A Finite Element Model of the Human Head for Simulation of Bone-conducted Sound

Abstract: Bone conduction is usually understood as the hearing sensation based on the vibrations of the skull bone and surrounding tissues. The fact that vibration of the skull bones can result in a sound percept has been known for a long time. However, it is difficult to give a general definition of BC sound. Normally, BC sound is described as the sound energy transmitted through the body (comprising the solid and fluid parts) then the outer, middle and inner ear are involved and finally produce a perception of sound.Even if BC sound perception has been studied for more than a century, the whole pattern of BC sound transmission is still not complete. There are limitations for experimental investigation of BC sound, such as the complexity of experimental manipulations and individual differences between subjects resulting in difficult to interpret outcomes. One way to overcome some of those issues is the use of a simulation model for BC sound. However, until now, the published models are unable to provide a holistic response of BC sound in the human. Therefore, the primary aim of this thesis is to develop a finite element model that could simulate BC sound transmission in the human. Based on cryosectional images of a female, the LiUHead was developed as a FE model of the human head with the structure and material properties of real human. Most the structures and tissues which could contribute to the BC transmission were included in the LiUHead. The simulation results of the LiUHead agreed with experimental data obtained in both cadaver heads and live humans.After the development and validation of the LiUHead, the model was used to investigate BC sound.  Since BC sound is transmitted in and between the tissues, the power transmission of BC sound was investigated in the LiUHead in the frequency domain. When the stimulation was applied on the surface of the skull at the mastoid position, the results of the simulations show that, as the name suggest, the skull bone dominants the BC sound transmission. The soft tissues and cartilages are as the second most important media of the BC sound while the skull interior is the least important for the BC transmission. Moreover, according to the power flux in the skull, the BC vibrations are mainly concentrated at the skull base. Other important transmission pathways are located at the occipital bone at the posterior side of the head, but the power transmitted over the face, forehead and vertex is minor. There is power interaction between the skull bone and skull interior near the stimulation position but the transmission of sound power through the brain seem to be minimal. Since the power or energy is difficult to measure in an experimental setting, this investigation gave unique knowledge about BC sound transmission in the head and the interaction between the tissues.As a common application for BC sound, bone-conduction devices are used to stimulate the hearing and is a method for hearing loss rehabilitation. Nowadays many different kinds of BCDs are available. However, most studies failed to compare the different types of BCDs in the same conditions as well as between several BCDs as it is not possible to compare several BCDs within the same subject due to the implantation required for several BCDs. The model gives a unique opportunity to evaluate various BCDs in the same head. Eight different BCDs, including four kinds of skin-drive BCDs, three kinds of direct-drive BCDs, and one in-the-mouth device, were applied to the LiUHead and the simulation results were evaluated. The results proved that the direct-drive BCDs and the in-the-mouth device gave similar vibration responses at the cochlea. At low frequencies, the skin-drive BCDs had similar or even better cochlear responses than the direct-drive BCDs. However, the direct-drive BCDs gave stable responses at mid-frequencies and gave higher responses than the skin-drive BCDs at high frequencies. These results are beneficial evaluating and for designing and improving current BCDs.The ultimate goal of this thesis is to provide a computational model for BC sound that can be used for evaluation of BC sound transmission. This was accomplished by the LiUHead that gave results comparable to experimental data and enabled investigations that cannot easily be conducted in experiments.