Spatial variation in soundscape frequency components across an urban area: A case study of the City of Southampton, UK

There is growing evidence that continuous exposure to noise can lead to premature hearing loss, reduced cognitive performance, insomnia, stress, annoyance, hypertension, cardiovascular diseases and stroke. Currently, over 125 million people in the European Union are exposed to levels of road noise that affect their health. In order to mitigate environmental noise, Member States are required to identify major noise concentrations (hotspots), selected on the basis of being located in major agglomerations or alongside major roads, railways and airports. Although these strategic noise maps have proved beneficial in addressing noise pollution, they do not take into account the differences in sound frequencies across a city. The authors therefore conducted a survey of the entire city of Southampton, UK during the summer of 2016 to capture spatial variations in sound characteristics across multiple frequencies. Southampton is an ideal city to study because it suffers from noise pollution arising from road traffic, a busy port, a rail network and an airport, but also has several green spaces that could potentially mitigate the worst effects. Using moving observers, over 52,000 calibrated and georeferenced sound clips across the audible spectrum from 11 Hz to 22.7 kHz were captured at three different times of the day. Here, the focus is on the contrast between the sound pressure levels in frequencies that characterise the “anthrophony” (sounds from anthropogenic sources) and the “biophony” (sounds from nature). Hotspot maps were developed for octave analogues of various soundscape indices (e.g. acoustic diversity, Normalised Difference Soundscape Index) and dominant octave bands. Results showed that most of the city is dominated by broad-spectrum noise, clustered at three different mean decibel levels (50, 58 and 66 dBA). There was some evidence that noise decreased with increasing surrounding vegetated cover but patterns of sound characteristics with land cover and land use were complex. Although most anthropogenic sounds occur in low frequency ranges, the results showed that human-generated urban sounds are also present in the upper frequencies and therefore a well-defined biophony is difficult to distinguish. Opportunities for the development of enjoyable soundscapes are therefore few in Southampton because of the dominance of traffic noise.