20Hz-20kHz is considered the normal range of human hearing. Little can be detected by humans over 20,000Hz. If one is over forty years of age, dramatic high frequency reduction occurs to human ears. Some humans can, but 20Hz-20kHz is typically how audio gear is rated, at least to show the flatness of the curve in this range. Ideally, 20-20kHz would show as a flat curve from one end to the other. This is rarely acheived, so often you will see it expressed something like 20-20kHz, +/-3dB. All devices can produce sound outside of this range, but many frequencies outside will be down greater than 3dB. Khadas shows the T2P gets close to flat in this range, Frequency Response, 20Hz～20KHz, ±0.15dB.
For perspective and in amplifier’s terms, a 3dB increase in volume requires a doubling of amplifier power. If a speaker’s efficiency is rated as 85dB@1W at 1 meter, in order to get 88dB from that speaker it would require 2 watts, 91dB requires 4W, 94dB requires 8 watts, 97dB requires 16 watts, 100dB requires 32 watts etc. So something down more than 3dB becomes less audible compared to signals within the 3dB.