Acoustic distribution study inside centrifugal pump impeller under different blade outlet angles using the Powell vortex sound theory

The acoustic distribution characteristics inside centrifugal pump impeller under different blade outlet angles are explored based on the Powell vortex sound theory. It is shown that the increase in the blade outlet angle narrows the fluctuation ranges of the acoustic source and acoustic pressure perturbation at the impeller outlet far away from the tongue, while extends those near the tongue. The total acoustic source intensity and total acoustic pressure level at the impeller outlet far away from the tongue decrease by 1.23% and 1.17%, respectively, while those near the tongue increase by 0.64% and 0.94% when the angle increases from 15° to 35°, respectively. Additionally, under different blade outlet angles, the total acoustic source intensity and total acoustic pressure level on the streamline far away from the tongue increase rapidly first, then increase slowly from inlet to outlet of the impeller. On the streamline near the tongue, a mutation of total acoustic source intensity and total acoustic pressure level appears near the impeller outlet, and the mutation range moves towards the middle section of the impeller with increasing blade outlet angle and rotational speed. In addition, the A-weighted average acoustic source intensity level and A-weighted average acoustic pressure level increase by 0.4% and 1.22% when the angle increases from 15° to 35°, respectively. The study reveals the rotor–stator interaction intensity evolution characteristics, and provides a reference for the optimization design of centrifugal pumps.

Signal transfer through the hearing protectors with the regulation of damping in the presence of the impulse noise

Hearing protectors with the regulation of damping are used ever so more frequently to protect the hearing of persons endangered with the presence of impulse noise. A clear advantage coming from the use of this type of hearing protectors, compared to other types, lies in the improvement of the perception of useful signals. However, in spite of such advantages it is during their use one can observe the effect of “blocking” of the sound transfer underneath the protectors (meaning the limitation of the signal transfer through the protectors), occurring directly after the appearance of the acoustic impulse. The article presents an analysis of the acoustic signal transfer underneath the hearing protectors with the regulation of damping in the situation of the acoustic impulse appearance. The analysis aimed at the definition of the time of the “blocking” of noise transfer underneath the protectors effect as well as the decrease of the acoustic pressure level resulting from the mentioned effect. Towards that goal a special work stand was prepared, consisting of impulse signal source, a set noise and an acoustic tester (artificial head). The research had shown the time of the “blocking” effect during the use of the protectors with the regulation of damping to be reaching 1.2 seconds and the decrease in the value of the mean acoustic pressure level, resulting from such effect, to be exceeding even 16 dB.

Vibration and Noise Transmitted by Agricultural Backpack Powered Machines Critically Examined Using the Current Standards

European Directives 2002/44/EC and 2003/10/EC establish the exposure limit values for preventing operators’ risks to vibration and noise transmitted by machines. Few studies studied noise and vibration of agricultural backpack powered machines (as mist blowers and blowers), but nobody critically studied them. This work analyzed the field back vibration, hand-arm vibration (HAV), and noise transmitted to ten operators by eight blowers and mist blowers. Unweighted and weighted vibration were analyzed, using the standards ISO 2631-1 (back), and ISO 5349-1 and ISO/TR 18570 (hand-arm system). The noise was evaluated by recording the acoustic pressure level at the operators’ ears using the ISO 9612. With the ISO 2631-1, the vibration to the operators’ back was low (0.38 ms−2), but the unweighted vibration measured along y and z-axes (not used by the ISO 2631-1) were high (>11 ms−2). HAV were also low when using the ISO 5349-1 (the highest value was 2.51 ms−2 in mist blowers), but high with the ISO/TR 18570 for the onset of vibration white finger (1446 ms−1.5 in blowers). Noise levels were always high: more than 100 dB(A), excluding the blower with the exhaust inside the blower hose. This last machine had noise levels lower than 86 dB(A), but its specific feature could increase environmental pollution.