To be clear, this curve set is for the SPL of the sound to be heard with equal intensity, not the ear's sensitivity at those frequencies which, according to that idea, would mean that we're far less sensitive to the 1KHz-5KHz range than we are to the extreme low and high frequencies. Human hearing is more sensitive in the range where this curve shows the dip between 2KHz and 5KHz. According to this link (and the other information they have, human hearing acuity is highest between 3.5KHz and 4KHz.
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/maxsens.html
From the text below the last graph, "Where the curve dips between 2000-5000Hz, this implies that less sound intensity is necessary for the ear to perceive the same loudness as a 120dB, 1000Hz tone. In contrast, the strong rise in the curve for 0 phons at low frequencies shows that the ear has a notable
discrimination against low frequencies for very soft sounds.". This is the reason loudness controls boost the bass and treble and specific to Yamaha, their variable compensation curve produces more bass and treble at lower levels.
Here's another link-
http://www.extron.com/company/article.aspx?id=loudnesscontrol_ts
I remember curves from past reading that were the inverse of this- not sure why they don't appear with searches, now.