This problem relates to abnormal frequency resolution and aberrant patterns of growth in loudness, each of which reduces speech intelligibility under challenging listening conditions, such as noisy surroundings.
For normal listeners, the dynamic range from sensing soft sounds to the loudest tolerable noise is more than 100 dB.
In contrast, the dynamic range of patients with SNHL is often narrowed by both an increase in the threshold of audibility and a lowering of the ceiling of tolerance to high-intensity sounds.
This compaction of dynamic range leads to recruitment, an abnormal growth in loudness as sound intensity increases.
Recruitment thus remains one of the principal challenges of hearing aid rehabilitation, and it is responsible for a common phenomenon most clinicians who deal with hearing loss have witnessed or experienced: at average speaking levels, an individual with recruitment may ask a speaker to talk more loudly, yet with even a slight increase in intensity, the speech becomes intolerably loud, and the speaker is told not to shout.