The noise generated by vacuum cleaners is one of the dominant noise sources among household appliances with its suction unit as the key element of noise generation. The dominant noise source in the ...suction unit is its impeller, and that is why there were numerous efforts made to reduce its noise. The years of its optimization have consequently led to a stage, where the possibilities for further noise level reduction are limited. Because human perception of noise is not only influenced by sound pressure levels, it is possible to alter the sound of the impeller to be less annoying to the user. A new centrifugal impeller design was optimized for sound quality, resulting in triangular-shaped flow channels. The optimization process of impeller geometry was based on the sound quality calculated from measured psychoacoustic metrics using two noise annoyance models, Guoqing’s and Lipar’s. The validation of the result was performed by listening tests. Forty individuals participated in the listening tests of the suction units tested and rated the sounds using two sets of semantic differentials. The first set was used to evaluate the sound quality of the impeller, while the second set was used for the validation of these listening tests by evaluating psychoacoustic metrics. Validation results confirm that Lipar’s model of noise annoyance is suitable for evaluating centrifugal blower impeller noise. The subjective evaluation of psychoacoustic metrics, via listening tests of impeller noise, suggests that fluctuation strength and tonality are important features for the sound quality of impeller noise.
Eutypella parasitica R.W. Davidson & R.C. Lorenz is the causative agent of Eutypella canker of maple, a destructive disease of maples in Europe and North America. The fungus E. parasitica is known to ...cause wood stain and decay. However, it is not known how effectively it decomposes the wood of the most widespread maple species in Europe. Wood samples of Acer pseudoplatanus L., A. platanoides L., and A. campestre L. were exposed to four isolates of E. parasitica and nine other fungal species for comparison, according to the modified EN 113 standard. After 15 weeks of incubation, mass loss and microscopical analysis of samples showed evidence of colonization and different wood decay potentials among fungal species. A highly significant positive correlation was found between mass loss and moisture content for all fungal species. Similarly, the measured cell wall thickness correlated well with the calculated mass loss of the samples. On average, the fungal species caused the lowest mass loss in A. pseudoplatanus (10.0%) and the highest in A. campestre (12.6%) samples. Among the samples exposed to E. parasitica isolates, the highest mass loss was recorded in A. pseudoplatanus (6.6%). Statistical analysis showed significant differences in mass loss and moisture content between different E. parasitica isolates. Based on the results of staining, we discuss the type of decay caused by E. parasitica. Although E. parasitica isolates caused smaller mass loss of samples compared to other more effective decay species, we should not disregard its capability of degrading maple wood. Because E. parasitica usually infects the lower portion of the trunk, which is the largest and most valuable part of the tree, any damage can cause significant economic and resource loss.
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