most speakers (and probably many amps, including those on the output stages of many nos dacs) aren't really wide range either.
Speakers, probably. Amplifiers I have to disagree with. It would take a serious effort to make an amplifier with hifi pretensions and yet with an appreciable rolloff below 40kHz, let alone 30kHz.
One thing seems clear enough though- if you really could hear anything over 20Khz, it would be somewhat annoying.
More elegance, of nature this time. High frequencies are detected at the entrance to the cochlea, where the basilar membrane is narrowest and tuned highest. The membrane is divided into frequency bands along its length, called ERBs (used to be Barks, but this has been refined) (*). Frequency discrimination AFAIK (#) is effected by sound triggering several neighbouring ERBs simultaneously (they overlap), allowing the auditory system to triangulate the original pitch.
The highest-HF ERB, some 3-4 kHz wide, is unique in that it lacks a neighbour near the entrance, and thus loses triangulation capabilities with progressive higher frequencies entering the ear.
What does this mean?
1) from 12kHz on (give or take) most adults cannot distinguish pitch. 13kHz and 15kHz sound exactly the same.
2) in digital audio: mild cases of an-harmonics due to aliasing or imaging (if audible at all) are not perceived as anharmonic (due to above pitch-deafness) but only as an overall brightening of treble, which is innocuous. Yes, aliasing is mostly harmless when confined to one ERB!
3) and does it surprise that ultrasonic detection by bone conduction (which is proven up to at least 100kHz) results in a perception identical to sound received on the upper ERB? I.o.w. bone-injected 100kHz sound is not distinguished from, say, 13kHz sound. Which makes a lot of sense if you look again at the physical structure of skull and cochlea.
(* I am not sure if the division into bands is static and defined by physical structure, or dynamic and defined by the lowest-frequency signals received, i.e. the lowest, deepest ERB that is triggered defines where the actual divisions start, back to the cochlear entrance. Which would be funky as sound needs 3-4 ms to travel down the membrane and reach that ERB. Can someone enlighten me?)
(# And if I am wrong then the underlying mechanism is certainly functionally similar to what I describe, in which case I am functionally right ;-)