Volume 117, Issue 2, February 2005
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
117(2005); http://dx.doi.org/10.1121/1.1841571View Description Hide Description
A closed-form, two-dimensional analytical solution is developed to investigate the acoustic performance of a concentric circular Helmholtz resonator lined with fibrous material. The effect of density and the thickness of the fibrous material in the cavity is examined on the resonance frequency and the transmission loss. With the expressions for the eigenvalue and eigenfunction in the cavity, the transmission loss is obtained for a piston-driven model by applying a pressure/velocity matching technique. The results from the analytical methods are compared to the numerical predictions from a three-dimensional boundary element method and the experimental data obtained from an impedance tube setup. It is shown that the acoustic performance of a Helmholtz resonator may be modified considerably by the density and thickness of the fibrous material without changing the cavity dimensions.
117(2005); http://dx.doi.org/10.1121/1.1848072View Description Hide Description
Sound radiated by a computer cooling fan consists of tones which are phase locked with the rotation, and other less deterministic tones and broadband random noise. This paper demonstrates the feasibility of globally eliminating the rotation-locked tones by applying a very simple destructive interference to a modified cooling fan with the number of struts equal to the number of rotor blades. The rig consists of a miniature electretmicrophone used as a rotation sensor, an ordinary loudspeaker, and a bandpass filter with adjustable amplitude and phase delay. The microphone is located at the inlet bellmouth of the fan to pick up the fluctuating aerodynamic pressure caused by the passing rotor blades. The pressure spectrum is rich in the blade passing frequency (BPF) and its low-order harmonics. It provides much better performance than a pulse-generating tachometer. Analysis of the original fan noise shows that about 90% of the radiated tonal sound is phase locked with rotation, and this portion is almost completely eliminated in all directions. The reductions of the radiated sound power in the first two BPFs are 18.5 and 13.0 dB, respectively, and the overall sound power reduction is 11.0 dB.
117(2005); http://dx.doi.org/10.1121/1.1850208View Description Hide Description
In the study the inverse problem of deducing the modal structure of the acoustic field generated by a ducted turbofan is addressed using conventional farfield directivity measurements. The final objective is to make input data available for predicting noise radiation in other configurations that would not have been tested. The present paper is devoted to the analytical part of that study. The proposed method is based on the equations governing ducted sound propagation and free-field radiation. It leads to fast computations checked on Rolls-Royce tests made in the framework of previous European projects. Results seem to be reliable although the system of equations to be solved is generally underdetermined (more propagating modes than acoustic measurements). A limited number of modes are thus selected according to any a priori knowledge of the sources. A first guess of the source amplitudes is obtained by adjusting the calculated maximum of radiation of each mode to the measuredsound pressure level at the same angle. A least squares fitting gives the final solution. A simple correction can be made to take account of the mean flow velocity inside the nacelle which shifts the directivity patterns. It consists of modifying the actual frequency to keep the cut-off ratios unchanged.
117(2005); http://dx.doi.org/10.1121/1.1847872View Description Hide Description
Two methods of calculating the effective impedance spectra of acoustically hard, randomly rough, two-dimensional surfaces valid for acoustic wavelengths large compared with the roughness scales have been explored. The first method uses the complex excess attenuation spectrum due to a point source above a rough boundary predicted by a boundary element method(BEM) and solves for effective impedance roots identified by a winding number integral method. The second method is based on an analytical theory in which the contributions from random distributions of surfacescatterers are summed to obtain the total scattered field. Effective impedance spectra deduced from measurements of the complex excess attenuation above 2D randomly rough surfaces formed by semicylinders and wedges have been compared to predictions from the two approaches. Although the analytical theory gives relatively poor predictions, BEM-deduced effective impedance spectra agree tolerably well with measured data. Simple polynomials have been found to fit BEM-deduced spectra for surfaces formed by intersecting parabolas corresponding to average roughness heights between 0.25 and 7.5 m and for five incidence angles for each average height. Predicted effects of sea-surface roughness on sonic boom profiles and rise time are comparable to those due to turbulence and molecular relaxation effects.