Applying 3D scanning and printing technology in the replication of pinnae for head-related transfer function measurements

Abstract

An important application of a head and torso simulator is to obtain head-related transfer functions (HRTFs) because it eliminates the methodological challenges inherent in human-based measurements. However, the use of these non-individual HRTFs to simulate virtual environments can lead to sound source localization errors in the listener. The emerging 3D scanning and printing technology make it possible to fabricate such intricate elements as the outer ear, thus several studies were initiated to explore the fabrication of individual dummy heads for various uses in acoustics. In this study we evaluate the feasibility of using low-cost 3D scanning and printing technology in the replication of human pinnae for use in HRTF measurements. A pair of commercially available standardized pinna simulators (silicone-rubber material) was scanned with a 3D scanner. Then two replicas were printed using two easily accessible materials of different hardnesses: acrylonitrile butadiene styrene (ABS, hard material) and thermoplastic polyurethane (TPU, soft material). A set of HRTFs was measured for each pair of pinnae to perform technical and perceptual evaluations, taking the commercial one as a reference. First, a numerical evaluation and a binaural cues analysis were performed on the HRTFs, followed by a psychophysical experiment of discrimination between auralized stimuli with the different HRTFs. The findings revealed overall similarities between the reference pinna and both replicas, leading to difficulties in their perceptual discrimination. However, the flexible soft material (TPU) demonstrated superior performance in both the technical and psychophysical validations. This discrepancy seems mostly attributable to a superior fit of the pinna with the head simulator. The harder material (ABS) had small irregularities in the fit preventing the formation of a resonance tube of adequate length to effectively reproduce the ear canal. These findings indicate the effectiveness of affordable 3D technology in the construction of pinna simulators for HRTF measurements as an alternative for commercial ones.