Jet Noise Reduction

Near-field acoustical holography (NAH) is used to reconstruct three-dimensional acoustic fields from a two-dimensional planar measurement. During previous work at BYU, a method has been developed called energy based near-field acoustical holography which reduced the number of needed measurements by 75%. Other recent advances have expanded the theory to interior spaces where multiple sources and/or reflections are present. This paper presents a new method for reconstructing interior acoustic parameters using Fourier NAH and a single plane of energy density measurements. Energy density is measured using a six-microphone array. First, the probe measurements are used to create a Hermite surface pressure interpolation on two separate planes. These two planes are used to approximate the normal particle velocity as well as to separate the incoming and outgoing waves using the spatial Fourier-transform method. Once separated, traditional Fourier NAH is used to reconstruct the pressure and normal particle velocity at any point in space. Analytical and experimental results are shown and compared to exterior Fourier NAH approximations. Other drawbacks and benefits are discussed.

Given the benefits of using acoustic energy density for active noise control in enclosures, it was hypothesized that active structural acoustic control (ASAC) might also benefit by incorporating an energy-based structural error quantity. Power flow, or structural intensity, and structural energy density were studied for use in an ASAC system. Power flow was found to be unsuitable for general-purpose use in this application. Its minimization properties are such that for a general case, it may be impossible to predict structural or acoustic response based on a minimization of power flow amplitude in a two-dimensional setting. Additionally, sensor placement is complicated by the large changes in power flow field orientation caused by a small mass loading for a lightweight structure. Structural energy density was found to be a suitable error metric, and provides a slight improvement over velocity-based ASAC in enclosed spaces. A genetic algorithm was used to study structural energy density sensor placement on a simply supported plate. At modal frequencies, optimum control was achieved by placing the sensor at antinodes. The placement of the control force was found to be less critical, but showed a slight tendency towards locations remote from the disturbance force with low velocity cross-derivative.

Active structural acoustic control has been an area of increasing interest over the past decade with an increase in the search for a suitable error quantity. Current errors require the use of large amounts of accelerometers distributed across the entire structure to provide a suitable estimate of the volume velocity, as research has shown this quantity to be highly related to the overall acoustic radiation. Other methods involve a previous knowledge of the number of contributing acoustic radiation modes as well as an array of accelerometers to extract these modes. The purpose of this paper is to investigate a new structural quantity which when used in an active control situation attenuates the acoustic radiation over a large range of frequencies. The benefits of this technique are that it involves only a single point measurement and the placement of this sensor on the structure is fairly arbitrary. The results given are based on a simply supported plate and are purely analytical.

Near-field vector intensity measurements have been made of a 12.7-cm diameter nozzle solid rocket motor. The measurements utilized a test rig comprised of four probes each with four low-sensitivity 6.35-mm pressure microphones in a tetrahedral arrangement. Measurements were made with the rig at nine positions (36 probe locations) within six nozzle diameters of the plume shear layer. Overall levels at these locations range from 135 to 157 dB re 20 mu Pa. Vector intensity maps reveal that, as frequency increases, the dominant source region contracts and moves upstream with peak directivity at greater angles from the plume axis. (C) 2010 Acoustical Society of America

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