I couldn't find if this is the case in the paper but unless they created a pure tone at 20Hz (i.e. a sine wave oscillating at 20Hz) any sound will have higher h

I couldn't find if this is the case in the paper but unless they created a pure tone at 20Hz (i.e. a sine wave oscillating at 20Hz) any sound will have higher harmonics. Even though the fundamental is 20Hz, it'll also sound 40Hz, 60Hz, 80Hz (in decreasing volumes) etc and these higher harmonics can or will be audible. Besides, especially in Western classical music, when we use double bass at very low frequencies, it's not always meant to be audible, it's sometimes meant to create a physical effect i.e. feel it through your skin/body instead of your ear, similar to percussion in some settings.