Vibro-Acoustic Education at Michigan Tech

Chuck Van Karsen Associate Professor

Mohan D. Rao Associate Professor

Jason R. Blough Assistant Professor

Dynamic Systems Laboratory

Michigan Technological University Houghton, MI 49931

Abstract Michigan Tech has had a strong program in the area of vibro-acoustics for over 30 years with both graduate and undergraduate courses in vibrations, noise control, dynamic measurements and signal processing, acoustics, and modal analysis. This paper will explain how this technology is presented in both undergraduate and graduate courses. The paper addresses course sequences and content along with a description of how the material is integrated into projects and assignments. The program includes projects from industrial sponsors. This cooperative effort will be explained along with several examples. Introduction The character and nature of a specific program of study at the graduate level is usually a reflection of the nature and character of the participating faculty. New, untenured faculty are interested in building a research program in their area of interest. One of the key ingredients of a successful research program is a group of well trained graduate students. This means that new faculty offer courses in their area of interest, while established (tenured) faculty continue to refine their courses to meet the needs of their latest research efforts. In addition the fundamental and classical subjects must continue to be presented. A program of graduate study in Michigan Tech’s Vibro-Acoustic program involves classic theory, fundamental experimental techniques, commercial software experience tempered with self computation and programming, the latest analytical and experimental methods, and real projects/problems. The goal is to produce graduates who can ‘hit the ground running’ and be productive their first day on the job. MTU’s Vibro Acoustic Program Many of the students in MTU’s graduate program earn their undergraduate degree at Michigan Tech as a first step. MTU’s bachelor degree in Mechanical Engineering includes three required courses in the dynamic systems area: Dynamics, Mechanical Vibrations, and Control Theory. Students who plan on entering the Vibro-Acoustics graduate program are encouraged to take additional senior elective courses which include: Analytical and Experimental Modal Analysis, and Acoustics and Noise Control. Students entering the graduate program who have earned their undergraduate degrees from universities other that MTU are encouraged to take these senior elective courses as part of their graduate course work. At the graduate level, the program includes a three course sequence in Vibro-Acoustics: Analytical Vibro-Acoustics, Experimental Vibro-Acoustics, and Computational Vibro-Acoustics (new Fall 2007), along with Dynamic Measurements and Signal Analysis and three credits of advanced math. Elective courses are available in Experimental Stress Analysis, FEA Theory, Intermediate Dynamics, and Linear Systems.

Course Descriptions: Analytical and Experimental Modal Analysis

This course is a combined experimental and analytical approach to mechanical vibration issues. It covers the characterization of the dynamic behavior of a structure in terms of its modal parameters through both analytical and experimental methods. Digital data acquisition and signal processing, experimental modal analysis procedures, parameter estimation for obtaining modal parameters and modal model validation are also presented.

Acoustics and Noise Control This course covers the analysis and solution of practical environmental noise problems. The Fundamental concepts of sound generation and propagation, the unwanted effects of noise, assessment of sound quality, and source-path-receiver concepts in noise control are presented. The course consists of lecture, measurement laboratory, and team project directed at solving a real noise problem under a client’s sponsorship.

Dynamic Measurements and Signal Analysis This course presents material on the assessment of measurement system requirements including: transducers, conditioners, and displays of dynamic measurands. Signal Analysis topics include; Time-, frequency-, probabilistic-, and correlative-domain approaches to dynamic signal analysis: sampled data, discrete Fourier transforms, digital filtering, estimation errors, system identification, calibration, recording. All concepts reinforced in laboratory and simulation exercises along with a final project.

Analytical Vibro-Acoustics This is the first in a series of three courses on vibro-acoustics that provide a unified approach to the study of noise and vibration. The course emphasizes interaction between sound waves and structures. It presents advanced vibration concepts with computational tools.

Experimental Vibro-Acoustics This course covers operating data measurement and analysis, including multi-source ODS methods, signature analysis and order tracking. Experimental modal analysis techniques including,FRF estimators and measurements; multiple input excitation techniques; parameter estimation methods are presented. Additional topics include; sound measurements and acoustic intensity; sound quality; field data acquisition, binaural recording and playback with equalization. An industry sponsored project covering all aspects of the course material is required as well.

Most of the courses in the MTU Vibro-Acoustics Program have a significant laboratory or project component as part of the course requirements. In almost all cases the projects are sponsored by companies or other organizations from outside of the university. This activity gives the students a chance to experience the issues connected with real structures and situations in addition to the ‘canned’ material and experiments associated with traditional course work. Students in the Analytical and Experimental Modal Analysis course are required to complete two experimental modal analyses. The first, a simple plexi-plate structure, and the second, on a structure of their choice, or one from an industrial sponsor. Students have performed modal analysis on wine glasses, tennis rackets, musical instruments, golf clubs, skis, and SAE competition vehicles such as Formula One and Mini Baja. Industrial sponsored tests have included dryer cabinets from Whirlpool, small electrical generators from Briggs & Stratton, and transfer case housings from Borg Warner. All projects are reviewed before testing begins to insure that a successful modal analysis is possible. Heavily damped, highly non-linear structures are not considered for this course. All testing and analysis uses Siglab data acquisition systems and modal analysis software from Vibrant Technology and Spectral Dynamics. The goal of class projects in the Acoustics and Noise Control course is to tackle a noise problem from “initial assessment” to “implemented solution”. A good example of this concept is a class project that was completed for a local community. A local theater group had recently restored a small theater and realized that they had a

speech intelligibility problem due to long reverberation times. Students from the Acoustics and Noise Control class made an initial assessment of the theater’s acoustic condition through reverberation time and speech intelligibility measurements. Using this data the group made recommendations for modifications to the acoustic properties of the theater consisting of sound absorption treatments for the walls of the theater. The goal of the modifications was to reduce the average reverberation times to 1.0 second. The theater group purchased the recommended material and, with the help of members of the class, installed it in the theater, shown in the Figure on the right. Subjectively this made a large improvement in the theater’s acoustic properties. To complete the project the students re-measured the acoustical properties of the theater. Average reverberation times dropped from 1.3 to 1.0 seconds. Data acquisition and analysis was completed with a system

from 01-dB Inc. Dynamic Measurements and Signal Analysis is a graduate level course that attracts students from all areas of mechanical engineering and engineering mechanics. The background of these students ranges from “never used an oscilloscope” to “experience with the latest data acquisition systems”. The project portion of the course is adjusted appropriately; however each project must meet 3 fundamental criteria. The students must: make a measurement of a dynamic phenomena, determine the measurement system’s affect on the acquired data, and finally they must interpret the data. Students are encouraged to choose projects that are closely associated with their research topics. Examples include: ‘Tonal Analysis of a Baritone Horn’, Figure on the left, ‘Examination of High Frequency Limitations of an Elastomer Test Machine’, and ‘Evaluation of Accelerometer Mounting Techniques for Human Interfaces of a Snowmobile’. Data acquisition is completed with Siglab systems from Spectral Dynamics and data analysis is performed in Matlab.

The Experimental Vibro-Acoustics course is designed for students who have chosen the fields of noise and vibration or structural dynamics as the focus of their graduate education. The laboratory and projects portion of this class give the students the opportunity to learn all aspects of experimental vibro-acoustic methods. Class projects, which are typically industry sponsored, involve the characterization of operating data, measurement of path properties (FRF’s) and direct or indirect determination of operating loads. A good example of this type of project was one involving a boat/motor system sponsored by Mercury Marine and Boston Whaler. The sponsor provided the class with a particular boat/motor combination that experienced sever helm vibration at various operating conditions. The class members used Signature Analysis and Operating Deflection Shape measurements to determine the specific operating problems. Multiple input modal analysis methods characterized the dynamic properties of the system. Operating loads from the motor were predicted using Transfer Path Analysis techniques. Testing of the system was done both ‘in the lab’ and ‘on the water’. Upon completion of the project several students presented the results directly to the sponsors. Software and hardware from Lms International is the primary measurement and analysis system. Conclusions The overall goal of the Vibro-Acoustics program at MTU is to produce graduates with a good balance of theoretical and practical knowledge who can be productive members of the company or organization that employs them. This is accomplished through a comprehensive program of study that includes all aspects of vibrations and acoustics, and a faculty that is actively engaged in vibro-acoustics research. This is coupled with close cooperation with industrial sponsors and suppliers of tools for both analytical and experimental vibro-acoustics. This combination places MTU graduates among the respected members of the structural dynamics community.

Acknowledgements The authors would like to thank all of our sponsors who have helped to make our program successful: PCB Piezotronics, LMS International, 0-1dB Inc., Vibrant Technology, Spectral Dynamics, Whirlpool Corp., General Motors, Ford, Diamler Chrysler, Borg Warner, Mercury Marine, Briggs & Stratton, Polaris, Caterpillar, John Deere, and all others who have provided support.