Damage to high-threshold auditory nerve fibers can escape threshold detection yet impair speech-in-noise perception.

Evidence

For thresholdlevel stimuli, the only fibers available to code the vibration are the high-SR/low-threshold fibers. These are the only fibers that respond to sound levels below 20 dB SPL, and therefore are the only fibers active during threshold … Recent studies in animals have suggested that the low-SR/high-threshold auditory nerve fibers and their associated synaptic connections may be more easily damaged than the high SR/low threshold auditory nerve fibers when exposed to high levels of sound (Kujawa & Liberman, 2009). Auditory threshold testing may not reveal damage to the high-threshold fibers, due to the low stimulus levels associated with threshold. Consequently, it seems possible that supra-threshold tests would be necessary to identify loss of or damage to the high-threshold fibers. Scientists have speculated that loss of these fibers might contribute to speech perception difficulties in noise, even in the absence of elevated auditory thresholds. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 258