# Amplitude of power spectral density

Why is the amplitude of the Power spectral density higher for shorter period of time as compared to a longer period of time when calculated for any vibration data?

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I suggest you try looking at http://en.wikipedia.org/wiki/Periodogram. What you're calling the PSD is in fact a periodogram. What the Wikipedia page I cite above explains is that the periodogram is in general not a good estimator for the PSD, even though, from http://en.wikipedia.org/wiki/Spectral_density, "The ensemble average of the average periodogram when the averaging time interval $T\rightarrow\infty$ can be proved (Brown & Hwang) to approach the Power Spectral Density (PSD)."