Method has been developed for predicting interaction between components and corresponding support structures subjected to acoustic excitations. Force environments determined in spectral form are called force spectra. Force-spectra equation is determined based on one-dimensional structural impedance model

Kao, G. C.

English

NASA Technical Reports Server

February 1973	 B73-10059 
NASA TECH BRIEF 
Marshall Space Flight Center 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Mechanical Impedance and Acoustic Mobility Measurement Techniques

of Specifying Vibration Environments 
The problem: 
Component packages of rocket vehicles are tradi-
tionally qualified for acoustic environments by motion-
control (or response-control) testing. Vibration environ-
ments used in this type of testing are obtained by con-
sidering the envelope of the peak amplitudes of measured 
or predicted response data, but ignoring effects of com-
ponent-primary structure coupling. Consequently, com-
ponents qualified under motion-control criteria would 
probably be overtested or undertested as compared to 
actual inflight environments. A more realistic approach 
would be to first determine the interaction forces 
between a component and its support structure, and 
then to test the component under the force-control 
environments. 
The solution: 
A method has been developed for predicting inter-
action forces between components and corresponding 
support structures subjected to acoustic excitations. 
Such force environments which are determined in 
spectral form are referred to as force spectra. 
How it's done: 
In this method, the force-spectra equation is derived 
based on a one-dimensional structural impedance model. 
By applying Thevenin's and Norton's theorems to this 
model, the equation which relates the force-spectra to 
external excitation forces as well as component imped-
ance and dynamic properties of support structures, such 
as structural impedances and acoustic mobilities, is 
given by
	 - 
I ZZsl 	 I	 12 
= I	 I	 a(w)	 (w) 
I ZL + ZS	 p
where 1L is the predicted force spectrum, Z is the 
input impedance of the support structures, ZLIS the input 
impedance of the component package,4. 0 is the blocked 
pressure spectrum, and is the acoustic mobility of 
structural systems at mounting locations. 
This equation provides more realistic vibration en-
vironments than the standard motion-controlled specifi-
cations. In addition, it can be extended to acquire force 
spectra for multidimensional structures. 
The parameters employed in the computations can be 
obtained by two methods. In the first method, the 
support structure impedances are estimated by simple 
impedance equations ,
 developed for complex cylinders. 
The acoustic mobility and blocked pressure spectra are 
computed or predicted by methods outlined for stiffened 
cylindrical shells. The component package impedances 
could be predicted by a one-dimensional system model 
by employing classical multi-degree of freedom systems. 
The second method employs measurement techniques to 
acquire data on prototype or scale model structures. It 
employs an analog/digital data acquisition system to 
acquire the measured data in digital form which are 
subsequently analyzed to compute force environments. 
Good agreements between the predicted and measured 
data were observed by the latter approach. Such an 
approach can be used to specify vibration environments 
for new vehicles with greater efficiencies, and sub-
stantial savings in time and labor can be realized. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&PS-TU 
Marshall Space Flight Center, Alabama 35812 
Reference: B73-10059
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from priVately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000059 2020-03-17T06:40:29+00:00Z
Patent status: 
NASA has decided not to apply for a patent. 
Source: G. C. Kao of

Wyle Laboratories
under contract to
Marshall Space Flight Center

(MFS-2201 6) 
B73-10059	 Category 06


Mechanical impedance and acoustic mobility measurement techniques of specifying vibration environments

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