The effect of both modifications was evaluated by placing a simulation of the Agena forward structure and each of two adapters (one with diaphragm and one without) in a reverberation chamber and measuring the acoustic susceptibility at the flight accelerometer location with various diaphragm and mounting block configurations. The test was conducted in the large JPL reverberation chamber of Section 374. The Agena forward structure was placed on rubber pads on the floor of the chamber and each of the adapters (one with and one without a diaphragm) bolted to it. The tops of the adapters were plugged with a one-inch plywood board with two inches of fiberglass insulation attached to the adapter's side. The porkchop fitting at Foot A on both adapters was drilled for both the Block II and Block III (RA 1-4 and RA-6) accelerometer mounting blocks. The acoustic field within the chamber was set at 142 db (re 2 x 104 μB) for each test. Two microphones were located six inches from the outside of Feet A and C and two within the adapters. The output of the microphones and the flight accelerometers were recorded on magnetic tape and reduced by tape loop analysis, using a 10 cps analysis filter. The test consisted of four basic configurations: 1) Adapter with no diaphragm and Block II accelerometer mount. 2) Adapter with no diaphragm and Block III accelerometer mount. 3) Adapter with diaphragm and Block II accelerometer mount. 4) Adapter with diaphragm and Block III accelerometer mount. The effects of the diaphragm and the mounting block are compared by using the ratio of acoustic susceptibility measurements. The susceptibility at the flight accelerometer locations for each configuration was measured by calculating the ratio of the measured vibration excited by the acoustic field to the sound pressure level measured at the external microphone nearest the accelerometer. The 10 cps tape loop analysis of the microphone and accelerometers was used to calculate susceptibility vs. frequency. The effect of the various configuration changes is compared by ratioing the susceptibilities; by this method, the effect of changes in the acoustic field is eliminated. 4. Discussion a. Effect of changing accelerometer mounting block Figure F-12 is the ratio of the acoustic susceptibilities measured without a diaphragm using a Block II and a Block III accelerometer mounting block. The comparison is obvious; at low frequencies the ratio approaches unity (as it should) with a maximum difference of -10 to -15 db at 1000 cps and +10 db at 1600 cps. This shows, therefore, that in the 400-1200 cps range, the acoustic susceptibility of the Block III mounting block is greater than the Block II mounting block. By simply using this in place of the Block II mount, the vibration, as measured on RA-6, would be higher than that measured on RA 1-5 for the same acoustic excitation. b. Effect of diaphragm Figure F-13 is the ratio of the susceptibilities measured with a Block III mount both with and without a diaphragm. The ratio tends to oscillate about a value below unity with several peaks reaching unity. It may be concluded that some increase in measured vibration may be caused by the removal of the diaphragm but not in the clear way indicated for the mounting block change. C. Effect of both diaphragm and mounting block change Figure F-14 is the ratio of the measured susceptibilities of a Block II mount with diaphragm to a Block III mount without diaphragm. This comparison would, therefore, directly compare the configuration as used on RA 1-4 to that used on RA-6. The ratio generally follows the trend caused by the mounting block change (Figure F-14) with the addition of the low frequency roll off and peaks and valleys caused by the diaphragm curve (Figure F-15). d. Comparison of RA-6 vibration with RA 1-4 vibration Figure F-14, then, presents a method for comparing the measured vibration on RA-6 with the measured vibration on RA 1-4. The two dotted lines on Figure F-14 were used to correct the RA-6 vibration for comparison with RA 1-4. The upper curve is the most conservative (minimum) correction and the lower curve is a mean value of the correction. The corrected RA-6 flight data for liftoff and transonic along with the 95 percent curve of RA 1-4 are shown in Figures F-15 and F-16. The corrected RA-6 data is generally within the 95 percent curve of RA 1-4 with the exception of the 1000-1600 cps area of the transonic data. This discrepancy might be explained by the more localized excitation of the transonic buffeting. |