The hovercraft noise problem

Abstract

This paper deals with the hovercraft noise problem from two distinct points of view. At the outset, hovercraft noise is reviewed in relation to the noise associated with other means of transport. While hovercraft appear to be rather less noisy than aircraft of comparable installed power, they tend to be much noisier than conventional surface transport. It is concluded that, for the first generation of commercial craft, the distance between a terminal and the nearest houses in a quiet residential area should be at least 800 yd to completely avoid increase in the normal level of noise. On the other hand, in a busy urban locality, a distance of 150 to 200 yd is sufficient to bring the noise from the hovercraft down to the existing level of traffic noise. Estimates made in connection with the design of the SR.N 4 show that these distances will be substantially reduced for the next generation of hovercraft. It is pointed out that the noise problem at a terminal can be alleviated by minimizing the need for manoeuvring. The sources of hovercraft noise and the means for their reduction are then discussed. The external noise from existing hovercraft is dominated by that originating from the propeller and attention is drawn to the fact that actual measurements reveal that this noise is appreciably higher than would be expected from theoretical predictions. It is considered that reduction of the tip speed is the most effective way of reducing propeller noise. Large numbers of blades and the use of devices such as shroud rings are rejected as either ineffective or actually detrimental. For turbo-shaft engined hovercraft, compressor noise usually ranks next to propeller noise in importance. However, noise from this source can be reduced by muffling the air intakes. Noise from lifting fans is relatively unimportant at present but, as developments reduce the level of propeller and engine noise, it could become significant and thus merits attention. Inside a hovercraft, transmission and engine noise are usually predominant. However, the high frequencies involved can be dealt with quite effectively by aircraft soundproofing techniques and there are few fundamental problems involved.