The sound pressure wave reaching the tympanic membrane is not the same as that of its source in free-field space. Along the path from source to tympanic membrane, sound pressure is linearly transformed by reflections off of and diffraction around the head and body; constructive and destructive interference due to reflections off of features of the pinna; and resonance in open cavities of the outer ear. This transformation of pressure magnitude and phase, termed the head-related transfer function (HRTF), is frequency- and direction-dependent; therefore, sound pressure will be amplified at some frequencies and attenuated at others depending on the direction of the source. The set of HRTFs across locations in space contain the cues that are known to be perceptually relevant to the determination of the direction of a sound source: interaural time and level differences (ITDs and ILDs) for determination of source azimuth and shape of the magnitude spectrum for determination of source elevation (Middlebrooks and Green, 1991).

Knowledge of the HRTFs of a species is scientifically useful for at least two reasons. First, HRTFs contain information on how sound waveforms that reach the tympanic membrane are differentially weighted across frequency for that species. For example, many mammals exhibit a broad and largely non-directional peak at a particular frequency in the HRTF magnitude spectrum, attributed to acoustic resonance within the ear canal (Shaw and Teranishi, 1968). Therefore, frequency components of the source signal at and around the resonant frequency for that species are emphasized in the signal input to the organism. Second, HRTFs contain information on the range of binaural cues and the characteristics of monaural spectral cues available to a species for sound localization. For example, the range of ITDs and ILDs across spatial locations varies across species and across frequency within a given species. This information is important when interpreting neural or behavioral responses of a given species to sounds presented over headphones where it is possible to create ITDs or ILDs outside of the range for free-field sources.

HRTFs have been measured in humans, many other mammalian species, and some non-mammalian species. In the present study, we report HRTFs of rabbits—a species known for their relatively long pinnae. Rabbits have a hearing range that largely overlaps with that of humans (Heffner and Masterton, 1980) and are commonly used in neurophysiological and behavioral studies of the auditory system (Barzelay et al., 2023; Carney et al., 2014; Fan et al., 2022; Haragopal et al., 2020; Kim et al., 2020; Kuwada et al., 2015; Wagner et al., 2022; Zhai et al., 2020). There has been one previous study of rabbit HRTFs (Kim et al., 2010). Like the present study, Kim et al. (2010) measured HRTFs of the Dutch-belted strain of rabbits in the front horizontal plane and reported magnitude spectra, ILD spectra, and ITD spectra. However, unlike the present study, their study focused on changes of the HRTF with distance of the sound source and comparisons to a spherical head model. Our data complement that of Kim et al. (2010) by characterizing features of magnitude spectra shared across rabbits and examining the dependence of those features on source azimuth. While Kim et al. (2010) reported data from two rabbits and made no comparisons between them, we examined inter-individual differences across nine rabbits. Additionally, we assessed the potential utility of monaural spectral cues for determination of source azimuth and investigated the contribution of the pinnae to binaural cues and features of magnitude spectra.

HRTFs can be used to filter sound waveforms presented over earphones so that they are perceived as indistinguishable from an externalized, free-field source (Kulkarni and Colburn, 1998). This “virtual acoustic space” technique is advantageous because it allows presentation of free-field-like sounds monaurally or with augmented localization cues in order to dissect function in neurophysiological or psychophysical experiments (Dorkoski et al., 2020). Experiments utilizing virtual acoustic space are difficult because a subject's or animal's own HRTFs must be used to minimize experimental error, assuming HRTFs are significantly different across individuals. To assess this assumption for rabbits, we quantified inter-individual differences of HRTFs and differences between individual HRTFs and an average HRTF. Our results may be of use to researchers who wish to perform neurophysiological or behavioral experiments on rabbits using virtual acoustic space, but who do not have the capability of measuring individualized HRTFs.