Essay on on Hovercraft Noise!
From the point of view of noise nuisance, the hovercraft presents a particularly unfortunate combination of a vehicle. This is due to fact that it uses power on a scale normally associated with aircraft but, unlike the latter the hovercraft operates at ground level, usually near built-up areas around harbours, river estuaries and beaches.
Fortunately, there is no problem of hovercraft noise in India and other developing countries so far due to their total absence or rarity in these countries.
In advanced nations on the other hand, where hovercraft present the problem of noise pollution, there are specific recommendations on their noise level. In the United Kingdom, for example, there is a recommendation that hovercraft should not produce a noise level, more than 60-65 dB at a distance of 100 feet (about 30 m). But this represents an unrealistic requirement in the present state of the art.
Earlier hovercraft, for example required a distance of some 600 m to achieve this noise level. Recent advances in propeller design and hovercraft propulsion have reduced this distance considerably, but not yet to the recommended value (30 m). In addition, the problem of hovercraft noise is aggravated due to poorly sited hovercraft terminal areas.
Airscrew is the chief source of noise with conventional hovercraft. There are methods other than propellers for hovercraft propulsion, e.g., marine screws or utilizing some of the lifting air as a propulsive jet. Hovercraft using these methods of propulsion are generally judged to be less noisy. However, as they are developed further, noise from sources other than propulsion could become more important.
A typical noise spectrum generated by a first generation hovercraft is shown in Fig. 1. The most prominent feature of this spectrum is the strong discrete frequency content due to propeller noise, with the fundamental blade passage frequency (shaft rotational frequency multiplied by the number of blades in the propeller), and up to six harmonics being present.
Also evident from Fig. 1 are various discrete frequency components.
These are generated by the fan drive shafts, the main gearboxes, turbo shaft engine compressors and turbines. All these discrete components are super-imposed upon a broad-band spectrum which is probably a combination of vortex noise from propellers, lift fans, engine compressors and the exhaust jets.
The type of jet that is likely to be present on present day commercial hovercraft can be classified under one of the following functions:
(B) Turbo shaft engine exhaust
(C) Lift air efflux.
A pure jet propulsion system of the type normally associated with aircraft is not likely to be practicable on a large hovercraft. The reason is that, apart from any economic consideration, the noise level of such a jet-propelled hovercraft alone would probably prohibit an approach to the vicinity of built-up areas under anything but minimum power.
In fact, the sound power level of a typical propulsion jet would be greater than 150 dB, which would produce a noise level of more than 80 dB(A) at 150 m. This noise level will not be acceptable by any standards, even for short periods.
On the other hand, the exhaust jets of turbo shaft engines are not so much of a problem since these engines have lower jet efflux velocities. The sound power level produced by turbo shaft engines may be in the range of 100-110 dB, which would imply a sound pressure level of around 40 dB (A) at 150 m.
The noise from lifting air jets can usually be disregarded, at least some distance away from the hovercraft. This again is due to the fact that low velocities are involved.
The compressors of turbo shaft engines and the lifting fans are the most important fans on hovercraft. The spectrum of fan noise contains both discrete-frequency rotational noise and also broad-band noise. In this respect, fan noise in the hovercraft is similar to propeller noise.
On the other hand, there is not likely to be any strong discrete-frequency radiation of noise from the hovercraft with lifting fans of the multi-vane centrifugal type.
Some form of transmission system (shafts and reduction gearboxes) between the prime mover and the lift fan or propeller is utilized by most hovercraft. Unfortunately, there are no simple and general expressions to predict the levels of transmission noise that may be expected inside or outside the hovercraft.
The only best thing one can do at this stage of development is to recognise the transmission system as a potential noise problem and to be aware of the design principles that have to be followed in order to reduce the transmission noise to a minimum level.
A transmission shaft has to be used where direct drive is not possible. In such situations, there is always the possibility of an out-of-balance force or some other periodic force being imposed on the shaft as it rotates. Slight misalignment of the bearings, excessive spacing between bearings, universal joints or flexible couplings, for example, may all impose a load on the shaft once per revolution.
While this load may be small and well within the capacity of the shaft, it will be transmitted through the bearing housings to the relatively light-weight structure of the hovercraft.
When this happens, quite significant radiation of noise may result from vibrating skin panels, particularly if any of them is excited near resonance. After doing as much as possible to minimize the level of excitation caused by these mechanisms, one can then look to the regions of the structure which are affected.
Shafts, for example should be supported by the heaviest parts of the structure rather than by light load- carrying members. Similarly, skin panels attached to the same structure should be stiffened or at least designed to have resonant frequencies well away from the shaft frequencies. The amount of noise radiated by these parts may also be reduced by using damping compounds.
The problem of gear noise, on the other hand, is a more complicated one. Much of the noise in a gear train seems to arise from the tooth contact frequency and its harmonics. There is also the possibility of “ringing” in a gear wheel at the natural frequency following contact between teeth.
Similarly, any eccentricity in the wheel will cause vibration at the wheel rotational frequency, which will then be transmitted through the casing and shafts. In addition to these, there may also be some noise from trapping and compressing air or oil between teeth, and from “splashing” of the lubricant, in the case of high-speed gears.
The problem of gear noise is so involved that there is a need for much more research in this area. But there are certain general principles which can be applied with advantage to control gear noise. Some tests have shown, for example, that gear noise is strongly dependent on the accuracy of gears. As a general rule, the closer tolerances give the least noise. There also appears to be a reduction of noise with increased loading.
The removal of “ringing” frequencies by using damped metals does not compensate, in general, for their generally poorer load carrying properties and manufacturing difficulties. It is likely that further research may reveal new possibilities in this area.
For example, it may be worthwhile to make the gearbox castings of such material (damped metals), particularly where casing resonance frequencies coincide with any of the excitation frequencies in the gear train. Gear cases should generally be designed for large mass and stiffness to give high transmission loss for the air- borne sound generated in the gears.
Besides transmission shafts, gear-boxes should also be supported by the most rigid parts of the vehicle structure. The excitation of the structure by the gearbox may also be reduced by the use of resilient mountings.
The air-borne sound from the gears may be reduced to some extent by isolating the whole gearbox with an acoustically designed enclosure to provide both absorption inside and transmission loss through the walls. As a matter of fact, where gear frequencies are relatively high, and the noise is truly air-borne, the method of acoustic isolation could be most effective.