1208 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



3.5 



3.0 



D 



^2.5 

 < 



;5 2.0 



1.5 



1.0 



Q. 



< 



1.2 



2.0 



2.4 



Fig. 2 — Amplification of forced vibration on mass for two degrees of damping. 



(c) damping of the system is small. If damping is considerable, equa- 

 tion (2) becomes 



2/1 



(ST- 



Sin wt 



{v-m-Hsr 



(5) 



In this expression c/cc = fraction of critical damping. However, since 

 by construction, the natural frequency of the system is large compared 

 to the forcing frequencies, and damping of the active elements employed 



in these accelerometers is small, the term ( 2 — • —y becomes negligible. 



\ Cc Wn/ 



Equation (5) therefore simplifies to equation (2). 



The useful upper frequency limit is generally fixed at J^ con, because, 

 as shown by Fig. 2* amplification of the impressed signal results as co/con 

 increases. If the disturbances contain frequencies beyond the useful limit, 

 low pass electrical filters must be employed in the associated amplifier 

 to suppress these frequencies. The fact that this type of pick-up has very 

 low damping offers some disadvantage in recording complex waves be- 



• Figs. 2 and 3 were obtained from material given in Reference 2. 



