I agree with the foregoing. In 1979, when Willis J. Willoughby "borrowed" that graph from Grumman at Bethpage, NY (now NGC) "everyone" used single-axis-at-a-time ED or electrodynamic shakers on which spectra could be shaped. Simultaneous multiaxis shaking on multiple RS (repetitive shock) platforms was 10+ years off in the future. RS platforms are certainly cheaper, for highly beneficial simultaneous multiaxis testing and screening, than several ED shaker systems. Unfortunately, RS offers little spectral control. The forthcoming "G" revision to MIL-STD-810 will offer a new Test Method 527 on multi-exciter testing.
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