An important concept when considering the hearing capabilities of animals is that of the auditory
threshold (also called hearing threshold) [Johnson, 1967b]. The hearing threshold is the average
sound pressure level that is just audible to a subject under quiet conditions. An audiogram is a plot
of hearing threshold as a function of frequency. Audiograms are generally derived using two
principal techniques; behaviourally (using measured behavioural responses to acoustic signals), and using Auditory Evoked Potentials (AEPs; based on electro-physiological responses to acoustic
signals). It is important to highlight that the results from these two methods may not be directly comparable and a recent study by [Yuen et al, 2005] suggests that AEP measures of hearing do not yield as sensitive a measure as behavioural thresholds; thresholds gathered using behavioural methods [Yuen et al, 2005] were usually about 10·dB more sensitive than those obtained using AEPs with the same technique as that used in the present study.