Different auditory nerve fiber types have distinct response thresholds.

Evidence

Figure 4–2 illustrates that each of the three different fiber types (low-, medium-, and high-SR fibers) increase their firing rate as the stimulus level increases. The threshold of each of the fiber types, the stimulus level that causes a detectable increase in firing rate from spontaneous, is also different. As a result, different fibers will respond by different amounts depending on the level of the stimulus. For example, a sound at 15 dB SPL will only cause the high-SR fibers to fire. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 700