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What Is the Difference Between Dip & SRS?

Mechanical systems, including those used in aeronautics, astronautics and nautical vessels, are subject to sudden acceleration or deceleration, otherwise known as mechanical shock. The response of a system to mechanical shock can be represented, graphically, by what is known as the Shock Response Spectrum. Under certain circumstances, the SRS produces unusual results due to a phenomenon known as Shock Response Spectrum Dip.

Shock Response Spectrum
- The Shock Response Spectrum illustrates how a random, transient acceleration or deceleration affects a simple vibratory system, such as a single mass connected to a spring. Such a system, in which the mass is allowed to travel only along the direction in which the spring elongates, is known as a Single Degree of Freedom system. In fact, the Shock Response Spectrum shows the maximum response of an infinite number of SDOF systems -- each of which has its own natural frequencies -- to acceleration.
Shock Spectrum Dip
- The phenomenon known as Shock Spectrum Dip was discovered by accident during mechanical shock trials on naval vessels in 1957. The reed gauges -- simple mechanical instruments that record the essential features of transient motion -- employed during the trials produced unusual results. In particular, the reeds corresponding to the fixed natural frequencies of equipment produced unexpectedly low results and heavy equipment produced a lower shock response than lighter equipment. The Shock Spectrum Dip is the same phenomenon that occurs in earthquake engineering and, onboard a ship or submarine, produces a pronounced on the Shock Response Spectrum that individual items of equipment experience.
Experimentation
- It was believed, initially, that the equipment interacted with the structure of naval vessels, producing a ̶0;dip̶1; in the Shock Response Spectrum at its base. The initial observation of Shock Spectrum Dip led the Naval Research Laboratory to develop more sophisticated experiments during the 1960s to demonstrate the phenomenon and the effect of modal mass -- the ̶0;effective̶1; mass in each mode of vibration upon it.
DDAM
- One such experiment involved three double cantilever beams -- of the same weight, but different stiffnesses -- to which mechanical forces were applied. Equipment mounted on flexible foundations produced Shock Spectrum Dip at the fixed base natural frequency of the equipment, corresponding to troughs, rather than peaks, in the shock spectrum. The Navy has since conducted many shock tests and analyzed the results at the base of equipment, culminating in a technique known as Dynamic Design Analysis Method. DDAM estimates the response of individual components to the motion of the hull of a naval vessel.

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