Noncontact excitation of sound in metals by a video pulse of electric field

An acoustic signal that is excited without acoustic contact by an excitation electrode in a metal sample has been studied. A constant electric voltage close to the breakdown value was applied between the electrode and the sample. A video pulse of electric field with a duration of 0.4 mu s was used to excite sound in the sample. The ratio k of signals that were registered with noncontact and contact excitation proved surprisingly large (k = 0.14) when compared to the value expected from the literature data. A strong dependence of the noncontact acoustic signal on the filling frequency f of the excitation pulse has been observed. For f > 10 MHZ, the signal dropped to the level of noises. An explanation for this frequency dependence is proposed. It is based on the assumption of quasi-atomic thickness of the charged metal layer that is created by the electric field.