Technically this is not aliasing; rather, the large and varied non-linearities of a speaker can act like a frequency mixer, which is why you'll get a 3 kHz soun

Technically this is not aliasing; rather, the large and varied non-linearities of a speaker can act like a frequency mixer, which is why you'll get a 3 kHz sound (the difference) when playing, say, 20 and 23 kHz.