Abstract
For a programme of research into hydraulic noise generation in centrifugal pumps, it was necessary to develop special measuring equipment for use in conjunction with existing equipment designed for the measurement of air-borne and structure-borne noise.
Small hydrophones, less than 0.5 in in diameter, in which the piezo-electric sensitive element was an axially polarized lead-zirconate-titanate disc, were designed and developed. In the design, reliability, implying both mechanical and electrical stability, was considered to be more important than extreme sensitivity; ease of construction and low cost were also taken into account. The final design, in which a thin stainless steel diaphragm was used, proved very stable and reliable and has a sensitivity of over 250 mV/lb in2 into a 200 megohm load.
Commercial tape recorders, considerably modified, were used as a cheap method of recording all data on 1/4 in magnetic tape for subsequent analysis. A time-delay tape deck was also constructed for use in conjunction with a small special purpose computer to evaluate the cross-correlation function from the outputs of two hydrophones spaced a known distance apart.
Methods of hydrophone calibration were devised, and the results agreed closely with the theoretical figures derived from the piezo-electric constants of the ‘crystals’ used.
Special consideration was given to methods of automatically recording the results of one-third octave and discrete frequency analyses in forms which would provide the maximum amount of easily assimilable information.
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