Jet Noise Reduction

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.

Structural fatigue, hearing damage, and community disturbances may all result from jet noise, especially as jet aircraft become more powerful. Noise-reduction technologies require accurate characterization of the noise sources within jets. Array-based sound pressure measurements were made in the jet exhaust region of an F-22 raptor to allow for sound-field visualization using near-field acoustical holography (NAH). This is one of the largest-scale applications of NAH since its development in the 1980s, and the most detailed near-field measurements made of high-power jet noise to date. The measurement was made using a large, dense microphone array, which scanned sound pressures over several measurement surfaces near the jet, resulting in more than 6000 measurement points. Fixed reference microphones, measuring simultaneously with each scan, were used to perform partial field decomposition (PFD) of the measurement planes. Guidelines for multi-reference jet-noise measurements in current literature are qualitative at best. The PFD allows for an analysis of reference microphone requirements. A method for determining the adequacy of the reference array using near-field coherence measurements is examined. [Work supported by Air Force SBIR.]

The nonlinear propagation of a pure sinusoid is considered using time domain statistics. The probability density function, standard deviation, skewness, kurtosis, and crest factor are computed for both the amplitude and amplitude time derivatives as a function of distance. The amplitude statistics vary only in the postshock realm, while the amplitude derivative statistics vary rapidly in the preshock realm. The statistical analysis also suggests that the sawtooth onset distance can be considered to be earlier than previously realized. (C) 2011 Acoustical Society of America