Advanced measurement techniques, particularly sound intensity measurement, are becoming an important design and diagnostic tool in aircraft design. The utilization of these tools in aircraft measurement presents unique challenges that must be overcome to obtain meaningful data. Once the data are obtained, careful analysis must be utilized in order to interpret the measurement results. A working group under SAE Committee A-21 is currently drafting a document intended to provide guidelines on sound intensity measurement under flight conditions. This document will be discussed in the light of some recent sound intensity measurements made in the Douglas Aircraft Fuselage Acoustic Research Facility as well as under flight conditions to highlight some of the measurement challenges involved in this type of measurement.
An analysis of the transmission of sound through sandwich panels (consisting of lightweight, flexible cores between relatively stiff skins) is presented here. First, previously given results for the free vibrations of such panels are analyzed and unified. Then, a traveling-wave analysis is used to obtain impedances and transmission-loss formulas, which are in turn compared with previously published experimental data. The comparison is relatively good, and confirmation of a symmetric coincidence effect is obtained. Further, the numerical results indicate that this symmetric coincidence generally occurs at a much lower frequency than does antisymmetric (shear wave) coincidence.
The influence of source location and room modifiers are examined with respect to the effectiveness of source position averaging in reducing measurement error for sound power measurements in reverberation rooms.
An analysis of the relationship between source location and measured sound power output of discrete frequency sources in reverberation rooms is presented, based on data obtained during the qualification of the 94 m3 reverberation room at the Centre for Building Studies. Total measurement uncertainty is evaluated at several source positions for both a bare chamber and for the final configuration with a rotating diffuser and low-frequency absorption. A high total measurement uncertainty at low frequencies proved to be the major obstacle to qualification as has been observed in numerous other studies (see for example the Sept.–Oct. 1976 issue of Noise Control Engineering). The modifications to the room and averaging over some positions reduced the measurement uncertainty to acceptable limits. It was observed that source position averaging was ineffective for the bare chamber configuration as pressure/frequency responses were highly correlated for different source positions. The addition of diffusing elements and low-frequency absorption increase modal overlap and result in less correlated pressure/frequency responses between source positions and hence improve the averaging effectiveness. [Work supported by NSERC.]
A reconsideration of a previous study of the transmission loss of sandwich panels is presented. An erroneous calculation of the transmission loss is corrected, and new results are presented. Also, some interesting implications for the design of walls of high transmission loss are pointed out. Subject Classification: [43]55.75.
The interior noise technology program to improve the noise environment in the aft cabin of the MD-80 twin jet aircraft is discussed. Two potential noise control treatments were identified: vibration absorber devices for the airframe and for the engine. A series of ground and flight tests using in-service aircraft was then conducted. These tests showed that the vibration absorbers for the airframe and engine decreased aircraft noise significantly.
This chapter contains sections titled: Introduction Principle of Operation Precision and Accuracy Installation Process Control Using pH Sensors References
An optimization study to investigate the feasibility of an optimal acoustic design process for sandwich panels is presented here. Using a pattern search procedure it is shown that the average transmission loss of a panel may be improved through optimization over a range of frequency, and that the optimization procedure results in a shift of the panel symmetric coincidence frequency beyond the range of interest. Subject Classification: 55.75.
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