Frequency Response of L.P's

Amps...no problem. Many speakers roll off by a few dB at 20KHz. Some supertweeters are good to 30Kc and higher. "Ultrasonic" transducers go much higher, but noone (except bats) cares about sonic fidelity at these frequencies.

One method of quadraphonic recording on vinyl involved recording from 20Khz to 40Khz, but this information was Frequency Modulated, using a special preamp to decode it. Because it was FM, flat frequency response and low distortion was not required. (Like FM radio).

Nevertheless, the requirement for some kind of sensitivity up to 40KHz caused large improvements in pickup technology, which has benefited us long after quad vinyl died.

Some audiophile recordings have claimed flat response to 22KHz. This is fairly easy to measure simply by looking at the grooves with a microscope.

Zaikesman...As you say, response to several octive above the highest frequency of interest is an indication of good performance up to that frequency.

When I was in college (many moons ago) I was a subject in some research project relating to hearing, and, as part of this project, my ears were "calibrated". At that time my hearing did extend well beyond 20 KHz, which was the highest frequency of interest to the experiments. Perhaps because of this early experience I have maintained an interest in the subject of human hearing ability.

Over the years the highest frequency pure tone that I can hear has moved steadily down. By now I guess it is around 12 or 14KHz. But, while I cannot hear a 16 KHz tone, I can sense the effect on white noise of a low pass filter at 18 KHz. This is why honest response to 20 KHz is necessary, and why a supertweeter good to 30KHz can be desirable if the source can match this.

Response to 100 KHz is no big deal for an ultrasonic transducer, but it would be a stretch to call it a loudspeaker.

A vinyl disc can produce higher frequencies than a CD. The CD limit is 22 KHz, and it is a "brick wall"...nothing at all above it. However, fidelity does not degrade until this limit is reached. The fidelity of vinyl degrades gradually, and what you can get higher than 20KHz depends on the pickup used, and in any event will be of degraded fidelity. Also, high frequency information on a vinyl record degrades with repeated playing. Finally, there is some argument as to whether response over 20KHz is audible (to the majority of listeners).

"Superior" involves a lot more than frequency response. For use in an automobile, a CD is clearly superior.