Jet Noise Reduction

A study of the eight metallophone pairs from a Balinese gamelan semara dana has been conducted. Acoustical recordings of metallophone bars being struck were used to examine ratios of overtone frequencies to the fundamental. Results showed large variability in the number and ratios of overtones present. Scanning laser Doppler vibrometry measurements made on several bars also revealed great variability in mode shapes present. The distribution of prominent overtones and their modal shapes do not appear to match those of Western metallophones. Notably, the overall gamelan metallophone characteristics are quite dissimilar to the glockenspiel, which disagrees with previous studies. (C) 2010 Acoustical Society of America

Fans are used to dissipate heat from electronic devices to keep the components working properly but create noise as a byproduct. Multiple fans are often needed in computer servers and other types of electronic equipment to adequately cool them. The fan array increases the cooling rate but also increases the noise produced. In addition to the added noise, differences between fans due to manufacturing cause the fans to rotate at different speeds, creating an annoying beat frequency. Feed forward active noise control was studied for an exhaust mounted two fan array using near field error sensors and control speakers. The tonal noise content of the blade passage frequency and the first harmonic was controlled at the error sensors for each fan. The tonal noise content was also reduced in the far field and the beat frequency reduced.

Although BYU’s acoustics heritage began over 75 years ago, noise control education and research was not a specific emphasis until the mid-1990’s. Physics and engineering undergraduate students interested in acoustics and noise control typically have little or no background in acoustics before they take graduate-level courses. This creates difficulties at times because students are not introduced to many important practical topics in noise and acoustics that are not regarded as “graduate-level” topics. Undergraduate students also attempt to take the graduate courses before they are prepared. To bridge the curriculum gap and to better prepare students that enter the workforce after receiving their undergraduate degrees, a new undergraduate course in acoustics and noise control has been introduced. This course combines teaching physics-based conceptual and mathematical principles with applications-based assignments and practical laboratory exercises. In this paper, a description of the course, its objectives, and its place in the overall scope of acoustics and noise control education at BYU is given. 

The desire to better characterize the jet noise source region for high-performance military aircraft has resulted in an effort to apply near-field acoustical holography (NAH). The extended and partially correlated nature of the source poses challenges. To overcome these challenges, scanbased patch NAH, coupled with partial field decomposition, is being investigated. Partial field decomposition uses multiple stationary reference microphones, but specific guidelines for determining proper reference microphone number and placement relative to the jet are needed. To help determine appropriate reference microphone configurations, numerical and physical laboratory experiments have been performed. The numerical and physical sources consist of series of partially correlated point sources and loudspeakers, respectively. Statistically optimized NAH is employed as the reconstruction method. Reference microphone placement is not found to be as significant of a factor as number of reference microphones for reconstruction error. Partially correlated sources decrease the required number of reference microphones from the uncorrelated case.

The Air Force commissioned a construction and engineering company to build an indoor test facility for the GAU-8 Avenger at Hill Air Force Base in Layton, Utah. The blast pressures from this 30-mm Gatling gun, however, are large enough to cause spallation of the concrete walls over time. The facility is being designed and constructed to last for 20 years, requiring several acoustical treatments. The pressures from the gun were measured outdoors, with maximum pressures exceeding 3000 Pa (163 dB) at a distance of 30 ft (9.1 m). A computer model of the room was designed using EASE, and impulse responses were generated at several positions. These impulse responses were convolved with an ideal blast wave pulse train to mimic the sound of the gun in the room. From these data and results collected from preliminary tests in the range, recommendations have been provided as to placement and types of necessary treatments. Final data confirm that the test facility meets all acoustical and occupational safety requirement.

Military jet aircraft are exposing both ground maintenance personnel and the community to high levels of noise. The US Department of Defense is funding research to develop advanced modeling tools for noise reduction techniques and community noise exposure. For these tools to achieve their full potential, innovative measurement and analysis methods are necessary to characterize the jet noise source region. To meet this need a portable near-field acoustic holography (NAH) system is under development to characterize full scale jet noise emissions. This paper will describe the basic design of the measurement array and data acquisition system for the NAH system, which will employ a patch measurement approach. With the patch measurement approach, multiple NAH reconstruction techniques can be used in tandem to provide improved confidence in the resulting reconstructions.