• In a continuous system subjected to broad-band excitation, the natural frequencies should be separated as widely as possible so that not too many modes participate in the response.
  
  
• Jet noise often generates broad band excitation of modern aircraft fuselage (refer to Example 1 of lecture 4)
  
  
• To reduce the effect of vibration, the fuselage structure is stiffened by using stringers in between the shell panels (see fig 7.1)
  

Figure 7.1: Shell panel with stringers

• As the stringers are identical and equally spaced and the excitation is predominant along the X-direction (Fig. 7.1), neglecting the curvature of the panel, the structure can be modeled as a periodically supported beam (Fig. 7.2).

Figure 7.2: Periodically supported beam

• In Fig. 7.2, represents the torsional stiffness and the bending stiffness of the stringers with representing the uniform spacing between the stringers. Such a periodic structure, as we know, has alternate propagation and attenuation frequency bands.