Jet Noise Reduction

Exploding hydrogen-oxygen balloons are popular chemistry demonstrations. Although initial research experimentally quantified potential hearing risk via analysis of peak levels [K. L. Gee et al., J. Chem. Educ. 87, 1039-1044 (2010)], further waveform and spectral analyses have been conducted to more fully characterize these impulsive noise sources. While hydrogen-only balloons produce inconsistent reactions and relatively low, variable levels, stoichiometrically mixed hydrogen-oxygen balloons produce consistent high-amplitude noise waveforms. Preliminary consideration is also given to the potential use of these exploding balloons in architectural acoustics applications. (C) 2012 Acoustical Society of America

This paper presents a physical demonstration of an audio-range line array used to teach interference of multiple sources in a classroom or laboratory exercise setting. Software has been developed that permits real-time control and steering of the array. The graphical interface permits a user to vary the frequency, the angular response by phase shading, and reduce sidelobes through amplitude shading. An inexpensive, eight-element loudspeaker array has been constructed to test the control program. Directivity measurements of this array in an anechoic chamber and in a large classroom are presented. These measurements have good agreement with theoretical directivity predictions, thereby allowing its use as a quantitative learning tool for advanced students as well as a qualitative demonstration of arrays in other settings. Portions of this paper are directed toward educators who may wish to implement a similar demonstration for their advanced undergraduate or graduate level course in acoustics. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3676723]

Exploding gas-filled balloons are common chemistry demonstrations. They also provide an entertaining and educational means to experimentally verify nonlinear acoustical theory as described by the Earnshaw solution to the lossless Burgers equation and weak-shock theory. This article describes the theory, the demonstration, and the results of a propagation experiment carried out to provide typical results. Data analysis shows that an acetylene-oxygen balloon produces an acoustic shock whose evolution agrees well with weak-shock theory. On the other hand, the pressure wave generated by a hydrogen-oxygen balloon also propagates nonlinearly, but does not approach N-wave-like, weak-shock formation over the propagation distance. Overall, the experiment shows that popular demonstrations of chemical reactions can be extended from chemistry classrooms to a pedagogical tool for the student of advanced physical acoustics. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3676730]

Research in physics education has indicated that the traditional lecture-style class is not the most efficient way to teach introductory physical science courses at the university level. Current best teaching practices focus on creating an active-learning environment and emphasize the students' role in the learning process. Several of the recommended techniques have recently been applied to Brigham Young University's introductory acoustics course, which has been taught for more than 40 years. Adjustments have been built on a foundation of establishing student-based learning outcomes and attempting to align these objectives with assessments and course activities. Improvements have been made to nearly every aspect of the course including use of class time, assessment materials, and time the students spend out of the classroom. A description of the progress made in improving the course offers suggestions for those seeking to modernize or create a similar course at their institution. In addition, many of the principles can be similarly applied to acoustics education at other academic levels. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3676733]

Research in physics education has indicated that the traditional lecture-style class is not the most efficient way to teach science courses at the university level. Current best-teaching practices focus on creating an active-learning environment and emphasize the students' role in the learning process. Several of the recommended techniques have recently been applied to Brigham Young University's acoustics courses. Adjustments have been built on a foundation of establishing student-based learning outcomes and attempting to align these objectives with assessments and course activities. Improvements have been made to nearly every aspect of the courses including use of class time, assessment materials, and time the students spend out of the classroom. The progress made in bringing two of the courses, specifically an introductory, descriptive acoustics course for a general audience and a junior level introduction to acoustics course for majors, is described. Many of the principles can be similarly applied to acoustics education at other academic levels. Suggestions are made for those seeking to modernize courses at their institutions.

In 1905, Heinrich Rubens and Otto Krigar-Menzel published a paper describing a unique acoustics teaching apparatus. They developed a flammable gas-filled tube with holes in the top that revealed the acoustic standing wave behavior via the height of flames above the tube. Interestingly, their article holds the distinction of being printed immediately following Einstein's Nobel-prize winning paper on the photoelectric effect. From that auspicious beginning, the "Rubens tube" has been used for over a century in the teaching of acoustical resonance behavior. This article describes some of the history around the tube's development and its operation, as well as some of the commentary and investigations involving the flame tube found in the literature.