A theoretical and experimental study of a proposed hybrid passive–active plane-wave system to provide broadband acoustic absorption is presented. The passive absorber consists of a microperforated panel (MPP), used in place of conventional fibrous materials, in front of an air layer. The active system uses an active transducer (a loudspeaker), an error sensor (a microphone), and an adaptive controller. MPPs are thinner than fibrous materials and provide a better solution in hygienic environments. For two such systems, the dependence of performance on MPP parameters is studied for two control strategies: impedance matching and pressure release. The first condition is found to be better for cases where the acoustic impedance of the system approaches that of air. Otherwise, the pressure-release condition is better, and a wideband solution is the implementation of the active control system up to a frequency where the natural band of the passive system provides higher absorption. Therefore, the use of a low-pass filter in the error signal is implemented to afford hybrid passive–active wideband absorption. The control effort of active system is also considered. Experimental verification shows good agreement with theory, and an average absorption coefficient of 0.82 in the frequency bandwidth 100 to 1600 Hz was achieved.

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In order to contribute to the solution of the environmental problem created by the tire wasteways, the possibilities of using rubber crumbs of waste tires as acoustic absorbent materials have been studied in a systematic way. Experimental absorption coefficients of rubber crumb granulates with mesh diameters between 1.4 and 7 mm have been tested in Kundt’s tube and in a reverberation room. Absorption coefficient values so obtained are compared with computational values afforded through mathematical prediction models that use the intrinsical parameters of porous material such as porosity, tortuosity, and flow resistivity, which were measured on layers of the respective materials. The good broadband acoustic absorption of the material encourages its use as an alternative to the current absorbent acoustic screens, used for the protection against traffic noise.

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A single number rating index to qualify the acoustical protection of a noise barrier that accounts for the influence of the ground is developed. This is an extension of a previous index for semi-infinite barrier developed by Pfretzschner ͓Pfretzschner et al., Acust. Acta Acust. 82, 504-508 ͑1996͔͒. This index is based on the asymptotic value found for the insertion loss or the acoustic attenuation as physical magnitude, and it is uniquely dependent on the barrier, noise source, and acoustic characteristics of the ground in the source semi-space. The new index agrees well with measured data on a scale model under laboratory conditions, and a direct experimental measuring procedure could be envisaged. In this way the proposed index, which accounts for extrinsic properties ͑acoustical protection͒ of barriers, would complete the two previous indices dealing with intrinsic properties ͑transmission loss and acoustical absorption͒.

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