Engineering noise control at source

1. Engineering noise control at source

2. Lecture outline 1. Summary 2. Introduction 3. Engine Noise 4. Gear Noise 5. Bearings 6. Hydraulic System 7. Fan Noise 8. Air Noise. 9. Engineering Vibration at Source 10. Active Noise Control

3. 1. Summary Engineering noise control at source and in the transmission path is one of the primary physical means of noise control with the other being elimination, substitution and isolation.

4. The most common sources of engineering noise are engines, gears, bearings, hydraulic systems, fans, high velocity gas and vibration. Control of engineering noise can be achieved by reducing the driving force, reducing the distance between components, balancing rotating equipment or installing vibration isolation fittings.

5. Liquid or gaseous sources may be controlled by diminishing turbulence, reducing flow velocity, smoothing flow and attenuating pressure pulsation. These control methods are illustrated using numerous case studies and illustrations.

6. 2. Introduction Before any decisions can be made on how to address the noise source it is necessary to identify the cause of noise, i.e., how it is generated. Noise is created by mechanical impacts, high-velocity air or fluid flow, vibrating surfaces of a machine or quite often by vibrating surfaces of the product being manufactured.

7. For excessive noise generated by mechanical impacts, the control options may include reducing the driving force, reducing the distance between components, balancing rotating equipment or installing vibration isolation fittings.

8. Reduction of the driving force can be achieved by reducing the velocity of moving parts or by applying elastic layers to increase the duration of impacts. Liquid or gaseous sources may be controlled by diminishing turbulence, reducing flow velocity, smoothing flow and attenuating pressure pulsation.

9. 3. Engine Noise The two main noise sources in a combustion engine are the cylinder cover and the exhaust system. A change in the construction of the covers by using laminated steel or a high density plastic can reduce the engine noise.

10. The exhaust system can be quietened by installation of a silencer. The types of silencer available include dissipative, resonator (Helmholtz) and absorptive. Plate x: Three stage reactive silencer suitable for large diesel engines.

11. Engine Noise The exhaust system can be quieted by installation of a silencer. The types of silencer available include dissipative, resonator (Helmholtz) and absorptive. The acoustic performance of a silencer is best characterized by the transmission loss, which is the difference between the sound pressure level at some defined measuring point with and without a silencer. Sometimes one silencer cannot provide a desired noise reduction and there is a need to use a secondary absorptive type silencer. Most engine manufacturers specify the most effective type of silencer that should be used with their engines.

12. Use of Silencers and Covers ReducesNoise. Baulderstone Clough J V has an internal noise policy to keep daily noise exposure levels to 85 dB(A) or below. To achieve this goal it was important to keep background noise levels inside the tunnel being built as low as possible. The background noise levels ranged from 80 dB(A) to 85 dB(A) due to the ventilation fans and the very reverberant character of the tunnel itself. To reduce these noise levels ventilation fans were fitted with intake and exhaust silencers, and ducts were lined inside with absorptive fibrous material and wrapped outside. This reduced the noise levels from around 85-88 dB(A) down to 81 dB(A) at 1 m from the fans.

13. Plate x: Acoustically treated ventilation fan Plate x: Acoustically treated ventilation duct

14. 4. Gear Noise Gear noise is a very common source of noise encountered in many workplaces. Gear noise is caused by friction between meshing teeth. The friction can be reduced by ensuring correct meshing of gear teeth.

15. Sometimes, stiffening the supporting shafting or increasing or decreasing one of the gear weights helps to reduce generated noise levels. Changing from metallic spur gears to non metallic or helical gears can reduce the noise created during meshing.

16. The solution to gear noise can be summarized as follows: • Casing improvement • Better manufacturing of gears • Design changes of gears • Damping • Isolation • Selection of suitable gears

17. Plate x: Metallic spur gears

18. Plate x: Helical gears

19. 5. Bearing Noise Bearing noise results from the friction generated by sliding or rubbing surfaces caused by lack of lubrication. Selection of a lubricant to provide full hydrodynamic lubrication is essential to achieve low noise levels.

20. Another source of noise is worn, off balance bearings. In general, roller bearings are noisier than ball or sleeve bearings. Sleeve bearings are the best for low noise and vibration, however, they are more expensive and require special installation and maintenance.

21. Also available are special bearings made with a plastic housing which produce less noise than those with a metal housing. Plate x: Examples of different types of bearings: roller, sleeve and ball

22. 6. Hydraulic System Noise The main source of noise in a hydraulic system is a pressure pulsation in the pipe work. This pulsation is transmitted throughout the system causing vibration in the pipes, valves, pumps and motors that is radiated by vibrating surfaces as noise.

23. There are various methods to deal with these noise sources and they can be summarizedas: • Use generously sized pipes on the pump inlet, and ensure either a good gravity head from the reservoir or use a boost pump • Use a large reservoir with baffles to encourage the release of air from the return fluid

24. Fit flexible pipes where possible, and particularly on the suction side of the pump • Consider a pressurised return system • Mount pumps and motors on suitable isolators and avoid attaching them to resonant panels • Minimise flow restrictions and sudden changes in pipe diameters

25. Fit a pulsation damper or a silencer close to the pump • Avoid lengths of pipe which are close to the wavelength of the pump pulsations or its multiples • Choose valves, motors and pumps that have been designed to minimisepressure pulsations

26. Consider the design of an enclosure around the noisy pumps and valves • Use isolating pipe clamps • Use servo valves which give a controlled opening and closing, thus avoiding the pressure transients

27. Plate x: Example of noise control in a refrigeration plant

28. 7. Fan Noise Fan noise results from intake turbulence and vortex generation related to the fan geometry and rotational speed. A reduction in fan speed can significantly reduce the generated noise. However, this reduces the volume of delivered air as well.

29. Another way to reduce the noise is to use backward-curved blades, which produce a lower pressure. About 8-10 dB noise reduction can be achieved by replacing straight fan blades with backward-curved blades. Another option is to replace radial flow by axial flow fans.

30. Plate x: Axial type of fan Plate x: Radial, backward curved blade type of fan

31. The intake turbulence noise can be reduced by not placing fans directly behind components producing turbulence. The noise can also be avoided by leaving a maximum possible distance between the fan and any fixed or moving surfaces in the flow direction. Fan silencers can also be considered.

32. Plate x: Fan silencer

33. 8. Air Noise Air noise is generated when a high velocity gas interacts with the ambient air or solid surfaces causing turbulence and shearing stress. Silencers are used for the control of high-velocity airflow noise.

34. There are six basic types of silencers: • Absorptive silencers • Reactive expansion chambers • Reactive resonators • Plenum chamber • Lined bend • Diffuser

35. Plate x: Basic silencers

36. Plate x: RoboWashmachine Plate x: Special cup fitted over air gun nozzle

37. 9. Engineering Vibration Control at Source 9.1. Vibration Isolation Airborne noise can be produced by any solid, vibrating part of a machine. The vibrating part may be driven into vibration by direct contact with a moving part or by contact with an intermediate solid linkage which is in contact with the moving part.

38. To control it, vibration isolation techniques are applied. In general all vibration isolation methods aim at disassociating the vibrating part from the force causing it to vibrate. Plate x: Application of vibration isolators with no isolation.

39. There are various vibration isolators commercially available in the form of springs, rubber mounts, elastomer types (compressed or shear, ribbed Neoprene), other compressible materials such as cork, or fibrous mats made of felt or fibres.

40. Plate x: Different types of vibration isolators Plate x: Application of vibration isolators with isolation Different types of vibration isolators

41. Plate x: Bench grinder placed on special pedestal and mounted to the floor using rubber mounts Plate x: Feed table disconnected from saw table

42. Plate x: Guillotine after full service with collecting tray lined with carpet Plate x: Pedestal grinder fitted to the floor on rubber

43. 9.2. Vibration Damping Another way of controlling acoustic radiation from vibrating surfaces is vibration damping. The term "damping" refers to a property of materials which converts vibration energy into heat energy. • There are two types of damping: • External surface damping • Constrained layer damping

44. 9.3. Use of External Surface Damping External surface damping can be in the form of a sheet bonded to the structure being damped, or a layer which is sprayed or painted on.

45. Plate x: Application of damping layer Plate x: Application of damping layer

46. 9.4. Constrained Layer Damping The second type of vibration damping is constrained layer damping, which involves sandwiching a layer of visco elastic material between the structure being damped and an outer constraining layer.

47. This type of damping finds application where a large vibration reduction is required. Plate x: Constrained damping material.

48. 10. Active Noise Control - "Anti- Noise" "Anti-Noise" Is Used To Cancel the Existing Noise Active Noise Control is a technique that uses the principle of wave cancellation by generating a pressure wave that is exactly 180 degrees out of phase with the original wave and of the same magnitude.

49. The basic principle of active noise control is tom create "anti-noise" in order to cancel the existing noise. Plate x: Basic active noise control system

50. An active noise control system consists of the input microphone that picks up the pressure variation and the controller that generates a signal to the loudspeaker which will create an opposing pressure precisely when the noise wave reaches the loudspeaker. An error microphone downstream from the speaker monitors the residual acoustic pressure after cancellation and signals the controller to adjust itself for optimum results.