Measurement and Analysis of Random Vibration

You want to measure and analyze random vibration? Fine. But first, let’s review some of ebook 11. A figure shows our old friend in the time domain, as on your oscilloscope. The lower view shows him after he is Fourier-transformed into the frequency domain, on your spectrum analyzer. All his energy is at one frequency f. Now let’s add a bit of complexity. We see (upper) a complex wave in the time domain and (lower) in the frequency domain. Note that now a third harmonic, at frequency 3f, has been added. In this very simple example of a line spectrum, all the vibratory energy is at the discrete, identifiable frequencies f and 3f.

Now we will commence to explore an entirely different kind of vibration, one in which there are no discrete, identifiable components. Rather, the vibratory energy exists in a continuum, over a continuous range of frequencies.

Repeating a bit from ebook 20, the concept of a continuous spectrum, a spectrum with no “holes”, of vibration occurring simultaneously at all frequencies, is somewhat difficult for newcomers to random vibration to accept. Think back to your high school physics class; remember how you beamed white light through a prism and generated a continuous spectrum of colors? Random vibration is something like that.

21.1 Review the familiar sine wave

21.2 Fourier transform pair

21.3 Review the complex wave

21.4 Random vibration in two domains

21.5 Random vibration in the time domain

21.6 Gaussian distribution

21.7 Meaning of term RMS

21.8 True RMS (TRMS) metering

21.9 Variance

21.11 More on amplitude probability density

21.11 Is your field vibration Gaussian?

21.12 Is your test vibration Gaussian?

21.13 A mixture of sinusoids?

21.14 How can we examine a spectral “slice”?

21.15 Function of bandpass filter

21.16 Earthquakes are random vibration