An active free-field equalizer for headphones used in functional magnetic resonance imaging

The free-field response of NordicNeuroLab AudioSystem electrostatic headphones for use in functional magnetic resonance imaging (fMRI) is determined by loudness comparisons with free-field equalized Beyerdynamic DT 48 headphones. Based on these measurements, an active equalizer with resonances at 0.55, 1.5, and 6.9 kHz is developed, realized, and tested. A free-field equivalent level independent of frequency within ±3 dB between 63 Hz and 10 kHz is obtained when using the AudioSystem headphones with the described free-field equalizer.