The prefix “nHL” conventionally refers to sound level (dB) of a signal referenced to the thresholds for a group otologically normal hearing people – it is the same concept as dB HL, however nHL is often associated with a short duration signal that has been calibrated using the peak-to-peak equivalent SPL method (via an oscilloscope). Short duration signals are of course used in objective hearing assessment via evoked potentials like ABR, ASSR, AMLR and ALR (as well as other applications like OAE and WBT). eHL is the estimated behavioural hearing level.
Typically, the behavioural hearing threshold is lower than the threshold given by the objective hearing assessment (e.g. ABR, AMLR, ASSR, ALR). Therefore, a correction factor is applied to the objective hearing threshold to better estimate the (all important) behavioural threshold.
Temporal integration is one factor that gives rise to the nHL-to-eHL difference. Temporal integration is the way in which the auditory system integrates energy over time and is particularly relevant for short duration sounds below about 200ms. (This encompasses most sounds used in objective hearing assessment.) However, the pure tones used in behavioural pure tone audiometry are typically much longer than 200ms (e.g. several seconds in duration), so these sounds produce greater temporal integration which acts to increase loudness and lower threshold at an equivalent level (perceptual threshold improves by about -3dB per doubling of sound duration up to 500ms according to the Zwislocki model). There might be other factors influencing the nHL-to-eHL correction too. For example, the signal presentation rate in the EP recording (which also influences temporal integration, particularly at faster rates), the amount of residual noise in the EP recording, and calibration differences in the ear of the patient. This last feature is of particular relevance when using insert phones amongst infant populations, since the behavioural hearing level is referenced to that of otologically normal adults. Other relevant factors might include the electrode montage, the anatomy of the head and orientation of the neural generators, and arousal (particularly in auditory evoked potentials arising from the cortex).