the very soft overtones present at very high frequencies.
Also, in every day situations, there is only noise at 12 kHz. So you´d hear much more noise. And if you use the 12 kHz amplification for music only, you won´t be used to it and find everything sharp and tinny.
People with young, healthy ears have 0 dB loss at, say, 15 kHz. With ears like that I´m quite sure that there are interesting things to hear, especially for music. But for people - even with a mild loss only - those frequencies are out of reach.