Jet Noise Reduction

Selected Publications

Kent L. Gee (et al.)

Jet engine technology is more sophisticated than ever, building on decades of learning and growth in the field. However, even the latest advances in jet engine design and technology have not been able to counter an age-old challenge noise. Noise issues persist and adversely impact both ground maintenance personnel and surrounding communities. There is continued research to combat this issue, but for these emerging tools to achieve their full potential, innovative measurement and analysis methods are necessary to characterize the jet noise source region. A near-field acoustic holography system has been developed to meet this need and provide high-quality acoustic data. These data can be used for model refinement and benchmarking, evaluation of noise control devices, and predicting ground maintenance personnel and community noise exposure. The design of the 150-channel measurement array and data acquisition system is presented here. The prototype system was used recently to perform jet source noise measurements of an F-22 at Holloman Air Force Base located near Alamogordo, NM. The measurement approach and sound pressure level measurement maps detailing the near-field levels, spatial extent, and frequency content for four power conditions are featured.

David W. Krueger, Kent L. Gee, and Jeremy Grimshaw
The Balinese gamelan gong ageng wadon produces distinct acoustic beating (called ombak) when struck. This phenomenon is explored using both acoustical and vibrometry measurements. The measurements have revealed the beating has two sources. First, there are four closely spaced modes that, given their asymmetric vibration patterns, might have been deliberately hammered into the response of the gong. Second, and more importantly, a nonlinear structural response of the gong causes the fundamental axisymmetric mode to produce harmonics. The second harmonic of the fundamental mode interacts with the second axisymmetric mode with relative amplitudes such that strong beating is produced. (C) 2010 Acoustical Society of America
Kent L. Gee (et al.)

The Challenge:
Developing a portable near-field acoustic holography (NAH) system for measuring high-amplitude jet noise from current and next-generation military aircraft to provide model refinement and benchmarking, evaluate performance of noise control devices, 

and predict ground maintenance personnel and community noise exposure.

The Solution:
Developing a system based on cost-effective NI PXI dynamic signal acquisition (DSA) devices to take advantage of portability, flexibility, scalability, and accuracy; scale the number of data channels; and move the microphone array to increase the area under test and decrease measurement time while integrating the technical requirements for NAH with the environmental conditions and safety constraints associated with jet noise measurements.

Kent Gee (et al.)
Accurate measurement of the noise fields emitted by a full scale high performance jet engine and jet plume (with supersonic jet flow) requires detailed planning and careful execution. The apparent acoustic source can be very large, more than 50 feet long and 20 feet high and wide. The jet plume contains many noise generating sources, the main two being shock (broad band and shock cells) and turbulent mixing. This paper is an initial description of a detailed method to accurately measure and describe the near-field noise while simultaneously measuring the far-field noise. For a large high performance jet engine, the acoustic far-field may not be formed until more than 1000 ft away from the plume. The paper also describes proposed methods to measure the non-linear propagation of the noise from the nearfield to the far-field. The proposed methodology described with vetting will be considered as an US military standard (MILSTD) with possible later consideration as American standard measurement technique to describe noise fields for personnel noise exposure and for measuring the performance of jet engine noise reduction technologies.
Jarom H. Giraud, Kent L. Gee, and John E. Ellsworth
A 1 mu m diameter platinum wire resistance thermometer has been used to measure temperature fluctuations generated during a static GEM-60 rocket motor test. Exact and small-signal relationships between acoustic pressure and acoustic temperature are derived in order to compare the temperature probe output with that of a 3.18 mm diameter condenser microphone. After preliminary plane wave tests yielded good agreement between the transducers within the temperature probe's similar to 2 kHz bandwidth, comparison between the temperature probe and microphone data during the motor firing show that the +/- similar to 3 K acoustic temperature fluctuations are a significant contributor to the total temperature variations. (C) 2010 Acoustical Society of America
Molly E. Jones, Kent L. Gee, and Jeremy Grimshaw
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