Curiosity piqued, I dug around for info on FM transmitters and receivers--- the 10khz or so limit was breached in the 1940s. Most modern smaller transmitters can keep distortion below 0.5db distortion at 15khz, and the bigger (broadcast) units can manage the same distortion level up through 20khz. And another nerdy-tech note, most humans cannot differentiate between tonal variations above 12,000-14,000hz-- they can hear the sounds, but they all just sound alike. We may be able to walk upright, but we have a really narrow hearing range.