David W. Krueger

Cylindrical acoustical holography applied to full-scale jet noise

Near-field acoustical holography methods are used to predict sound radiation from an engine installed on a high-performance military fighter aircraft. Cylindrical holography techniques are an efficient approach to measure the large and complex sound fields produced by full-scale jets. It is shown that a ground-based, one-dimensional array of microphones can be used in conjunction with a cylindrical wave function field representation to provide a holographic reconstruction of the radiated sound field at low frequencies. In the current work, partial field decomposition methods and numerical extrapolation of data beyond the boundaries of the hologram aperture are required prior to holographic projection. Predicted jet noise source distributions and directionality are shown for four frequencies between 63 and 250 Hz. It is shown that the source distribution narrows and moves upstream, and that radiation directionality shifts toward the forward direction, with increasing frequency. A double-lobe feature of full-scale jet radiation is also demonstrated.

Aperture extension for near-field acoustical holography of jet noise

It is generally true that larger measurement apertures (relative to the source size) produce more accurate reconstructions of sound fields in near-field acoustical holography (NAH) applications. When such apertures are infeasible, numerical extrapolation of the sound field can be implemented to recover data, allowing a successful NAH reconstruction. When the limited aperture is due to a large standoff distance from the source, accurate recovery of data outside the measurement region becomes increasingly critical. In the present paper three data extrapolation methods are implemented on a simulated sound field from two extended, coherent sources. The methods are compared based on their relative accuracy of extrapolated data with distance from the measurement aperture.

Acoustical and vibrometry analysis of a large Balinese gamelan gong

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.

Cylindrical Fourier near‐field acoustical holography applied to a high‐power jet.

Near‐field acoustical holography as a method to reconstruct sound fields near military jet aircraft is being investigated. Measurements were recently made on the F‐22 Raptor using a linear, ground‐based array of microphones at a distance of 11 m from the centerline. Analytic continuation is used to extend the measurement aperture, and, assuming axisymmetry, cylindrical Fourier near‐field acoustical holography is used to reconstruct the sound field. Because of the highly directional nature of the source and the effects of wrap‐around error from the Fourier transform, the desired reconstruction distance (10–20 m) and the number of points to analytically continue the measurement are directly proportional. Furthermore, given the measurement standoff distance, inverse propagation to the source requires near‐complete regularization of the evanescent wave components. [Work supported by Air Force SBIR.]

Aperture extension for near-field acoustical holography applied to jet noise

Near-field acoustical holography (NAH) techniques are used to investigate noise source characteristics of high-power jets on military aircraft. Sound field reconstruction of large sources, measured with an aperture of limited size, may generally be performed with the use of patch NAH methods. Patch methods, such as statistically optimized near-field acoustical holography (SONAH), help to mitigate the effects of a truncated measurement aperture by avoiding the use of the spatial discrete Fourier transform operation. However, the lack of information outside the measurement aperture may lead to other errors, particularly when large propagation distances are required. Some missing data must be recovered to propagate beyond the immediate measurement region. Numerical aperture extension methods, in conjunction with SONAH, are employed to characterize high-power jet noise sound fields. These methods include complex interpolation and extension, analytic continuation, and in-plane holographic projection. [Work sup...

Acoustic and vibrometry analysis of beating in a large Balinese gamelan gong.

A large Balinese gamelan gong, the gong ageng wadon, is similar to other Indonesian gongs in that it is tonal. A previously undocumented phenomenon is the fact that this gong produces distinct beating, which is essential to Balinese gamelan music. In this study of Brigham Young University’s gong ageng wadon, acoustical and vibrometry measurements were performed to analyze the source of the beating. Scanning laser Doppler vibrometer results show a beat frequency of about 3 Hz is produced near 150 Hz by closely spaced structural modes. A slightly slower beat frequency (around 2.5 Hz) is also produced at about 120 Hz. The difference in this case is that there is only one structural mode measured near this frequency. It appears that the acoustic radiation from this mode beats with a harmonic of the radiation from a lower structural mode and that this harmonic is caused by nonlinear acoustical phenomena.

Near field placement of error sensors in an active noise control application of axial cooling fans using flow visualization techniques

The use of error sensors in the near field of an axial fan can be used to achieve global attenuation of the blade passage frequency and its harmonics. The pressure field produced by the minimized sound power radiation of the fan and control source configuration dictates possible locations of the error sensors by creating pressure nulls in the near field. By minimizing pressure at these locations, the minimized sound power field can be reproduced. Near field locations can be further investigated using qualitative flow visualization techniques including smoke visualization and quantitative particle image velocimetry (PIV). Analysis of the flow field will help to understand the presence of turbulent flow at the error sensors which can be a cause of a decreased signal to noise ratio. Optimal locations as well as mounting techniques for the error sensors will be discussed based on the flow field analysis. The effects of active noise control on the flow field produced by the fan will also be discussed.