Identification of viscoelastic material properties by ultrasonic angular measurements in double through-transmission.

Recent advances in additive manufacturing (AM) of viscoelastic materials have paved the way toward the design of increasingly complex structures. In particular, emerging biomedical applications in acoustics involve structures with periodic micro-architectures, which require a precise knowledge of longitudinal and transverse bulk properties of the constituent materials. However, the identification of the transverse properties of highly soft and attenuating materials remains particularly challenging. Thereby, the present work provides a methodological framework to identify the frequency-dependent ultrasound characteristics (i.e., phase velocity and attenuation) of viscoelastic materials. The proposed approach relies on an inverse procedure based on angular measurements achieved in double through-transmission, referred as θ-scan. Toward this goal, a forward modeling of the double transmitted waves through a homogeneous solid is proposed for any incidence angle based on the global matrix formalism. The experimental validation is conducted by performing ultrasound measurements on two types of photopolymers that are commonly employed for AM purposes: a soft elastomer (ElasticoTM Black) and a glassy polymer (VeroUltraTM White). As a result, the inferred dispersive ultrasound characteristics are of interest for the computational calibration and validation of models involving complex multi-material structures in the MHz regime.

On the variation of interaural time differences with frequency.

Interaural time difference (ITD) is a major cue to sound localization in humans and animals. For a given subject and position in space, ITD depends on frequency. This variation is analyzed here using a head related transfer functions (HRTFs) database collected from the literature and comprising human HRTFs from 130 subjects and animal HRTFs from six specimens of different species. For humans, the ITD is found to vary with frequency in a way that shows consistent differences with respect to a spherical head model. Maximal ITD values were found to be about 800 µs in low frequencies and 600 µs in high frequencies. The ITD variation with frequency (up to 200 µs for some positions) occurs within the frequency range where ITD is used to judge the lateral position of a sound source. In addition, ITD varies substantially within the bandwidth of a single auditory filter, leading to systematic differences between envelope and fine-structure ITDs. Because the frequency-dependent pattern of ITD does not display spherical symmetries, it potentially provides cues to elevation and resolves front/back confusion. The fact that the relation between position and ITDs strongly depends on the sound's spectrum in turn suggests that humans and animals make use of this relationship for the localization of sounds.

Estimation of the low-frequency components of the head-related transfer functions of animals from photographs.

Reliable animal head-related transfer function (HRTF) estimation procedures are needed for several practical applications, for example, to investigate the neuronal mechanisms of sound localization using virtual acoustic spaces or to have a quantitative description of the different localization cues available to a given animal species. Here, two established techniques are combined to estimate an animal's HRTF from photographs by taking into account as much morphological detail as possible. The first step of the method consists in building a three-dimensional-model of the animal from pictures taken with a standard camera. The HRTFs are then estimated by means of a rapid boundary-element-method implementation. This combined method is validated on a taxidermist model of a cat by comparing binaural and monaural localization cues extracted from estimated and measured HRTFs. It is shown that it provides a reliable way to estimate low-frequency HRTF, which is difficult to obtain with standard acoustical measurements procedures because of reflections.