704 MECHANICAL DESIGN AND PACKAGING 



Let 5rnax bc thc maximum deflection of the shock isolator. Then 



5n,ax = Vl^. (13-14) 



Equation 13-13 indicates that a low natural frequency favors good shock 

 isolation, as it favors good vibration isolation. Equation 13-14, however, 

 shows that substantial velocity shocks will be accompanied by large 

 deflections unless 12 is high. For example, with an isolator natural frequency 

 of 15 cps (a common value for vibration isolation), a drop from a height of 

 only 5 inches with inelastic impact will cause a maximum isolator deflection 

 of more than Y% i^^h if isolator damping is small. Such a large deflection 

 would not normally be permissible, and such a shock would cause a limiting 

 action in the isolator known as snubbing. This action increases the severity 

 of the shock by a factor which approaches 1 as the isolator clearance is 

 reduced. Therefore, in many cases an isolator which is beneficial during 

 vibration may be worse than useless during shock. 



If the natural frequency of the isolator is sufficiently high to prevent 

 snubbing and if it thus provides some shock protection, the mounted 

 equipment will probably undergo severe vibration at this frequency. Unless 

 the shock isolator is heavily damped, this will require that the structure and 

 components have resonant frequencies about 2 to IY2 times that of the 

 isolator. 



It has been pointed out that in order to produce a design that will be 

 reliable under conditions of shock and vibration, it is necessary to consider, 

 from the beginning until the end of the packaging design eff^ort, the expected 

 environment, the response characteristics of the equipment and its parts, 

 and the fatigue characteristics of the materials used. The topic of testing 

 is beyond the scope of this chapter; however, it may be stated that the 

 vibration and shock testing of early units and assemblies is now an essential 

 engineering technique for the development of an airborne radar. Isolators 

 may in some degree protect delicate equipment from the environment; their 

 application, however, has pitfalls and is to be undertaken with caution. A 

 design with high natural frequencies is often necessary in connection with 

 isolator application. This is in any case the generally applicable approach 

 to successful design for shock and for vibration. 



13-7 ACOUSTIC NOISE 



Supersonic speeds and high engine power are responsible for an induced 

 environmental condition termed acoustic noise. Acoustic noise can be 

 defined as vibration waves carried through the air. 



In aircraft the two major sources generating these sound pressure waves 

 are the power plant and the aerodynamic effects of the air passing over the 

 various surfaces of the aircraft. Most of the power plant noise from 



