The Impulse Testing for Natural-Frequency DeterminationĪs discussed in Chapter 5, one very convenient way to determine the natural frequencies of a structure is to do a dual-channel impulsive (or, more properly, pulsive) test. The effect that this energy shift has in some areas of everyday life will now be discussed. Note that, as the width of the pulse increases, the distribution of the energy caused by the pulse shifts to lower frequen- cies. The key to all that follows lies in comparing the plots for the three pulse widths of Figures 1, 2, and 3. Succeeding lobes have relatively little energy. Almost all of the energy of the pulse is located in the first lobe, going from 0 to 1/T in frequency (note that 1/T is a lower frequency than for the narrower pulse discussed above because T is larger). Notice what happens when the FFT of a still wider pulse is taken, as in Figure 3. Each successive lobe has considerably less energy than the previous one and approaches zero amplitude at har- monics of 1/T.įigure 3B - A Still Wider Pulse, Frequency The first lobe contains most of the total energy of the pulse and rolls off toward the zero amplitude line at a frequency of 1/T.
#Locked heart behind a wall series#
Examination of Figure 2B shows a series of lobes in the frequency domain. Note that the frequency domain of the pulse looks very different form the pure white noise spectrum caused by an impulse. Figure 2A shows a pulse of time with T sec.
Since we know that nothing in nature happens over a zero time duration, it makes sense to talk about a pulse of finite width. Since the white noise has equal amplitude at all frequencies, the energy of the impulse is spread rather thin, yielding a low ampli- tude of white noise at all frequencies (see Figure 1B). The Fourier transform of a single impulse is pure white noise in the frequency domain. The unit impulse is a mathematical fiction and is shown in the time domain representation of Figure 1A as having an amplitude of 1 unit and a width of 0 sec. The most commonly discussed pulse is probably the im- pulse.
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