The spectral density of a signal is a statistical quantity very useful in determining the mean-square value, or power spectral density, for frequency-domain signals. For mechanical input impedance at the hammer input force location, a spectral peak indicates that a large input force is needed to have a significant structural velocity response at the output location on the structure. Input–output coherence will be very high allowing a consistent measured transfer function, even if the spectral resolution of the fast Fourier transform (FFT) is less than optimal. The system impulse response is longer than the FFT buffers, some response from the input buffer will be reverberating into the time frame of the subsequent buffer, thus appearing as noise incoherent with the present input. The time buffers are synchronized so that the input buffer recording starts at exactly the same place on the input signal waveform with each repetition of the input excitation.