The element z 22 {\displaystyle z_{22}\,} is the open circuit mechanical impedance, that is, the impedance presented by the mechanical side of the transducer when no current (open circuit) is entering the electrical side. The element z 11 {\displaystyle z_{11}\,} , conversely, is the clamped electrical impedance, that is, the impedance presented to the electrical side when the mechanical side is clamped and prevented from moving (velocity is zero). The remaining two elements, z 21 {\displaystyle z_{21}\,} and z 12 {\displaystyle z_{12}\,} , describe the transducer forward and reverse transfer functions respectively.