Confronted with noise disturbance during transmission of vehicle exhaust air, several mufflers have been designed to control the noise and improve the transmission characteristics. In order to optimize the noise control and attenuation quality, the internal geometry, inlet and outlet pipe diameters of a designed muffler were varied and the performance evaluated using finite element method (FEM). Furthermore, 2mm diameter equal circular perforations were introduced in the resonator chamber to verify the effect on the transmission loss characteristics. The results show that the performance of the muffler with inlet pipe diameter variation is significantly better (68dB) than the standard muffler (55dB) in controlling and reducing acoustic wave propagation within the same frequency range of (200~580Hz). The performance improved further to 70 dB by the introduction of circular perforations in resonator chamber. However, with the variation in the outlet diameter increase the performance of the muffler was 55dB but within a higher frequency range of 220~680Hz which is not reliable for acoustic wave propagation control. The average transmission loss performance of the designed mufflers were 48.62, 47.77, 39.01 and 37.77dB for the resonator chamber perforation, inlet pipe diameter increase, outlet pipe diameter increase and standard muffler respectively. Therefore, the designed muffler with resonator chamber perforations is the best for optimal acoustic wave control.

The emergence of acoustic metamaterials generated a lot of attention in the study of low-frequency vibration, noise control and reduction in engineering applications. As a result, the elastic wave bandgap characteristics of a two-dimensional microcavity local resonator structure for two soft rubber materials was investigated using finite element methods (FEM). The transmission spectrum of the displacement eigenmodes of the bandgap edges relating to the lowest bandgap was calculated. The results showed that the phononic crystal structure without a microcavity local resonator plate has bandgap characteristics of elastic wave propagation in the high-frequency range between 2200~2400Hz. However, with the introduction of microcavity resonator plates in the phononic crystal structure low-frequency bandgaps are obtained in the region of 0~198Hz and 0~200Hz respectively. The low-frequency bandgaps appeared as a result of the microcavity local resonator plate which increased the path length through which the wave is transmitted. The phononic crystal microcavity local resonator plate structure has varying transmission loss characteristics of-65dB,-85dB,-100dB and-150dB in the low-frequency range depending on the number of local resonator plates introduced into the cell structure and density of the cell structure. The study provided a good demonstration of wave propagation in artificially engineered materials with critical emphasis on the effects of local resonators in a microcavity structure.

To improve the quality of threshed cowpea (Vigna unguiculata L.), produced by rural farmers in Ghana, and determine the best machine-crop parameters combination for optimum performance, a cowpea thresher was developed and evaluated at different moisture contents. The cowpea after harvesting was divided into five units and dried to 20.7, 18.4, 15.5, 13.5 and 12.6 % seed moisture contents. The effect of seed moisture contents on threshing quality was assessed at 735 rpm rotor speed using the white with black eye cowpea. The performance test showed that 12.6 % had the highest throughput of 77.56 kg/h with the lowest of 66.28 kg/h from the 20.7%. The seed damage ranged from 1.64 to 9.46 % and threshing efficiency from 93.11 to 99.40 %. Analyses done at 5% level of significance indicated no significant difference in the throughput and threshing efficiencies. However, seed damage had significant difference. Subsequently, analysis performed on the data for seed loss of 12.6 and 13.5 % which were the best performing moisture contents showed no significant differences. Therefore, seed moisture contents less than 13.5 % and rotor speeds less than 735 rpm are the crop- machine parameters combination for optimum thresher performance. Key words: Design; Throughput; Impact Force; Seed Loss; Simulation

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