Gilles A. Daigle

Effective flow resistivity of ground surfaces determined by acoustical measurements

Following earlier work by Chessell [J. Acoust. Soc. Am. 62, 825–834 (1977)] it is shown that his single‐parameter theory can be used to predict the measured transmission spectra between a source and receiver located above ground surfaces having a wide range of acoustic impedance—or effective flow resistivity. Surfaces behaving essentially as locally reacting range from new‐fallen snow, effective flow resistivity σ=10–30 cgs rayls, through grass‐covered ground, σ=150–300 rayls, to mature asphalt, σ=30 000 rayls. The thermal‐conduction and viscous boundary layer of the surface limits the effective flow resistivity of even the hardest and most impervious surface to the range 105–106 rayls, depending on frequency: this value is appropriate to evaluate the complex reflection coefficient of the paint‐sealed surface of a thick slab of reinforced concrete.

Air‐based system for the measurement of porosity

An experimental system for the measurement of porosity, the volume fraction of air contained in a material, is described. Porosity is important as one of several parameters required by acoustical theory to characterize a porous material. As with the technique described by Beranek [J. Acoust. Soc. Am. 13, 248–260 (1942)], the isothermal pressure change in a closed volume containing a sample material is measured for a known change in the volume. The volume of air contained in the sample, and hence the porosity, is inferred from these two quantities. The new system, though, avoids the use of liquids, either directly in the technique or for temperature stabilization. Instead, a piston of accurately known diameter is used to produce the change in volume, and the change in pressure is measured with an electronic pressure transducer. Model calculations and measurements on real materials confirm that porosity can be measured rapidly and conveniently with this apparatus, with an accuracy of better than 1% over a b...

Porous road pavements: Acoustical characterization and propagation effects

Measurements of the acoustical properties of some porous road pavements are presented here and an acoustical method for monitoring the performance of these surfaces is presented. Porous road pavements have been used previously because of their driving qualities and drainage capacities during rainy days (i.e., the elimination of water splash and spray) but they have also been found to reduce traffic noise substantially. Reductions in A-weighted sound levels of 3–5 dB, compared to a dense pavement structure, have been measured. To study further their acoustical performance, measurements over real road surfaces have been carried out and the results compared to theoretical predictions based upon models describing the surface impedance and sound propagation. For the impedance characterization, both a phenomenological and a microstructural model were used. Both approaches introduce a viscous and a thermal dependence to account for the different phenomena inside the porous structure. By incorporating these model...

Line‐of‐sight propagation through atmospheric turbulence near the ground

Line‐of‐sight measurements of the log‐amplitude and phase fluctuations of pure tones between 250 and 4000 Hz propagated over distances between 2 and 300 m in the turbulent atmosphere close to the ground are compared quantitatively with simple theory using simultaneously measured meteorological variables. The theory is based on the assumption of homogeneous and isotropic turbulence and approximates the availability of eddy sizes in the source region of turbulence by a Gaussian spectrum. In particular the transverse or mutual coherence function (the coherence in a plane perpendicular to the direction of propagation) and the coherence in the direction of propagation which we call the longitudinal coherence, are also calculated and compared with the measurements. When the measured mean square phase fluctuations are compared with the theory using the meteorological measurements, good agreement is obtained. However the measured mean square log‐amplitude fluctuations are in general substantially smaller than pre...

Electronic system for the measurement of flow resistance

A measurement system has been developed that can determine flow resistances quickly and accurately and can be used to complement acoustical measurements on the same samples. The use of electronic components permits measurements to be made 20–50 times faster than with the commonly used Leonard apparatus. Variable‐capacitance pressure transducers are used to measure pressure differences across both the test sample and a laminar flow element with known flow resistance (these two in series): For steady nonpulsating flow, the ratio of flow resistances equals the ratio of measured pressure differences. The rate of air flow through the sample is adjusted using a mass flow controller; flows of 10−3–1 cm3 s−1 can be controlled to better than 1%. Flow resistances between 1 and 105 g cm−4 s−1 can be determined with an accuracy of better than 1.6%. The corresponding range of flow resistivities of porous materials that can be determined with the present sample holder is 1–106 cgs‐rayl cm−1. Initial measurements have b...

Effects of atmospheric turbulence on the interference of sound waves near a hard boundary

The mean sound levels resulting from the interference between direct waves and those reflected from the ground are strongly influenced, especially at frequencies near interference minima, by fluctuations in phase and amplitude of the sound waves induced by propagation through atmospheric turbulence. Since it was found experimentally that the correlation length (∠1.1 m) of the meteorological fluctuations is comparable to the separation between the interfering sound paths, previous theoretical work by Ingard and Maling [J. Acoust. Soc. Am. 35, 1056–1058 (1963)] has been extended to allow for partial covariance between the two waves. The theory has been further extended to use the calculations of fluctuations in phase and amplitude of spherical waves, and to include the explicit calculation of the fluctuating acoustical index of refraction from the fluctuating values of temperature and wind velocity. Measurements (1–6 kHz) have been made of the interference spectrum at 15, 30, and 45 m from a point source 1....

Propagation of sound in the presence of gradients and turbulence near the ground

Sound‐pressure levels of pure tones between 1 and 16 kHz, generated by a point source on hard ground, were measured as a function of height at horizontal distances up to 21 m. In another experiment, measurements were made above hard ground and ground of finite impedance at horizontal distances up to 250 m using frequencies between 250 and 4000 Hz. Five frequencies were generated simultaneously and measured at four (and sometimes five) heights. The sound‐pressure levels below the refractive shadow boundary are compared with a simple theory that assumes a linear change of sound speed with height. This vertical gradient was obtained by approximating the temperature and wind velocity profiles measured during the acoustical experiment. Good agreement between theory and experiment was obtained at all distances and frequencies. However, at the largest distances and highest frequencies, there was evidence of additional energy penetrating the refractive shadow from scattering by atmospheric turbulence. Therefore, ...

Heuristic model for outdoor sound propagation based on an extension of the geometrical ray theory in the case of a linear sound speed profile

Abstract In the case of outdoor sound propagation at relatively short ranges, it is reasonable to assume that sound follows straight ray paths. At longer ranges, refraction due to temperature and wind gradients results in ray paths that are curved, and calculations using straight rays are no longer valid and lead to erroneous results. In addition, atmospheric turbulence plays an ever-increasing role by degrading the coherence of the sound field. In this paper, the theory valid for propagation from a point source above an impedance plane is extended to include the effects of curved rays and the loss of coherence due to atmospheric turbulence. The model assumes a sound speed gradient that varies linearly with height above the ground. This assumption allows an analytical determination of the travel times of the curved rays and the modified angle of incidence of the ground-reflected paths. The additional reflected rays that may appear in the presence of a positive gradient are included in the calculation. The assumption also permits the determination of the position of the shadow zone in the presence of a negative gradient. The total sound pressure level is computed by summing the contribution from all the rays. In this summation, a fluctuating index of refraction is used to take into account the partial coherence between the rays caused by the effects of atmospheric turbulence. If the receiver is in the shadow zone, a diffraction theory based on a residue series solution is used to compute the sound levels. The results of calculations using the model are compared with experimental results.

Comparison of an analytic horn equation approach and a boundary element method for the calculation of sound fields in the human ear canal

The sound field inside a model human ear canal has been computed, to show both longitudinal variations along the canal length and transverse variations through cross-sectional slices. Two methods of computation were used. A modified horn equation approach parametrizes the sound field with a single coordinate, the position along a curved center axis-this approach can accommodate the curvature and varying cross-sectional area of the ear canal but cannot compute transverse variations of the sound field. A boundary element method (BEM) was also implemented to compute the full three-dimensional sound field. Over 2000 triangular mesh elements were used to represent the ear canal geometry. For a plane piston source at the entrance plane, the pressure along the curved center axis predicted by the two methods is in good agreement, for frequencies up to 15 kHz, for four different ear canals. The BEM approach, though, reveals spatial variations of sound pressure within each canal cross section. These variations are small below 4 kHz, but increase with frequency, reaching 1.5 dB at 8 kHz and 4.5 dB at 15 kHz. For source configurations that are more realistic than a simple piston, large transverse variations in sound pressure are anticipated in the vicinity of the source.

Insertion loss of absorbent barriers on ground

This paper presents a method for calculating the insertion loss (IL) of a thin barrier covered with absorbent material, either on the source side, on the receiver side, or on both sides. The method used combines a classical theory for the propagation of sound over ground with an approximate solution for diffraction around an absorbent barrier, which can take into account the specific impedance of each side of the barrier. The validity of the method was confirmed by comparing theoretical results with experimental measurements for various geometrical configurations and screen boundary conditions. The results show that, when the angles of diffraction are significant, the insertion loss (IL) of a hard barrier can be substantially increased by covering one of its surfaces with an absorbent material. This absorbent layer must be placed on the surface of the barrier associated with the greatest angle of the diffracted rays paths (source top‐edge or receiver top‐edge). When these angles are about the same on each...