Measuring the human head-related transfer functions: A novel method for the construction and calibration of a miniature ‘‘in-ear’’ recording system

A method for the construction of a small "in-ear" system for recording the human-free field-to-eardrum and headphone-to-eardrum transfer functions is described. Customized in-ear inserts were obtained by a simple ear printing and electroplating method, resulting in a thin (< 0.25 mm) outer shell that minimized obstruction of the entrance at the ear canal. The insert can be used to position a microphone probe tube deep within the auditory canal. The effects of this recording system on the sound field in the ear canal were calibrated using a model head equipped with a second internal microphone close to the eardrum. Transfer functions were recorded for 343 different stimulus locations in free space and for a headphone sound source. For the free-field stimuli the presence of the recording system resulted in a small attenuation with maximum effects around 3.5 and 12.5 kHz (-1.5 and -2.0 dB, respectively). Passing the data through an auditory filter model reduced the averaged attenuation to less than -1.4 dB. Phase was undistorted up to 2.5 kHz. These results suggests that the perturbations produced by the insert are unlikely to be perceptually relevant.