Whether we can hear above 20K is one valid issue but so is the question of what is on a record over 20k. Supertweeters can definitely reach the frequencies they claim - my own are pretty flat to 40K, but that's only part of the story. In my own case I built them because the tweeters in my main speakers roll off quite quickly above 10K. One would expect the addition of a driver partly crossing into the operating range of another to adversely impact image focus and make the sound brighter. In actual fact the sound does not seem any brighter and focus is actually improved!

