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105C - ABR CALIBRATION FOR MP150 WITH ACQKNOWLEDGE 4.1
This Application Note outlines the precise test equipment calibration required to ensure proper signal-to-noise levels (S/N) and signal time delays for Auditory Brainstem Response (ABR) measurements using an MP150 data acquisition system and AcqKnowledge 4.1 or above software.
Whenever a sound wave is detected by the ear, it triggers a number of neuro-physiological responses along the auditory pathway. An action potential is conducted along the eighth nerve, the brainstem, and finally to the brain.
About one third of a second after initial stimulation, the signal evokes a response in the area of the brain where sounds are interpreted. These response signals are so small that they are frequently masked by the "background noise" of electrical activity in the brain. This interference is random with respect to the initial stimulation.
If you find an average of this interference signal, it will approach zero. However, since the evoked response is dependent on the stimulus, it should occur at the same time relative to the stimulus when the process is repeated. Therefore, if you repeat this stimulus-response process many times the "background noise" will disappear and what will be left is the neuro-physiological response of principal interest.
For ABR studies, calibrate the OUT101 Tubephone with an Etymotic ER-7C Probe Microphone—this microphone provides a calibrated output voltage which is a function of applied Sound Pressure Level (SPL). The sensitivity is 50 mV/Pascal (-46 dB re: 1 V/uBar): 0 dB SPL = 0 dBuV. Place the Probe Microphone insert tube in the auditory canal prior to the insertion of the OUT101 foam tip. The OUT101 Tubephone sound delivery tube and the Probe Microphone sound input tube will then be exposed to the same auditory chamber. Accordingly, the SPL is recorded, via the Probe Microphone, simultaneously with applied auditory stimulus from the OUT101 Tubephone.