The outer and middle ear, not the cochlea, shape the absolute threshold curve.

Evidence

The outer and the middle ear have a large influence on the shape of the MAP and MAF. The valley in the MAP/MAF is generally thought to reflect the sound transfer characteristics of the outer and middle ears. Recall that the combined outer and middle ear transfer function has a peak between 600 and 6000 Hz, corresponding to the range of best absolute thresholds. Both structures have frequency transmission characteristics that vary as a function of frequency, as illustrated in in Figure 2–6. On the other hand, the cochlea is thought to transmit sound roughly equally at all frequencies (Rosowski, 1991). Of course, the cochlea is critically involved in the ability to detect sounds, but it has little effect on the shape of the MAP/MAF due to its flat frequency-transmission characteristics. Not surprisingly, then, there is a strong correlation between the MAP/MAF and the transfer characteristics of the outer and middle ears. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 255