ABR Calibration

There are no current national or international standards for evoked response equipment and stimuli.  There are however standards in working progress.  This includes ANSI Working Group S3-72 on  “Measurement of Auditory Evoked Potentials”,

Stimulus rate and duration 

The stimulus rate and duration, temporal extent and waveform of a click needs to be accurately reflected at the earphone.  To ensure this accuracy, an oscilloscope is required.  An oscilloscope is a device used for viewing an electrical signal.  The stimulus output from the AER (auditory evoked response) machine can be recorded directly if the signal is sent to the oscilloscope.  The waveform is displayed on the oscilloscope that has a calibrated time base.  Duration characteristics such as rise or fall times, or plateau can be calculated from the display data.  Rate can be determined using the following equation:

Rate (stimulus frequency) = 1 / time interval between stimuli

Stimulus Polarity

The initial direction of the pressure wavefront in the stimulus waveform is measured at the face of the transducer (ANSI Working Group S3.72).  With alternating-polarity stimuli, each single polarity is presented alternately and represented equally in a series of stimuli repetitions.  Therefore, a response averaged from 1000 stimulus repetitions (sweeps) is actually two responses, averaged each from 500 single-polarity stimuli, that are added together.

To determine stimulus polarity, one needs to have an earphone coupler, sound level meter and oscilloscope.  Gorga et al 1985 outline the following procedure.  First, the output of the sound level meter is sent to the oscilloscope.  Then earphone is coupled to the sound level meter.  When pressure is applied to the back of the earphone, a positive voltage (condensation) results by forcing the diaphragm away from the earphone.  The direction of the change in the waveform is noted on the oscilloscope.  Then, condensation polarity click is presented to the earphone, and the direction of voltage change is again observed.  If the change is in the same direction, the clicks are condensation polarity, otherwise, they are rarefaction clicks. 

Keep in mind that reversing the terminal connections on a set of earphones will reverse stimulus polarity.  For alternating-polarity stimuli, accuracy in polarity makes no difference because there will be an equivalent number of stimuli of each polarity presented.   

Equipment manufacturers rarely include documentation regarding stimulus polarity validity.  Some manufacturers use the term positive for condensation and negative for rarefaction stimulus polarity (Brey et al 1988).  Some manufacturers use the terms rarefaction and condensation in reference to polarity but produce a condensation click with a negative electrical pulse and a rarefaction click with a positive electrical pulse.  This is opposite of what most manufacturers use.  To correct for this, some manufacturers reverse the polarity of their earphones that are shipped with the machines.  The problem with this is that most clinicians don’t know this and when it comes time to replace the earphones, they will not know to get ones that reverse the polarity.

Stimulus intensity

To calibrate stimulus intensity a sound level meter is required. To calibrate the frequency accuracy, a sound level meter and a spectrum analyzer is required.    

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