The shape of the audiogram reflects outer and middle ear transfer, not cochlear transmission

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. That peak in the combined outerand middle-ear transfer function has a direct correspondence to the valley in the MAP/MAF curves in Figure 2–9, which is where humans have the best auditory sensitivity. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 255

Categorization

• XREF: Connects to the general understanding that best-hearing frequency range 600–6000 Hz matches speech-relevant frequencies.
• DEFINE: Clarifies why the audiogram valley exists — a mechanism-level explanation of normal hearing sensitivity.