Volume 117, Issue 3, March 2005
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
117(2005); http://dx.doi.org/10.1121/1.1859251View Description Hide Description
In many previous studies, energy-based methods are used to predict the attenuation of sound in long tunnels. However, these models do not address the interference effects of the sound fieldsgenerated by all image sources. A numerical model has been developed, in which the total sound field is computed by summing contributions from all image sources coherently. This numerical model also incorporates a correction term for calculating the atmospheric absorption of sound in air. To validate the numerical models in practical situations, two road traffic tunnels have been chosen for extensive measurements. The levels of the transmitted noise have been recorded in one-third octave band frequencies at various separations up to a maximum of 400 m. The predictions using the coherent model agree reasonably well with the measured data at all frequencies. The agreements between the field data and the theoretical predictions using the energy-based model are tolerable at high frequency, but less so at low frequency. In most cases, the predictions of the coherent model give the best results, with an accuracy to within 3 dB. On the other hand, the energy-based models are not able to predict the peaks and dips across the frequency spectra, the variance with the measurement results being up to 7 dB at low-frequency bands.
117(2005); http://dx.doi.org/10.1121/1.1850211View Description Hide Description
A “boss” formulation by Twersky [J. Acoust. Soc. Am. 73, 85–94 (1983)] enables prediction of the plane wave reflection coefficient from a surface composed of rigid-porous roughness elements embedded in an acoustically hard plane where the roughness elements and their mean spacing are small compared with the incident wavelengths. Predictions for air-filled porous roughness elements on a hard ground plane are compared with effective impedance spectra obtained from laboratory measurements over random distributions of polystyrene hemi-spheres, polyurethane pyramids, and sand hemispheroids on glass plates. Overall the predictions agree well with these data. To enable prediction of the effective admittance of rough porous surfaces, Twersky’s original formulation is extended heuristically. The resulting theory is compared with a previous model [J. Acoust. Soc. Am. 108, 949–956 (2000)], which is a heuristic extension of Tolstoy’s theory [J. Acoust. Soc. Am. 72, 960–972 (1982)] to include nonspecular scattering. The theories are found to give different predictions for relatively large bosses. The modified Twersky theory gives relatively good predictions of the effective impedance spectra obtained from complex sound pressure level measurements over sand surfaces containing semielliptical roughness elements and over uncultivated soil.