Auditory evoked potentials and auditory magnetic fields

Auditory evoked potentials (AEPs) are a subclass of event-related potentials (ERP)s. ERPs are brain responses that are time-locked to some “event”, such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. For AEPs, the “event” is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc. ¹ 

Auditory magnetic fields (AEFs) are event-related magnetic fields (ERFs) evoked by auditory stimuli. They are generated in the brain as a consequence of the synchronized activation of neuronal networks by external stimuli.²  It is the magnetic counterpart of the EEG or ERP.³

Auditory evoked responses are an objective way to study the reactivity of the brain to sound. They can be measured without the subject paying attention to the stimuli, which is advantageous in studies of young children, particularly children with problems in understanding the test instructions. Both auditory evoked potentials (AEPs) and magnetic fields (AEFs) reflect the brain’s activation pattern with millisecond time resolution.⁴   

An advantage of measuring AEFs over AEPs is that AEFs have a greater ability to more precisely identify the source of the activity being recorded.³

References: 
1.  http://www.audiospeech.ubc.ca/haplab/aep.htm
2. Steven A. Hillyard, in: The MIT Encyclopedia of the Cognitive Sciences, edited by: Robert A. Wilson and Frank C. Keil, 1999
3. Kolb & Whishaw: Fundamentals of Human Neuropsychology, chapter 7, 2003
4. Pihko, E., et al., Language impairment is reflected in auditory evoked fields, Int. J. Psychophysiol., 2008