Evaluation of surface waves in bone-conducted sound

In order to better understand the basic physical principles of bone-conducted sound, the skull is approximated as a thin spherical fluid-filled shell. Results for sound waves propagating through and around this structure have been determined for a range of frequencies, media, and shell sizes. Since the early 1950s researchers have studied the problem of what happens when waves hit solid targets. Most of the research is concerned with the back-scatter signal but recognize that the interaction of the wave with the object influences these results. For the bone-conducted sound study, the interaction between the incident signal and the object is the critical information. For acoustic noise interacting with the human skull, the ka is below 5. The structure supports the existence of surface waves as the predominant mechanism of energy transfer within the skull. Multiple types of surface wave for this structure of a thin, curved fluid-solid interface exist. Signals which propagate on the air side of the surface due to the curvature are known as Franz or Stoneley waves. Since the shell is thin compared to the typical wavelength of sound, waves within the shell are lower order Lamb or plate waves. All of these waves are dispersive in nature with group speeds considerably smaller than the speed of sound in bone.