The interesting thing about this is the JNDs of bass depends on sound pressure levels. The differences that can be heard in low frequencies can be finer than bands which our ears are most sensitive, 1 kHz to 4 kHz. Look how the lines of the equal loudness curves bunch up in the lows:
In many other ways our hearing in low frequencies is much worse than higher frequencies, but at the point where we start to hear low frequencies, our ability to distinguish differences in sound pressure is actually quite good.
Yes, that is the equal loudness chart.
David Clark has a JAES paper called High Resolution Subjective testing Using a Double Blind Comparator, JAES May 1982, page 330 to 338, fig 2, shows the degree of level matching for various bandwidths and center frequencies necessary to eliminate audio frequency response effect for music sources. The graph has been posted on line in the past I think here and elsewhere.
To me this is in essence a JND graph. At 20 Hz it shows 3 dB at the best to detect level difference.
I think JND and equal loudness are not the same and the latter cannot show JND.
But, I certainly could be way off base.
Known to happen, from time to time.
