B. MEASUREMENT OF PARAMETERS (SEE SECTION I) 



In all vibration surveys there is still a discrepancy of several percent between calcu- 

 lated and experimental values of frequencies. This is probably due to some of the effects 

 mentioned in IV. A, sprung mass, etc., but more probably due to inaccurate knowledge of cer- 

 tain parameters such as virtual mass of water, rigidity of the hull, and position of the center 

 of mass in a section. Methods are under study which will permit the calculation of these 

 parameters directly from ship data. Similar studies are also underway which allow the deter- 

 mination of spring constants of radar masts and similar structures. Careful planning will 

 allow the determination of damping characteristics. In this program, the use of impulsive 

 loadings is under study and looks promising. 



C. DEVELOPMENT OF EXPERIMENTAL TECHNIQUES 



1. Shakers 



The evaluation of hull frequencies and mode shapes, natural frequencies of substruc- 

 tures like bulkheads, thrust bearing, etc., is done by use of a number of shakers available 

 which were developed during past years. These facilities include a 40,000-lb and a 5,000-lb 

 vibration generator for frequencies up to 30 cps, small shakers like the TMB medium vibra- 

 tion generator, a Lazan shaker, etc. It also includes instruments which exert a known point 

 force and allow the measurement of the structural response at the same or any other point to 

 obtain mode shapes, frequencies, damping, and transfer functions. 



2. Measuring and Recording Systems 



The measuring system was drastically changed during the recent years by replacing 

 mechanical instruments with electrical instruments such as accelerometers, velocity meters, 

 combination of the latter, etc., to cover frequencies in the range from zero to about 8000 cps 

 with emphasis on the frequencies up to 50 cps. Recordings of the data are made by use of 

 recording oscillographs and magnetic tape recorders. 



3. Analysis Systems 



The analysis of vibration as recorded on paper is done in the well-known fashion 

 as described in many reports, by evaluating frequencies present and the corresponding 

 amplitudes. 



The electronic tape analysis allows a much broader and accurate evaluation by ob- 

 taining frequency-amplitude spectrum, power spectrum, average and rms amplitudes, etc. 

 It shows all frequencies and also the amount of random vibration present. The amplitudes 

 cover a range of nearly 40 db. 



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