Poor propagation is gas due to widely spaced molecules.The closer the molecules the faster the sound wave moves through the medium.1Because the crystal in the transducer is electrically pulsed, it changes shape and vibrates, producing a sound beam that propagates through the tissues.The crystal emits sound for a brief moment and then waits for the returning echo refected from the structures in the plane of the sound beam.When the echo is received, the crystal again vibrates, generating an electrical voltage comparable to the strength of the returning echo.2The strength of the returning echo is related to the angle at which the beam stries the acoustic interface.The more nearly perpendicular the beam, the stronger the returning echo.!mooth interfaces at right angels to the beam are nown as specular refectors."choes returning at other angles are nown as scatter.