Sound and vibration transmission in tissues

Sound pressure levels measured inside a body must account for the acoustic impedance of the medium in order to obtain information on acoustic intensity. However, there is some uncertainty about the acoustic signals that reach and stimulate the fetal ear. Large wavelengths of frequencies in tissue in the human audible range suggest that the fetal ear will be in the near field of signals transmitted across the air/tissue interface or from an electrolarynx vibrator over a large portion of the frequency range. There are numerous physical mechanisms which effect high-frequency signals in transmission. Vibrational shear waves generated by electrolarynx activation on the skin will influence the fetal sound environment. Further research is required to fully understand the significance of measured signals when both compression and shear waves are present. Data in the frequency domain could be misconstrued, unless peculiarities of some acoustical measuring systems are understood.