CD can capture anything between 20 Hz and 20 kHz and many speakers can handle this range, but there can't be many musicians, or recording studios out there trying to give people very high frequencies in their music, because it's just annoying and really not pleasant to listen to. As I''ve said before, most music contains instruments producing sounds falling within the most sensitive region of human hearing. Speech, brass and woodwind, with strings and perscussion taking advantage of this also. Even synthesisers try to mimic these instruments and there can't be many people who enjoy really high pitched sounds, similar to many screeching birds and other annoying sounds. A CD full of birdcall, or mosquitos would drive me nuts. Birds screeching and squawking is obviously a response to a possible threat and this could have been beneficial to humans way back, but you wouldn't want to hear these sounds all the time. Being threatened all the time, or hearing animals responding to being threatened would be really stressful. Nature obviously feels we need to hear up to around 20 kHz, but does anyone really want to create music containing these really high pitched sounds? When the world was a quieter place, the ability to hear faint, high pitched sounds may have been useful, but as we've created a very noisy society and our sense of hearing is tuned to mainly unnatural sounds, there is possibly no longer a need, nor ability to hear these faint, natural sounds. Because the world is very noisy now, our hearing has become lazy and I think is no longer fine tuned to appreciate natural sounds. I try to appreciate peace and quiet when I can, but many people love noisy distractions all the time and their hearing may 'recalibrate' to a point where their sensitivity to natural sounds is diminished. I bet our ancenstors, half a million years ago could hear loads of stuff in the daytime and especially at night that nobody could pick up now. Having an ancestor from this period, dropped into the centre on my city would probably make them go nuts with all the loud and screechy mechanical and metal sounds, not to mention sirens. The very lowest frequencies are very difficult to detect. Elephants can detect earthquakes and distant thunder and move towards storms in the search for water. It's diffiicult to predict how human hearing will evolve, after 200 years of industry and vehicles and all sorts of other sounds competing for our attention. I often wonder how our senses will change and adapt over the next 100 generations, but I prefer natural, soothing sounds, like wind and rain and the sea etc. I don't go out of my way to find screechy, shrill and scary sounds in music and I'm sure most people feel the same way. Listening to test tones, wthin your hearing range is not useful if you don't need to respond to these sounds. Our speech and language and our relationship with other animals has defined where human hearing is most sensitive and useful. Brakes screeching and sirens are useful, but that's a very recent change in human society. There would be nothing like this half a million years ago, because you wouldn't be in a busy city centre, surrounded by modern, metal threats etc.