Effects of fundamental-frequency separation and spatial configuration on speech intelligibility in the mid-sagittal plane under bone-conduction stimulation

This study investigated effects of spatial separation and fundamental-frequency separation (ΔF0) on speech reception thresholds (SRTs) and masking release (MR) with stimulation in the mid-sagittal plane using air conduction (AC) earphones and bilateral bone conduction (BC) transducers at the mastoids. Twenty-four participants with normal-hearing, 12 each in 2 separate experiments, were tested in simulated spatial configurations based on non-individualized head-related transfer functions. Stimuli were equalized between the two stimulation modalities. Results showed that increasing vertical spatial separation and separating fundamental frequency (F0) both significantly reduced SRTs. Under BC stimulation, the MR attributable to vertical spatial separation and to ΔF0 was approximately additive. AC stimulation exhibited a similar near-linear accumulation of benefits, with a locally supra-linear gain observed at ΔF0 = 2 semitones. Within the mid-sagittal plane, ΔF0 accounted for the majority of the observed improvement in speech intelligibility, whereas the independent contribution of vertical spatial separation was relatively modest. These findings indicate that both AC and BC stimulations benefit from spatial and F0 cues, with F0 producing a larger effect than vertical spatial separation within the tested parameter range. The results have implications for auditory modeling and for optimization of BC hearing device algorithms.