Patrice Boulanger

Ground effect over hard rough surfaces

Laboratory and outdoor measurements are reported of the relative sound pressure level spectrum over hard surfaces containing either random or periodically spaced arrays of 2-D roughnesses. The resulting data have been compared with predictions obtained analytically and with numerical predictions of a boundary element code. Effective impedances of the rough surfaces have been calculated from the boss theory developed by Twersky. A classical asymptotic approximation for propagation near grazing incidence from a point source over an impedance boundary has been modified, heuristically, to allow for diffraction grating effects. The resulting predictions are found to be in tolerable agreement with the data except for close and random packing. The boundary element code is found to give superior results for larger roughnesses, but computational restrictions on element size reduce its usefulness for roughnesses with small width or height.

Models and measurements of sound propagation from a point source over mixed impedance ground

Measurements of the excess attenuation of sound from a point source over mixed impedance ground in an anechoic chamber are compared with predictions obtained from models based on (a) the semiempirical theory due to De Jong, (b) Nyberg’s theory, (c) a Fresnel-zone approximation, and (d) a boundary element code. The impedance discontinuities studied in this work are perpendicular to the source–receiver line. When there is a single discontinuity between acoustically hard and finite impedance surfaces, the De Jong semiempirical model, the Fresnel-zone model and boundary element code are found to give satisfactory agreement with measured data. The frequency of the first maximum in attenuation is found to be highest at approximately 70% hard surface cover rather than at the 100% expected. It is argued that this is a result of edge diffraction. When extended to multiple impedance discontinuities, the De Jong semiempirical model performs poorly. However, both Nyberg’s theory and the boundary element code give goo...

Effective impedance of surfaces with porous roughness: Models and data

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. T...

Reflection of sound from random distributions of semi-cylinders on a hard plane : models and data

A new analytical theory for multiple scattering of cylindrical acoustic wave s by a n array of finite impedance semi -cylinders embedded in a smoot h acoustically hard surface is derived by extending previous results for plane waves [Linton and Martin , J. Acoust. Soc. Am. 117 (6) 3413 ‐ 3423 (2005 )]. Although the computational demands of the new theory increase as the number of the semi -cylinders in t he arrays and/or the frequency increases , t he theory offer s an improvement on analytical boss theories since the latter (i) are restricted to non -deterministic (infinite) random distributions of semi -cylinders with spacing /radii small compared t o the incid ent wavelength and (ii) are derived only for plane waves . The influence on prediction accuracy of truncation of the infinite system of equations introduced by the new theory is explored empirically . Laboratory measurements have been made over deterministic random arrays of identical varnished wooden semi -cylinders on a glass plate. The agreement between predictions and measured relative Sound Pressure L evel spectra is very good both for single deterministic random distributions and for averages representing non -deterministic random distributions. The analytical theory is found to give identical results to a Boundary Element calculation but is much faster to compute .

Effective impedance spectra for predicting rough sea effects on atmospheric impulsive sounds

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 surface scatterers 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.

Predicting acoustic and seismic pulses from outdoor explosions

To support the formulation of a simple model that can be used for assessment and prediction of ground vibration levels due to above‐ground explosions, a fast field program for layered air ground systems (FFLAGS), developed originally for continuous sound sources, has been extended to enable predictions of propagation from impulsive sound sources in a refracting atmosphere above layered porous and elastic ground. Using a mixture of measured and best‐fit parameters, the pulse version of the code (PFFLAGS) has been found to give predictions of the air pressure spectrum above ground and the vertical component of solid particle velocity near the ground surface that compare tolerably well with published data for the spectra and waveforms of acoustic and seismic pulses at short range (60 m) in grass‐ and snow‐covered ground and at longer range (3 km) in forested terrain. The predicted seismic pulses in the presence of the snow cover explain a phenomenon frequently observed for shallow snow covers whereby a high‐...

Measurements of acoustic and seismic pulses from outdoor explosions

Measurements of the ground vibrations produced by airborne detonations of C4 were conducted at locations with a variety of ground types, including concrete, soil, forest, tropical vegetation, and snow cover. The measurements show that, although two separate seismic (ground vibrational) arrivals can be detected in all cases, the early seismic arrival from an underground path is always much smaller than the vibration induced by the air blast arrival. The acoustic‐to‐seismic coupling ratio for the atmospheric wave is a constant with respect to distance and peak pressure at a given location, but varies from site to site, usually between 1 and 14 μm/s/Pa. A conservative empirical equation to predict ground vibration from explosions is derived. This equation predicts that the commonly used vibrational damage criteria of 12 and 25 mm/s will be exceeded when the peak positive pressure exceeds 480 Pa (147.6 dB) or 1 kPa (154.0 dB), respectively. Either of these levels is much higher than the Army overpressure damage criterion of 159 Pa (138 dB). Thus in most situations damage from blast overpressure will occur long before damaging levels of ground vibration are reached. [This research supported by the U.S. Army Corps of Engineers and SERDP Seed Project SI‐1410.]

Effective impedance of hard rough surfaces

The effective impedance of hard rough surfaces insonified from a point source has been investigated through boss models, boundary element simulations, and measurements. The complex excess attenuation measured or predicted has been fitted for effective impedance by means of a formulation for the sound field above a smooth finite impedance plane. It has been found that the effective impedance plane is higher than the nominal one and that the results are very sensitive to the location of the specular reflection point. Polynomial expressions for the real and imaginary parts of the effective impedance have been derived for various types of roughnesses.