Background & Aims: In recent years, with recording equipment becoming more accessible to researchers through improved performance and reduced prices, habitat and biodiversity assessment conducted through audio recording has developed rapidly. An acoustic index is the quantification of the overall characteristics of sound, which is influenced by both the habitat characteristics and the composition and abundance of species in the study area. Therefore, there is a correlation, at least in theory, amongst acoustic indices, habitat, and biodiversity. An acoustic index can be divided into two categories: reflecting the information within the recordings and comparing the differences between different recordings, which are named an alpha acoustic index and beta acoustic index, respectively. With the increase of utilizing recording monitoring equipment in studies, the demand for comparing acoustic indices at different times and places has also increased. Therefore, the development and application of the beta acoustic index is an important direction of acoustic index research. Here, we introduced some commonly used beta acoustic indices, and explored the mathematical properties (non-negativity, self-identity, symmetry, triangle inequality, finiteness) of these indices. We also summarized applications of these beta acoustic indices by conducting a systematic literature review.
Results & Discussion: In this paper, we introduced 11 commonly utilized beta acoustic indices: mutual information index (MI), relative frequency dissimilarity index (RF), temporal dissimilarity index (TD), spectral dissimilarity index (SD), acoustic dissimilarity index (AD), Kolmogorov-Smirnov distance index (KS), Kullback-Leibler distance index (KL), cumulative spectral dissimilarity index (CS), correlation-based dissimilarity index (CB), Itakura-Saito distance index (IS), and log-spectral distance index (LS). Three of these indices (RF, KS, CS) satisfy all the properties (non-negativity, self-identity, symmetry, triangle inequality, finiteness), and another three indices (TD, SD, LS) satisfy the metric properties (non-negativity, self-identity, symmetry, triangle inequality). Cross-reference searches were conducted in the Web of Science database (www.webofscience.com, accessed on 2022-10-20), and 14 articles were found concerning the application of beta acoustic indices. Amongst these articles, SD was used most frequently in seven articles; AD and CS were used in five and four articles, respectively, while other beta acoustic indices were used no more than three times. The application of beta acoustic indices mainly focused on three aspects: the changes in time rhythm, habitat characteristics, or biological composition.
Prospect: We pointed out three areas in need of urgent development in the study/application of beta acoustic indices: designing new indices, optimizing the algorithm for existing indices, and strengthening empirical research.