Imran Bashir

Outdoor ground impedance models

Many models for the acoustical properties of rigid-porous media require knowledge of parameter values that are not available for outdoor ground surfaces. The relationship used between tortuosity and porosity for stacked spheres results in five characteristic impedance models that require not more than two adjustable parameters. These models and hard-backed-layer versions are considered further through numerical fitting of 42 short range level difference spectra measured over various ground surfaces. For all but eight sites, slit-pore, phenomenological and variable porosity models yield lower fitting errors than those given by the widely used one-parameter semi-empirical model. Data for 12 of 26 grassland sites and for three beech wood sites are fitted better by hard-backed-layer models. Parameter values obtained by fitting slit-pore and phenomenological models to data for relatively low flow resistivity grounds, such as forest floors, porous asphalt, and gravel, are consistent with values that have been obtained non-acoustically. Three impedance models yield reasonable fits to a narrow band excess attenuation spectrum measured at short range over railway ballast but, if extended reaction is taken into account, the hard-backed-layer version of the slit-pore model gives the most reasonable parameter values.

Diffraction assisted rough ground effect: models and data

The destructive interferences observed in Excess Attenuation (EA) spectra over periodically and randomly spaced roughness elements with different cross-sectional profiles (semicylindrical, rectangular and wedge-shaped strips) have been investigated. If the roughness is spaced periodically, then two or three destructive interference maxima are observed in the same frequency range as the one or two observed with randomly distributed roughness. Roughness-induced surface waves are investigated also. Their amplitudes and the frequencies at which they occur are found to depend on the roughness height, mean center-to-center spacing and the extent to which the roughness is periodic. A semianalytical Multiple Scattering Theory and a numerical method (the Boundary Element Method) have been used to make predictions of the EA spectra which are compared with measurements. In addition it is found that the effective surface impedance spectra deduced from complex EA measurements over rough surfaces exhibit resonances similar to those observed for a hard-backed porous layer. On this basis a heuristic effective impedance model for rough hard surfaces is developed and the corresponding predictions of EA spectra are compared with data.

Aperiodicity effects on sound transmission through arrays of identical cylinders perpendicular to the ground

Results of laboratory measurements of sound transmission through 5 × 10 arrays of meter long polyvinyl chloride pipes with lattice constants of 5 and 10 cm with filling fractions of 13% and 50% located either on medium density fibreboard or a layer of felt are reported. Ground effects and sonic crystal effects are found to be additive. Measurements and predictions show that, while there is little broadband advantage in a periodic configuration compared with a random one, a quasi-periodic arrangement in which the perturbation has a standard deviation equal to the scatterer diameter gives the best overall attenuation.

Sound propagation over soft ground without and with crops and potential for surface transport noise attenuation

Growing demand on transportation, road, and railway networks has resulted in increased levels of annoyance from road traffic. Optimized use of green surfaces in combination with vegetation may be desirable as a method for reducing the noise impact of road traffic in urban and rural environments. Sound propagation over soft ground and through crops has been studied through outdoor measurements at short and medium ranges and through predictions. At lower frequencies, ground effect is dominant, and there is little or no attenuation due to crops. At higher frequencies above 3-4 kHz, the attenuation in crops is dominant. It was also found that the ground effects and the influence of crops can be treated independently and can be added to obtain the total effect. Sound attenuation by crops is the result of multiple scattering between the stems and leaves, loss of coherence, and viscous and thermal losses due to foliage. The major contribution is associated with viscous and thermal losses. A model for sound attenuation by vegetation is proposed. Insertion losses for a typical road traffic noise source have been calculated that result either by replacing hard ground with different types of acoustically soft ground or by growing crops along the road sides.

Exploiting ground effects for surface transport noise abatement

Abstract Growing demand on transportation, road and railway networks has increased the risk of annoyance from these sources and the need to optimise noise mitigation. The potential traffic noise reduction arising from use of acoustically-soft surfaces and artificial roughness (0.3 m high or less) is explored through laboratory experiments, outdoor measurements at short and medium ranges and predictions. Although the applicability of ground treatments depends on the space usable for the noise abatement and the receiver position, replacing acousticallyhard ground by acoustically-soft ground without or with crops and introducing artificial roughness configurations could achieve noise reduction along surface transport corridors without breaking line of sight between source and receiver, thereby proving useful alternatives to noise barriers. A particularly successful roughness design has the form of a square lattice which is found to offer a similar insertion loss to regularly-spaced parallel wall arrays of the same height but twice the width. The lattice design has less dependence on azimuthal source-receiver angle than parallel wall configurations.

Surface waves over periodically-spaced rectangular strips

Frequency- and time-domain measurements have been made on surfaces composed from parallel periodically-spaced rectangular strips (width: 0.0126 m, height: 0.0253 m) on an acoustically hard surface. The edge-to-edge spacing between the strips has been varied between 0.003 and 0.06 m. Frequency domain predictions show that when the spacing is small, these surfaces may be regarded as locally reacting rigid-framed hard-backed slit-pore layers with an effective depth slightly larger than the strip height, but when the spacing is comparable to the strip height or greater, the surfaces behave as periodically rough surfaces. Both frequency- and time-domain results show that surface waves of comparable magnitudes are created over the range of strip spacings studied but the frequency content of the acoustically induced surface waves decreases as the mean spacing is increased. It is found that surface wave dispersion is better predicted by the deduced effective impedance spectrum than by the slit-pore layer impedance model.

Surface waves over rigid-porous and rough surfaces

Surface waves are created by near grazing sound propagation from a point source over either a rigid-porous layer or a (slightly) rough surface. In theory, they have similar origins. Frequency- and time-domain measurements have been made on surfaces composed from parallel periodically spaced rectangular strips (width 0.0126 m, height 0.0253 m) on an acoustically hard surface. The edge-to-edge spacing between the strips has been varied between 0.003 and 0.06 m. Frequency domain predictions show that when the spacing is substantially smaller than the strip height these surfaces may be regarded as locally reacting rigid-framed hard-backed porous layers with an effective depth slightly larger than the strip height. When the spacing is comparable to the strip height or greater the surfaces behave as periodically rough surfaces. Both frequency- and time-domain results show that surface waves of comparable magnitudes are created over the range of strip spacings studied but the main frequency content of these acou...

Strips, slits, surface waves and slit-based MPP

Maa Dah-You was a pioneer of research into Microperforated Plates (MPP). A thin rigid-porous layer containing infinitely-long slits of small (1 mm or less) width may be regarded as a special case of MPP. Surface waves are created by near grazing sound propagation from a point source over either a thin rigid-porous layer or a (slightly) rough surface. Measurements have been made on surfaces composed from parallel periodically-spaced 1 m long rectangular strips (width 0.0126 m, height 0.0253 m) on an acoustically-hard surface. The edge-to-edge spacing has been varied between 0.003 m and 0.06 m. When the spacing is substantially smaller than the strip height these surfaces behave as locally-reacting rigid-framed hard-backed slit-pore layers with an effective depth slightly larger than the strip height. When the spacing is equal to or greater than the strip height the surfaces behave as periodically-rough surfaces. Surface wave magnitude is relatively unaffected by the strip spacing but the main frequency con...

Aperiodicity and ground effects on the sonic crystal noise barriers

Sonic crystal structures consisting of periodically-arranged solid vertical cylinders can act as sound barriers at certain frequencies. Their performance depends on the filling fraction which is determined by the spacing and cylinder radius. To be effective the filling fraction must be high. This means that periodic arrays with relatively low filling fractions such as in trees belts are not effective as traffic noise barriers. The effects of partially perturbing the positions of sonic crystal elements have been investigated by modelling and laboratory measurements and have been shown to improve the insertion loss of the periodic structure. It is argued that partial perturbation of regular tree planting near highways will improve their noise attenuation. Furthermore, much previous research assumes the sonic crystal structure to be in the free field, i.e. no account has been taken of the presence of the ground surface. With a conventional, wall type, barrier the ground effect is reduced by presence of the b...

Excess attenuation and effective impedance associated with rough hard ground

Although ground effect has been studied widely and is an accepted component of outdoor sound prediction schemes exploitation of ground effect has been restricted to the development of porous road surfaces which influence traffic noise generation as well as propagation. Relatively little attention has been paid to the potential for exploiting the effectively finite impedance associated with roughness small compared with the incident wavelengths on an otherwise acoustically-hard ground or to the potential usefulness of the wide variation in soft ground effects that are available. Results of recent laboratory and field measurements on artificially rough hard surfaces including the comparative acoustical performances of randomly- and periodically-spaced roughness are presented. The laboratory data have been used to study the influences of roughness shape and spacing and surface wave formation. Practical outdoor realisations of artificially roughened hard surfaces have utilised low brick wall configurations an...