Noise and Vibrations

This section gives an overview of the causes of helicopter noise and vibration, as well as of their effects on the aircraft and its environment. 

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The main advantage of the VFD is that once the operating poiat of the fan is selected, that efficiency is carried at all speeds and flows. Capacity is directly related to motor frequency and thus to speed. Another important benefit is that as the fan speed decreases, fan noise and vibration also decrease significantly. 

L. L. Beranek, Noise and Vibration Control, Institute of Noise Control Engineering, Cambridge, Mass., 1988. 

Catalog of STC andllC Ratings for Wall andEloorj Ceiling Assemblies, Office of Noise Control, California Department of Health Services, Berkeley, Calif., 1984. R. S. Jones, Noise and Vibration Control in Buildings, McGraw-HiU Book Co., Inc., New York, 1984. 

Collapse of vapor bubbles once they reach zones where the pressure exceeds the vapor pressure can cause objectionable noise and vibration and extensive erosion or pitting of the boundaiy materials. The critical cavitation number at inception of cavitation, denoted <7, is useful in correlating equipment performance data ... 

Cavitation has three negative side effects in valves—noise and vibration, material removal, and reduced flow. The bubble-collapse process is a violent asymmetrical implosion that forms a high-speed microjet and induces pressure waves in the fluid. This hydrodynamic noise and the mechanical vibration that it can produce are far stronger than other noise-generation sources in liquid flows. If implosions occur adjacent to a solid component, minute pieces of material can be removed, which, over time, will leave a rough, cinderlike surface. 

The effects of cavitation are noise and vibration. If the pump operates under cavitating conditions for enough time, the following can occur ... 

The main flare header should not be designed for critical flow at the entrance to the flare stack, or else noise and vibration will result. 

Landsrrom, U., Kjellherg, A., and Soderberg, L. (1991). Spectral character, exposure lei fcls and adverse eft ects of ventilation noise in offices. /. Low frequency Noise and Vibration 10(,i i. 

Landstrom, U., Soderberg, L., Nordstrom, B., and Kjellberg, A. (1994). Measure., against ventilation noise—Which tone frequencies are least and most annoying. /. Low Irequenc, - Noise and Vibration 13(3). 

Landstrom, Eh, Holmberg, K., Kjellberg, A.,. Soderberg, L., and Tesarz, M. (1996). Exposure time and irs influence on noise annoyance at work. /. Low Frequency Noise and Vibration 14(41. 

Normally there is some connection between the airflow rate and noise and vibration generation. This could modify the building construction either to prevent spreading or to diminish the levels of noise and vibrations from the air-handling units. This naturally includes all parts of these units, i.e., fans, pumps, and valves (see Chapters 5 and 9). The demands on noise insulation also include the noise and vibrations from the process equipment, which often has a higher level of noise and vibration than the ventilation system. 

Generally, adsorption, absorption, and biofilter units require more space than compact incinerators and condensers. If the plant room is restricted, a local roof-mounted system may be the best alternative. However, roof structural reinforcement may be required even for small and lightweight units. Consideration must be given to the effects of noise and vibration. Small adsorption systems, such as adsorption canisters, require an additional central regeneration unit on site, or they must be regenerated or disposed of off site. A central regeneration unit may require long runs of costly ductwork." ... 

Noise and vibration must be controlled in order to avoid... 

Medical Ultrasound Micro-positioning and Micro-motors Piezoelectric Transformers Active Noise and Vibration Damping SUGGESTED READING References on Silicon Devices Problems for Chapter 6... 

Quarrying, noise and vibration from, 15 75 Quartz, 1 1 5 650 22 402, 490. See also Silicon dioxide... 

In the above discussion it is assumed that the available NPSH in the system is adequate to support the flow rate of liquid into the suction side of the pump. If the available NPSH is less than that required by the pump, cavitation occurs and the normal curves do not apply. In cavitation, some of the liquid vaporizes as it flows into the pump. As the vapour bubbles are carried into higher pressure regions of the pump they collapse, resulting in noise and vibration. High speed pumps are more prone to cavitation than low speed pumps. 

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