Page 27 - Spring 2007

Fig. 20. Transmission loss as a function of frequency for a cavity-backed plate.
Fig. 19. Dimensions for the cavity-backed plate example problem.
mental measurements, thus demonstrating that dipole sources can be used to model both the interior and exterior acoustic fields simultaneously.
Summary
In Part 2 of this tutorial on structural acoustics, we have learned about how acoustic fluids interact with structures, both as an acceptor and a cause of vibrational energy. We have presented some simple and difficult concepts in a rel- atively short article (entire textbooks are devoted to the sub- jects we have discussed), and hope the information is useful as a handy reference. For those of you who are interested in learning more about these topics, please look through the references we have provided. You are also welcome to enroll in the Sound—Structure Interaction course offered by the Penn State Graduate Program in Acoustics for a more thor- ough treatment of this subject.
Of course, there is much we have not explained, but we
can refer you to other strong references on those subjects.
For example, we have focused almost entirely on the inter-
action of structures with exterior fluids. Acoustic cavities
contain resonances which can interact with the walls that
bound the interior space, particularly at low frequencies.
Some classic papers which introduce this topic are those by
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Pretlove and Dowell. During the Active Noise Control
(ANC) boom of the 1980’s and 1990’s, many people investi- gated how to control the sound within acoustic cavity modes by driving the enclosure boundaries with tuned forces. Nelson and Elliott’s textbook29 is a good reference on ANC.
Another relatively modern structural–acoustic topic is Nearfield Acoustic Holography (NAH), which is an inverse technique for inferring a structure’s surface vibrations from a complex pressure hologram measured near the surface. Once the surface velocities are known, numerical boundary value techniques can be used to compute the far-field sound radiation. We recommend the textbook by Williams30 to those interested in NAH.
Acknowledgments
We are grateful for our daily interaction with the ARL/Penn State Structural Acoustics Department (Andrew Barnard, Robert Campbell, Stephen Conlon, David Jenkins, and Tim McDevitt), along with some of our students in Penn State’s Graduate Program in Acoustics, in particular, Ben Doty, who measured the response of the elbowed pipe.AT
References for further reading
1 G. H. Koopmann and J. B. Fahnline, Designing Quiet Structures (Academic Press, San Diego, 1997).
2 R. D. Blevins, Flow-Induced Vibration, 2nd Edition (Krieger Publishing Company, Malbar, FL, 2001).
3 E. Naudascher and D. Rockwell, Flow-Induced Vibrations–An Engineering Guide (Dover Publications, Mineola, NY 2005).
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