A method for determining the starting instant (t0) of a periodically oscillating signal response (E2; E2′), wherein the signal response comprises a first set of half periods (E2a-d; E2′a-d) having a polarity equal to a polarity of the first half period (E2a; E2′a) in the signal response, and a second set of half periods (E2e-h; E2′e-h) having a polarity opposite to the polarity of the first half period (E2a; E2′a) in the signal response.The method comprises the steps of: determining a peak half period (E2e; E2′f) as the half period with the highest amplitude in a selected one of the first and second sets; determining a zero-crossing instant (ZC1; ZC′1) of the signal response occurring a known time distance from the peak half period (E2e; E2′f); determining the starting instant (t0) of the signal response (E2; E2′) based on the zero-crossing instant (ZC1; ZC′1) and a relationship between the peak half period (E2e; E2′f) and the starting instant (t0).