Course Related Activities For Mechanical Vibration In The Absence Of A Formal Laboratory

The Engineering Technology (ET) program at Middle Tennessee State University has approximately 350 students. Our Mechanical Engineering Technology (MET) concentration was started in the fall of 2004 and currently it has 120 majors. All MET students are required to take several senior level courses including Mechanical Vibration. The author started teaching this course formally in the fall of 2006. Although Vibration is a lecture/lab course currently we do not have a lab due to budget restrictions and therefore, the author decided to include a relevant hands-on project and an industrial visit. The student teams are required to design, build and test a Helmholtz resonator. A Helmholtz resonator, which can be modeled as a spring-mass system, consists of a body (cavity) and a neck whose dimensions can be selected to tune the resonator to respond at a desired natural frequency. In the fall of 2007 student teams as well as the author built resonators using steel and aluminum. We used a laptop computer and a freeware (software) to test the resonators. The testing included pressing the resonator neck opening against our lower lip and quickly blowing once and simultaneously recording the time domain data. The freeware was useful in recording the time domain data but the frequency response was not good because it did not display a well-defined peak. Therefore, we could not compare the calculated and measured natural frequencies satisfactorily. In spring of 2009 the author received an internal grant to purchase the frequency analysis software, SpectraPlus. This software can perform an FFT on an existing audio wave file using the sound card in a pc or laptop. It also has the capability of recording and performing fast Fourier transform (FFT) in real time. It can generate colorful frequency response and 3-D surface (signal, time and frequency) plots in a matter of seconds. In the summer of 2009 the author calculated the frequency response using SpectraPlus for the fall of 2007 and 2008 time-domain data. All 2007 and 2008 resonators were built/tuned to respond at a natural frequency of 1000 Hz. The FFT results showed a well-defined peak consistently at the same but lower than the calculated value for all cases. This means the software is reliable and that some damping was present in the resonator. The damping could be due to the viscosity of air or the fabrication techniques used. We wanted to investigate this matter further. In the fall of 2009 each student team was asked to design three resonators to respond at 1000 Hz, for consistency and comparison with the earlier results, using different metals. The students learned to use SpectraPlus as part of the laboratory activity. They were able to obtain several time domain data and generate the frequency response and 3-D surface plots. Each team submitted a formal report on their resonator project which included introduction, design, fabrication, testing, and discussion of results. In addition to the Helmholtz resonator project the students were given a tour of a local industry which performs dynamic balancing of multi-disk rotors that are used to produce corn flour for cattle feed.