Evaluating the influencers of acoustic indices in a zoo soundscape

Acoustic indices are promising tools for ecoacoustic studies and may be useful in zoos for passive acoustic monitoring of enclosures and their inhabitants. To validate their suitability for these applications, it is important to understand how different sounds in the environment affect them. Zoos have unique soundscapes with anthropogenic sounds such as human voices and mechanical noises often prevalent alongside high levels of animal-produced sounds; however, the influence of these sounds on acoustic indices in zoos has yet to be studied. This study investigates how different sound types recorded in aviaries at Chester Zoo in 2019 and 2020 affected three widely used acoustic indices: the Sonic Heterogeneity Index (SHI), the Acoustic Entropy Index (H), and the Normalised Difference Soundscape Index (NDSI). All three indices increased with avian vocalisations; H and SHI increased with human voices; only H increased with non-vocal anthropophonic sounds; SHI and NDSI increased with non-vocal biophonic sounds; and SHI increased but H decreased with the sound of wind. Because each index captures sound types in different ways, using all three in conjunction may provide researchers and practitioners with a more detailed understanding of the acoustic environments in zoos and other anthropogenically dominated environments.