I ran this test using two sets of fairly wideband headphones and got results that were consistent across headphones but different between an HP laptop and a MacBook. The headphones were rated, respectively: (1) 15 – 20,000 Hz, 47 Ω input impedance; and (2) 15 – 24,000 Hz, 35 Ω input impedance. On the HP laptop with “IDT High Definition Audio” (92HD93 chip), I could hear a range from 30 Hz to 18 kHz. On the MacBook Pro with mid-2014 hardware, I could hear a range from 20 Hz to 16 kHz. I was quite surprised at the magnitude of this difference. A headphone amplifier (e.g. one built into the headphones) driven by digital input would eliminate this difference.