Spann Vibroacoustic Method

Avionics components in aircraft and launch vehicles may be mounted to surfaces which are exposed to high intensity acoustic excitation. The external acoustic pressure field causes the panel and shell surfaces to vibrate. This vibration then becomes a base input to any component mounted on the internal side. Components must be designed and tested accordingly.

The component vibration input levels can be derived via analysis and testing for a given sound pressure level.

Acoustic testing of the structure can be performed in a reverberant chamber or using a direct field method. There is some difficultly in testing, however, because the simulated acoustic field in the lab facility may be different in terms of spatial correlation and incidence than that of the flight environment even if the sound pressure level can be otherwise replicated.

The vibroacoustic analysis techniques include finite element and boundary element methods, as well as statistical energy analysis. These are powerful tools, but they require numerous assumptions regarding external acoustic pressure field type, coupling loss factors, modal density, impedance, radiation efficiency, critical and coincident frequencies, distinguishing between acoustically fast and slow modes, etc.

As an alternative, simple empirical methods exist for deriving the structural vibration level corresponding to a given sound pressure level. Two examples are the Franken and Spann techniques. These methods may be most appropriate in the early design stage before hardware becomes available for lab testing and before more sophisticated analysis can be performed.

The Spann method provides a reasonable estimate of the acoustically excited component vibration environments when only the areas exposed to the acoustic environment and mass are known.

Spann_vibroacoustic_method.pdf

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The scripts for performing this calculation are given at:
Vibrationdata Signal Analysis Package

>> vibrationdata > Acoustics & Vibroacoustics > Vibroacoustics

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– Tom Irvine

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