title: Outdoor ground impedance models.
abstract: 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.
authors.affiliation: Department of Design, Development, Environment and Materials, The Open University, Milton Keynes MK7 6AA, United Kingdom. k.attenborough@open.ac.uk
authors.forename: Keith
authors.initials: K
authors.lastname: Attenborough
authors.forename: Imran
authors.initials: I
authors.lastname: Bashir
authors.forename: Shahram
authors.initials: S
authors.lastname: Taherzadeh
mesh_terms: D000162:Acoustics; D003252:Construction Materials; D004777:Environment; D006838:Hydrocarbons; D008433:Mathematics; D008962:Models, Theoretical; D009622:Noise; D009623:Noise, Occupational / Q000706:statistics & numerical data; D009624:Noise, Transportation / Q000706:statistics & numerical data; D006109:Poaceae; D016062:Porosity; D012987:Soil; D014197:Trees
chemical_list: D006838:Hydrocarbons; D012987:Soil; C006647:asphalt