The sound quality generated by the guitar depends on the vibration characteristics (i.e. natural frequencies and mode shapes) of this instrument. Thus, it is of particular interest to the guitar manufacturers to be able to obtain global information about the characteristics of the guitar. Traditional sensors can only measure at discrete locations. However, digital image correlation (DIC) can measure full-field data on the surface of the structure. In the current paper, a finite element (FE) model of a guitar with free boundary configurations was developed using quadratic solid elements. An eigensolution was performed on the FE model to determine its natural frequencies and mode shapes. In order to validate the numerical results, a modal impact hammer test was performed on the instrument in a free configuration. Furthermore, a measurement was performed on the guitar using the DIC technique. In this measurement as well, the guitar was suspended in a free configuration, but was driven using a broadband acoustic excitation generated by a sound source. The response of the guitar to the excitation was recorded using a pair of high-speed cameras. The recorded images were processed to extract the natural frequencies and mode shape of the guitar.

As part of the recent electrification of the transportation industry, internal combustion engines are being coupled with or replaced by electric motors. This movement towards an electrified drivetrain poses new noise, vibration, and harshness (NVH) challenges related to electric motors. In this paper, the acoustic signature of an electric motor was analyzed to obtain a better understanding of the sound generated by these motors. This work provides an insight into an acoustic measurement technique that can be used to identify certain frequency bands that significantly contribute to the perceived sound. In the first part, the structural response of the motor was correlated with its acoustic spectra. Furthermore, data from acoustic and structural measurements were used to analyze the order content of the signal and identify critical contributors to the overall perceived sound. The differences between data captured by microphones in different positions around the motor helped to localize components of the overall sound. The results provide some discussion about techniques to decrease the overall sound. The technique described in this paper can be extended to fan-cooled motors that are used in vehicles such as golf carts or as auxiliary motors in electric/hybrid vehicles, as well as across a wide range of industrial applications.

For decades, quality of assessment has increasingly become a part of the discussion of good teaching in higher education. New faculty (and older ones) may be overwhelmed at the thought of comprehensive assessment of student learning at the level of individual courses and entire degree programs, on top of keeping up with grading and feedback for each student. However, if student assignments are used effectively and efficiently to assess student learning, students perceive the process as fair, grades become more meaningful, and data is generated to inform evidenced-based improvement of instruction. While some graduate programs and early-career mentoring include course design, learning objectives, and assessment of student learning, not all Ph. D. programs require training in even basic elements. New faculty may benefit from an introduction to a “backwards” approach to course design, a survey of learning assessment tools, and current best practices in grading. Examples from acoustics courses at Kettering University illustrate these ideas, with lessons learned from recent ABET accreditation in engineering and applied physics programs.

In certain communities of engineering education, especially where intersections with entrepreneurship shape the discussion, “Learning from Failure” has become a catch phrase. Usually these refer to pedagogical approaches in which students recognize the benefits of trying, failing, reflecting, and trying again. This iterative process also describes a teacher’s journey over a decade, learning to implement and practice research-based educational techniques with patience and a focus on the students. However, the stakes can be much higher when teachers try and fail! Examples from an elective course designed for a wide engineering audience, Acoustics in the Human Environment, will trace its evolution. It began as an experiment in a studio classroom format and became an asynchronous online class intended to mirror an engineering work environment. Milestones marking the steps of this journey: parallels to iteration involved in the engineering design process; suggested elements that provide a safety net for teaching failure, and other lessons learned along the way.

The cajon is a percussion instrument with origins in 16th century Peru. Literally translated “crate,” the cajon was originally a crate that players sat upon and struck to make rhythmic patterns. The modern version is a wooden box, closed except for a hole in the back panel. The front panel is struck with the hands or one of several types of beaters. The front panel is thinner than the others and is often attached with a slightly loose top edge for a snare-like effect. Alternatively, the snare sound is obtained with wires or guitar strings placed against the inside surface of the front panel, and these rattle when the panel is struck. Measurements of acoustic transfer functions with impulse excitation and roving hammer structural measurements explore the acoustic and structural resonances of the cajon with rattling strings removed. Spectra from recordings establish timbral features of the instrument, both with and without the rattling strings. By comparing results from measurements and recordings, the important features of the particular instrument tested in this study are identified, and possible directions for improved the designs are suggested.

A physical demonstration apparatus, inspired by [K. Meykens, et. al., Am. J. Phys., 67(5), 400–406 (1999)] is used to demonstrate evanescent sound waves in a rectangular waveguide. The apparatus consists of a rectangular waveguide, approximately one meter in length, with one optically transparent wall. The waveguide is driven at one end by a pair of loudspeakers whose polarity may be switched. The other end of the waveguide is open. The sound field inside the waveguide may be interrogated using a microphone and an oscilloscope. The apparatus will be used to demonstrate several features of acoustic wave guides, including the propagation of plane waves, the cutoff frequency for non-plane waves, the propagation of non-plane waves above the cutoff frequency, and the exponential decay of evanescent non-plane waves below the cutoff frequency. The educational and teaching applications of this apparatus will be discussed.