Vibration tests on a single poplar. Typical changes in time of the variables obtained in the single frequency analysis (SFA). (a) Image correlation coefficient C(t). The vertical bar shows the end of the air pulse. (b) Autocorrelation function of this correlation, A(t). The vertical dashed line indicates the value of T the first maximum of A(t). (c) Distribution of frequencies derived in 48 tests, over 12 angles.