Helicopter noise experiments in an urban environment

Abstract

In two series of helicopter noise experiments, sound-pressure-level recordings were made on the ground while a helicopter flew over (i) an array of microphones placed in an open field, and (ii) a similar array placed in the center of a city street surrounded by tall buildings. For given helicopter altitude and airspeed, it was found that the flyover noise recorded in the street, although initially lower, built up rapidly as the aircraft approached such that the peak recorded noise was actually more intense than that recorded in the open field. This result is in qualitative accord with the results of previous laboratory scale-model experiments performed by Lyon and Pande. The differences between the two sets of field data are attributed in major part to the fact that a reverberant sound field builds up in the street during flyover. This enhancement is less pronounced for higher flight altitudes. A simple theory based on geometrical acoustics and statistical concepts is described that quantitatively explains the sound enhancement found for a helicopter flying over a city street.

Influence of mesoscale urban morphology on the spatial noise attenuation of flyover aircrafts

“…This is perhaps because although the increase in flight altitude results in larger source-receiver distances, it also decreases the shielding effects of buildings. It was shown in a previous study that the enhancement of sound level by streets relative to that in the open field decreases with the increase of flight altitude, from 5.0dBA at 200ft to 2.0dBA at 400ft [13]. In Fig.…”

“…To overview the above results about the correlations between urban morphological parameters and acoustic indices, Table 7 shows the number of correlations according to the acoustic indices, and Table 8 gives the number of correlations according to the horizontal distances between site and flight path. It can be seen from Table 7 that L 90 (15) and L avg (13) in open areas are more correlated to urban morphological parameters than L 10 , which indicates that control of urban morphological parameters can benefit aircraft noise attenuation for the relatively quiet areas and the whole area of a given site. Table 8 shows that when the distance is 1000m, urban morphology has greater influence on aircraft noise attenuation, both on façades and in open areas.…”

“…As the aim of this study is to examine the influence of urban morphology, the atmospheric effect is not taken into account, and generic source conditions were considered. The flyover aircraft was set as a line source, considering five horizontal distances from a given site, namely 0m, 100m, 300m, 600m and 1000m, and two flight altitudes, namely 60.96m (200ft) and 121.92m (400ft), according to previous studies [12,13]. The receiver height was set as 1.6m.…”

“…It is indicated through modelling that noise from an aircraft passing overhead in a city street is enhanced compared to that heard in an open area [14]. Kinney et al [13] carried out a series of field experiments which confirmed the enhancement and explained the phenomenon. It has been demonstrated that relative Effective Perceived Noise Level increases with the ratio of building height to flying altitude, but the street width has little influence [12].…”

“…While many prediction tools mainly focus on the noise from taking offs and landings, noise mapping tools for aircraft taxing have also been developed [11]. On the other hand, with the expansion of air transport and injection of airports and heli-pads into or close to city areas, the effects of morphology of urbanised areas, for instance, the effects of urban street pattern [12,13], have become a concern on aircraft noise distribution near airports. It is indicated through modelling that noise from an aircraft passing overhead in a city street is enhanced compared to that heard in an open area [14].…”