Effectiveness of Earphone Shielding for Artifact Control in Auditory Evoked Potentials

Control of the electromagnetic artifact produced by earphones is a major problem in the acquisition of surface-recorded auditory evoked responses, especially short latency responses (0-10 ms). Several investigators have used magnetically shielded earphones to reduce the artifact. In this paper, a method for measuring shielding effectiveness is described, and results obtained with a TDH-39 earphone are presented. Two layers of shielding were found to provide 10-15 dB of shielding effectiveness; the resulting artifact is less than 0.1 μV for acoustic stimuli below 120 dB sound pressure level (SPL). The shielding also affects the response of the transducer, especially at low frequencies. It is concluded that shielding is effective in controlling artifact for high-frequency transient stimuli. However, for low-frequency stimuli, for which the response begins while the stimulus is present, results obtained with shielded earphones should be interpreted with caution.