Acoustic Complexity Index Research Articles

The acoustic complexity index (ACI): theoretical foundations, applied perspectives and semantics

The acoustic complexity index (ACI) is a commonly used metric in ecoacoustics, demonstrating reliability across diverse environments and ecological conditions. However, this index requires specific procedures to be applied correctly. Based on the Canberra metric, the ACI is an unsupervised metric formulated to extract information from fast Fourier transform (FFT) sonic matrices. The ACI measures contiguous differences in acoustic energy of each frequency bin along temporal steps (ACItf) and a temporal interval along the frequency bins (ACIft). Aggregating data after an FFT with a clumping procedure allows for better scaling of sonic signals before computing the ACI. A filter must be applied to reduce the effects of nonenvironmental signals produced by microphone electrical noise . Due to the singularity of the index for values of 0, ACI requires ad hoc procedures to exclude element pairs for which one of the elements is equal to 0 from the comparisons. The spectral and temporal sonic signatures are vectors obtained from the sequence of ACItf and ACIft values, respectively. The comparison between sonic signatures using the chord distance index returns spectral and temporal sonic dissimilarities, allowing the evaluation of sonic patterns at different temporal and spatial resolutions. Sonic variability, sonic evenness, and the effective number of frequency bins are further derivative metrics that help interpret sonic heterogeneity by distinguishing the temporal and spatial heterogeneity of sonoscapes. Moreover, this paper proposes changing the terminology of ‘acoustic complexity index' to ‘sonic heterogeneity index.'

Unsupervised selection of acoustic indices: An experimental comparison for characterizing unlabeled audio recordings from sub-Andean forest soundscapes

We present an experimental comparison of applying six unsupervised (i.e. not relying on class labels) and almost parameterless feature selection methods for ranking acoustic indices. The study is aimed at guiding the practitioner in choosing appropriate acoustic indices when, in absence of class labels, a small but meaningful number of features to characterize soundscapes is desired. Forty acoustic indices were considered, which correspond to seventeen temporal, spectral and soundscape features, along with their basic statistics. Three publicly available soundscape datasets, registered in a sub-Andean forest, were used for the experiments; moreover, several subsets were considered according to the different times of the day. Results reveal that the Acoustic Evenness Index is the most important one in terms of representational power according to the six considered selection criteria, followed by the Acoustic Complexity Index when conditions are relaxed to examine features ranked among the top-five. Besides, the Bioacoustic Index, the Acoustic Diversity Index and the Root Mean Square Energy stand out as important features when characterizing days in their separated parts.

Continuous measurement of red wood ant (Formica rufa) outdoor behaviour using passive acoustic monitoring

Ants serve as ecosystem engineers that maintain important ecological processes within forests. Given their ecological importance, it is a clear scientific shortcoming that we lack non-invasive methods to survey their behaviour inside common opaque habitats such as mounds, litter, and soil. In this study, we assess if acoustic signals from red wood ant (Formica rufa) mounds are useful to infer temporal changes in ant activity within forested ecosystems. We found that acoustic indices used previously as a proxy for soil fauna in soil ecological studies (Acoustic Complexity Index, Bioacoustic Index) can indeed separate sounds generated by the ant's daily routines (biophony) from other forest sounds. Yet, we also show that these indices are problematic proxies for soil diversity as they increase not only due to an increased number of species but also due to an increased number of the same species. Acoustic measures that incorporated the strength of acoustic signals, Average Power Density (APD) and Peak Power Density (PPD) also increased with increasing ant abundance and constituted the conceptually best proxy for ant activity. For example, the PPD could i) track diurnal changes in Formica rufa activity with a high temporal resolution (minutes) and ii) detect altered behavioural responses to temperature changes. We conclude that microphones detecting biophony can provide high-resolution information about in situ ant behaviours in forested ecosystems. Thus, passive acoustics monitoring offers a promising avenue as a non-invasive monitoring tool for soil macrofauna studies.

Studying environment sounds using transfer entropy: Application to an urban park in the city of Milan, Italy

Sounds in large cities are usually characterized by multi-source overlaying components. This makes environment sound quality difficult to interpret, thus requiring different investigation approaches. This is the case of urban parks where the coexistence of anthropogenic and biophonic sound sources makes the evaluation of the local soundscape quality challenging. In this context, we applied the transfer entropy measure to extract additional information with respect to the more classical approach of eco-acoustics. The case study refers to the Parco Nord of Milan, Italy, for which classical soundscape analysis is already available. We used sound recordings (3.5h long each) taken over an area of approximately 20 hectares using 16 sensors. We calculated the classical eco-acoustic indices and studied the transfer entropy among all sites. The considered time series corresponded to three representative eco-acoustic indices (Acoustic Diversity Index, ADI, Acoustic Complexity Index, ACI and Dynamic Spectral Centroid, DSC calculated for all the 16 sites with 1-minute time resolution) obtained after a principal components analysis. A preliminary interpretation of the results is provided by selecting a representative index from one principal component and by comparing its behavior with a previous standard soundscape analysis performed by the authors for the same index and area.

Increased bird sound diversity in vineyards enhances visitors' tour experience

Abstract Biodiversity is rapidly declining, reducing the quantity and quality of human interactions with nature and constraining its contribution to human health and well‐being. Natural sounds are a key component of our experience of nature, but biodiversity losses are reflected in soundscapes, which are becoming less diverse and quieter. We characterised the soundscapes across 21 English vineyards using acoustic indices and related them to bird species richness and abundance. We found that higher bird species richness, but not abundance, led to more diverse and louder soundscapes, as reflected in higher values of Acoustic Complexity Index (ACI), Bioacoustic Index (BIO) and Normalised Difference Soundscape Index (NDSI), and lower values of Acoustic Entropy Index (H). Secondly, at three of the study vineyards that run vineyard tours, we measured visitors' experience of the tour in terms of sound enjoyment, soundscape connectedness and tour satisfaction and related these to ambient and experimentally enhanced soundscapes, where we used playback recordings of five additional birdsongs to increase the soundscapes' complexity and volume. Under ambient conditions, respondents' (n = 107) experience was significantly higher at sites with soundscapes that had higher ACI and BIO values, and lower H, Acoustic Diversity Index (ADI) and NDSI values, indicating a positive effect of more diverse and louder soundscapes. Natural sounds formed an important part of the tour experience at these sites, making visitors feel more present and connected to nature. Under experimental soundscape enhancement, respondents (n = 79) reported hearing significantly more bird species during the tour, and they reported significantly higher scores for sound enjoyment, soundscape connectedness and tour satisfaction than under ambient conditions. This effect was stronger in visitors who engaged more in pro‐environmental behaviours, such as purchasing organic foods. Our study demonstrates (i) the direct contribution of aural modes to our experience of nature and (ii) that the delivery of biodiversity conservation measures aimed at supporting bird diversity could simultaneously enhance the experience and well‐being benefits of spending time in nature. Natural soundscapes should therefore be recognised and valued as natural capital, and their protection should be incorporated into conservation planning and policy. Read the free Plain Language Summary for this article on the Journal blog.

Soundscape analysis can be an effective tool in assessing seagrass restoration early success

Restoration of vulnerable marine habitats is becoming increasingly popular to cope with widespread habitat loss and the resulting decline in biodiversity and ecosystem services. Lately, restoration strategies have been employed to enhance the recovery of degraded meadows of the Mediterranean endemic seagrass Posidonia oceanica. Typically, habitat restoration success is evaluated by the persistence of foundation species after transplantation (e.g., plant survival and growth) on the short and long-term, although successful plant responses do not necessarily reflect the recovery of ecosystem biodiversity and functions. Recently, soundscape (the spatial, temporal and frequency attribute of ambient sound and types of sound sources characterizing it) has been related to different habitat conditions and community structures. Thus, a successful restoration action should lead to acoustic restoration and soundscape ecology could represent an important component of restoration monitoring, leading to assess successful habitat and community restoration. Here, we evaluated acoustic community and metrics in a P. oceanica restored meadow and tested whether the plant transplant effectiveness after one year was accompanied by a restored soundscape. With this goal, acoustic recordings from degraded, transplanted and reference meadows were collected in Sardinia (Italy) using passive acoustic monitoring devices. Soundscape at each meadow type was examined using both spectral analysis and classification of fish calls based on a catalogue of fish sounds from the Mediterranean Sea. Seven different fish sounds were recorded: most of them were present in the reference and transplanted meadows and were associated to Sciaena umbra and Scorpaena spp. Sound Pressure Level (SPL, in dB re: 1 μPa-rms) and Acoustic Complexity Index (ACI) were influenced by the meadow type. Particularly higher values were associated to the transplanted meadow. SPL and ACI calculated in the 200–2000 Hz frequency band were also related to high abundance of fish sounds (chorus). These results showed that meadow restoration may lead to the recovery of soundscape and the associated community, suggesting that short term acoustic monitoring can provide complementary information to evaluate seagrass restoration success.

Complex and highly saturated soundscapes in restored oak woodlands reflect avian richness and abundance.

Temperate woodlands are biodiverse natural communities threatened by land use change and fire suppression. Excluding historic disturbance regimes of periodic groundfires from woodlands causes degradation, resulting from changes in the plant community and subsequent biodiversity loss. Restoration, through prescribed fire and tree thinning, can reverse biodiversity losses, however, because the diversity of woodland species spans many taxa, efficiently quantifying biodiversity can be challenging. We assessed whether soundscapes in an eastern North American woodland reflect biodiversity changes during restoration measured in a concurrent multitrophic field study. In five restored and five degraded woodland sites in Wisconsin, USA, we sampled vegetation, measured arthropod biomass, conducted bird surveys, and recorded soundscapes for five days of every 15-day period from May to August 2022. We calculated two complementary acoustic indices: Soundscape Saturation, which focuses on all acoustically active species, and Acoustic Complexity Index (ACI), which was developed to study vocalizing birds. We used generalized additive models to predict both indices based on Julian date, time of day, and level of habitat degradation. We found that restored woodlands had higher arthropod biomass, and higher richness and abundance of breeding birds. Additionally, soundscapes in restored sites had higher mean Soundscape Saturation and higher mean ACI. Restored woodland acoustic indices exhibited greater magnitudes of daily and seasonal peaks. We conclude that woodland restoration results in higher soundscape saturation and complexity, due to greater richness and abundance of vocalizing animals. This bioacoustic signature of restoration offers a promising monitoring tool for efficiently documenting differences in woodland biodiversity.

Seasonal sonic patterns reveal phenological phases (sonophases) associated with climate change in subarctic Alaska

Given that ecosystems are composed of sounds created by geophysical events (e.g., wind, rain), animal behaviors (e.g., dawn songbird chorus), and human activities (e.g., tourism) that depend on seasonal climate conditions, the phenological patterns of a soundscape could be coupled with long-term weather station data as a complimentary ecological indicator of climate change. We tested whether the seasonality of the soundscape coincided with common weather variables used to monitor climate. We recorded ambient sounds hourly for five minutes (01 January–30 June) over three years (2019–2021) near a weather station in a subarctic ecosystem in south-central Alaska. We quantified sonic information using the Acoustic Complexity Index (ACItf), coupled with weather data, and used machine learning (TreeNet) to identify sonic-climate relationships. We grouped ACItf according to time periods of prominent seasonal events (e.g., days with temperatures >0°C, no snow cover, green up, dawn biophony, and road-based tourism) and identified distinct sonic phenophases (sonophases) for groups with non-overlapping 95% confidence intervals. In general, sonic activity increased dramatically as winter transitioned to spring and summer. We identified two winter sonophases, a spring sonophase, and a summer sonophase, each coinciding with hours of daylight, temperature, precipitation, snow cover, and the prevalence of animal and human activities. We discuss how sonophases and weather data combined serve as a multi-dimensional, systems-based approach to understanding the ecological effects of climate change in subarctic environments.

Virtual Reality Technology in Analysis of the Sarek National Park Soundscape in Sweden

The paper presents an in-depth analysis of the soundscape within Sarek National Park, the oldest national park in Europe, situated in Lapland, northern Sweden. The comprehensive acoustic measurements, ambisonic recordings, and 360X video recordings were carried out during a scientific expedition in the summer of 2020. The aim of the paper is to show the soundscape analysis of carefully selected characteristic locations in various parts of the valley. The paper extensively discusses the findings derived from the recorded data using both classical acoustic methods and the soundscape approach. The classic acoustic parameters, commonly employed in environmental acoustics as well as eco-acoustic indices such as: ACI (acoustic complexity index), ADI (acoustic diversity index), AEI (acoustic evenness index), NDSI (normalized difference soundscape index), BIO (energy level of biophony), amplitude index (M), and total entropy (H) were calculated. To gain further insights, listening tests, facilitated through virtual reality tools, were conducted, enabling participants to engage in soundwalk experiences. By employing a combination of traditional acoustic methods and innovative soundscape approaches, the paper presents a holistic evaluation of the auditory environment in Sarek National Park. The main contribution of the presented research is providing new data from the unique, geographically inaccessible region of the world, the Sarek National Park. This research not only enriches our understanding of the national park’s soundscape but also offers valuable insights into the interaction between the natural environment and human perception of sound.

Hazardous wildfire smoke events can alter dawn soundscapes in dry forests of central and eastern Washington, United States

As global wildfire activity increases, wildlife are facing greater exposure to hazardous smoke pollution – with unknown consequences for biodiversity. Research on the effects of smoke on wild animals is extremely limited, in part due to the inherent logistical challenges of observing how animals respond to smoke in real time. Passive acoustic monitoring may be a powerful tool to safely and effectively monitor biodiversity before, during, and after major smoke events. In this study, we used data collected from a large-scale network of bioacoustic recorders at 92 sites in central and eastern Washington state during August–September, 2019–2020 to investigate the effect of wildfire smoke on dawn soundscapes and, by extension, acoustically active wildlife. We used acoustic indices to document and characterize changes in soundscapes related to smoke exposure, including the Acoustic Complexity Index (ACI), Bioacoustic Index (BI), and Normalized Difference Soundscape Index (NDSI). Higher values of these indices likely indicate higher levels of biodiversity in our study area. We hypothesized that wildfire smoke would reduce bird vocalizations, leading to declines in ACI, BI, and NDSI at dawn, when birds are most active. We used linear and quantile regression models to test for an effect of daily exposure to fine particulate matter (PM2.5), a marker of wildfire smoke, on the mean daily values and the upper 90th percentile of each index at dawn. We also conducted a before-during-after analysis of a particularly hazardous smoke event that impacted our study area on September 12–14, 2020. We did not observe linear effects of daily PM2.5 on average or peak daily values of acoustic indices; however, we did observe a significant reduction in ACI and BI during the three-day smoke event in 2020 and in the two weeks following this air pollution episode. Our results indicate that, on average, ACI and BI were reduced by 2.7 % and 15.9 % during and 1.5 % and 11.0 % afterward, respectively. These findings add further evidence that wildfire smoke alters soundscapes, likely due to changes in the presence, abundance, or behavior of acoustically active animals. Furthermore, our study demonstrates that wildfire smoke may have delayed and/or cumulative effects on acoustically active wildlife. Our study highlights the potential for passive acoustic monitoring to document wildlife responses to smoke pollution and identify potentially relevant exposure periods.

Soundscape characteristics of RAS tanks holding Atlantic salmon (Salmo salar) during feeding and feed withdrawal

Behavioural monitoring can provide crucial information on welfare and feeding in aquaculture. Passive acoustic monitoring of behaviour can be particularly useful in recirculating aquaculture systems (RAS), as they often have turbid water that impairs visual monitoring. Currently, little is known about the sounds that make up the soundscapes in RAS tanks holding Atlantic salmon (Salmo salar). In this study, hydrophones were used to continuously record the soundscape in eight single tank RAS holding Atlantic salmon parr for 15 days, with the fish in four of the tanks being fasted by feed withdrawal for five days from the sixth to the tenth day. The results show that soundscapes in RAS tanks are affected by feeding. Two main sound sources were identified during feeding in RAS tanks, one related to pellets delivery and the other to fish behaviour. The sound of pellets hitting the water surface had energy concentrated at frequencies between 1.7 and 4.0 kHz, with peak frequency decreasing and amplitude increasing with increasing number of pellets hitting simultaneously. The feeding sounds of Atlantic salmon had energy concentrated at frequencies between 6.5 and 9.4 kHz.More complex soundscapes were recorded during feeding events. These were characterized by variations in amplitude and frequency that have been described by using acoustic indexes in RAS tanks for the very first time. The Acoustic Complexity Index (ACI), the Acoustic Entropy Index (H) and the Normalized Difference Soundscape Index (NDSI) showed distinct changes in the soundscape related to feeding events; ACI increased while H and NDSI decreased compared to the times in between scheduled feeding times. The sound types identified in this study and the outcomes of the acoustic indices indicate a possibility to monitor system performance as well as fish behaviour in the tank soundscapes in RAS. Soundscape monitoring can contribute to match feeding closer to fish appetite, improve water quality, and reduce risks that deviations in the system performance can have on fish welfare during production.

Measuring biodiversity with sound: How effective are acoustic indices for quantifying biodiversity in a tropical dry forest?

AbstractLarge‐scale biodiversity conservation and management necessitate rapid assessment tools and technologies. Indices derived from passive acoustic data offer a novel solution for rapid biodiversity monitoring. Although these indices quantify vocalizing biodiversity at a site, previous studies indicate inconsistencies in the indices' performance across different biomes. We tested the efficacy of seven acoustic indices—acoustic complexity index, acoustic diversity index, bioacoustic index (BI), acoustic entropy index, total entropy (H), normalized difference soundscape index, and number of peaks in an understudied biome, tropical dry forests, in central India. We measured the relationship between every index and a combination of these indices with a biodiversity metric, avian species richness. We found a weak correlation between individual indices and species richness (0.00 ≤ R ≤ 0.35), while a combination of indices was comparatively better at predicting species richness (R2 = 0.54). Although BI performed better than all other indices, our results indicate that acoustic indices do not accurately quantify avian species richness in this forest in central India. However, combining multiple indices increases their efficacy, limitedly. We recommend evaluating the efficiency of acoustic indices, especially in underrepresented habitats, before their application in avifauna‐based rapid acoustic surveys.

Springtime spatio-temporal distribution of bird diversity in urban parks based on acoustic indices

Human-dominated urban ecosystems have encroached on native habitats, positioning urban parks as crucial sanctuaries that support bird diversity conservation. Bird communities tend to act as significant indicators of habitat quality. Birds play a key role in generating soundscapes in urban environments, where a healthy acoustic environment is essential not only for human well-being but also serves as a core focus for environmental monitoring and landscape enhancement. Previous studies have pointed out that acoustic indices have great advantages and strong feasibility for long-term environmental monitoring. However, few studies on the comparative analysis of bird diversity across urban parks subject to varying degrees of urbanization have been published in this field. In this study we recorded the springtime soundscape across 8 urban parks in Nanjing (China) from March to April 2023 for 10 hours each day. Six acoustic indices of Acoustic Complexity Index, Acoustic Diversity Index, Normalized Difference Soundscape Index, Bioacoustic Index, Acoustic Entropy Index and Power Spectral Density were used to quantify the monitoring results. To investigate the influence of vegetation habitats on bird activity, we utilized K-means clustering analysis to categorize the parks based on their developmental gradient, while Principal Component Analysis (PCA) was employed to simplify the dimensions of 12 vegetation factors. The results revealed that the distribution of birds in urban parks is a complex interplay of variables including the gradient of urban development, the functions of park plots, and the characteristics of vegetation habitats, among others. Day-to-day, human and bird activities show similarities, yet birds' responses to urban environments vary by location. The daily movement of visitors exerts a discernible influence on park birds, even those adapted to urban life. In instances of heightened human activity, birds demonstrate a proclivity for seeking refuge in less disturbed areas, and their activity space tends to be concentrated on higher plants. Vegetation communities in areas of high human disturbance are found to provide substantial support to bird habitats. Specifically, vegetation communities featuring tall trees or ground cover that form buffer zones have a significant positive effect on weakening the impact of human activities on bird communities. In terms of monitoring bird diversity with acoustic indices, Bioacoustic Index and Acoustic Diversity Index show more consistent performance, rendering them more reliable than other acoustic indices.

Sonotope patterns within a mountain beech forest of Northern Italy: a methodological and empirical approach

According to the Sonotope Hypothesis, the heterogenous nature of the acoustically sensed, but not yet interpreted, environmental sounds (i.e., sonoscape) is created by the spatial and temporal conformation of sonic patches (sonotopes) as recently been described in a Mediterranean rural landscape. We investigated the Sonotope Hypothesis in a mountain beech forest of the Northern Apennines, Italy that is notoriously poor in soniferous species. Our aim was to test whether sonotopes were temporally distinct over seasonal and astronomical timeframes and spatially configured in relation to vegetation variables. We used the Acoustic Complexity Index (ACItf) to analyze the heterogeneity of sonic information gathered from an array of 11 sound recorders deployed within a lattice of eleven 4-ha hexagonal sample sites distributed throughout a 48-ha managed beech forest. We visualized and described the temporal patterns of ACItf between seasons (May–June and July–August 2021), across six astronomical periods (Night I, Morning Twilight, Morning, Afternoon, Evening Twilight, and Night II), and according to two aggregated frequency classes (≤2000 and >2000 Hz). We introduced Spectral Sonic Signature (SSS) calculated from the sequence of ACItf values along frequency bins as a descriptor of the dynamic production of sounds across spatial and temporal scales. We calculated Mean Spectral Dissimilarity to compare SSS values across temporal periods and between sample sites. We identified sonotopes by grouping similar SSS for each sample site generated from cluster analyses and visualized their spatial arrangements. Frequencies ≤2000 Hz (mainly geophonies from wind and rain) were more prevalent than frequencies >2000 Hz (mainly biophonies from songbirds). Despite there being no strong relationship to vegetation variables and minimal biophony and anthropophony, distinct sonotopes still emerged for every astronomical and seasonal period. This suggests that the sonoscape expresses distinct spatial and temporal sonotope configurations associated with the temporal and spatial patterns of geophysical events that generate geophonies with minimal animal or anthropogenic occurrences. A new strategy based on the reintroduction of indigenous trees and shrubs in managed clearings should be considered for enhancing local biodiversity conservation along with ecoacoustic monitoring based on the Sonotope Hypothesis.

The impact of vehicular noise on acoustic indices within simulated bird assemblage soundscapes

ABSTRACT Passive acoustic monitoring (PAM) is a sampling technique that has gained increasing popularity in the field of wildlife monitoring and research since it allows for non-invasive and cost-effective collection of acoustic information. Retrieving biological information from PAM recordings can often involve time-consuming sound annotation methodologies, but the advent of acoustic indices can help expedite this process. While correlations between acoustic indices and species richness have been observed in a variety of ecological contexts, these relationships faulter in environments with increased vehicular noise. Here, we assessed the direct impact of vehicular noise on nine acoustic indices through controlled manipulation of vehicular noise within computer-generated bird assemblage soundscapes. Our results demonstrate that recording distance from roadsides and number of passing cars per minute have notable and persistent impacts on acoustic index values, but the magnitude of the effect varies across indices. Four acoustic indices demonstrated greater resilience to vehicular noise interference and may therefore be better suited for developed areas: Bioacoustic Index, Acoustic Complexity Index, Acoustic Diversity Index, and Acoustic Evenness Index. By contributing to the collective understanding of acoustic index behaviours under anthropogenic noise pollution, we hope to better inform their ecological application in human-developed contexts.

Unidentified fish sounds as indicators of coral reef health and comparison to other acoustic methods

The global decline of coral reefs is a major contributor to the global biodiversity crisis and requires improved monitoring at these critically important habitats. Non-invasive passive acoustic assessments may address this need, leveraging the rich variety and spatiotemporal variability of biological sounds present in coral reef environments and offering near-continuous temporal coverage. Despite this, acoustic metrics that reliably represent coral reef health are still debated, and ground-truthing of methods is limited. Here we investigated how the prevalence of low frequency biotic sounds (without species information) relates to coral reef health, providing a foundation from which one can compare assessment methods. We first quantified call rates of these low frequency sounds for three reefs exhibiting different community assemblages around St. John, U.S. Virgin Islands, by manually annotating presumed fish noises for 1 min every 30 min across 8 days for each site. Annotated days were selected at key points across lunar cycles. These call rates were then compared with traditional visual surveys, and several acoustic methods and indices commonly used in underwater soundscape research. We found that, overall, manually detected fish call rates successfully differentiated between the three reefs, capturing variation in crepuscular activity levels–a pattern consistent with previous work that highlights the importance of diel choruses. Moreover, fish vocal rates were predictors of hard coral cover, fish abundance, and fish species richness, while most acoustic indices failed to parse out fine distinctions among the three sites. Some, such as the Acoustic Complexity Index, failed to reveal any expected differences between sites or times of day, while the Bioacoustic Index could only identify the most acoustically active reef, otherwise having weak correlations to visual metrics. Of the indices tested, root-mean-squared sound pressure level and Acoustic Entropy, both calculated in the low frequency fish band (50–1,200 Hz), showed the strongest association with visual health measures. These findings present an important step toward using soundscape cues for reef health assessments. The limited generalizability of acoustic indices across different locations emphasizes the need for caution in their application. Therefore, it is crucial to improve methods utilizing fish sounds, such as automatic fish call detectors that are able to generalize well to new soundscapes.

Recognition of bird species with birdsong records using machine learning methods.

The recognition of bird species through the analysis of their vocalizations is a crucial aspect of wildlife conservation and biodiversity monitoring. In this study, the acoustic features of Certhia americana, Certhia brachydactyla, and Certhia familiaris were calculated including the Acoustic complexity index (ACI), Acoustic diversity index (ADI), Acoustic evenness index (AEI), Bioacoustic index (BI), Median of the amplitude envelop (MA), and Normalized Difference Soundscape Index (NDSI). Three machine learning models, Random Forest (RF), Support Vector Machine (SVM), and Extreme Gradient Boosting (XGBoost), were constructed. The results showed that the XGBoost model had the best performance among the three models, with the highest accuracy (0.8365) and the highest AUC (0.8871). This suggests that XGBoost is an effective tool for bird species recognition based on acoustic indices. The study provides a new approach to bird species recognition that utilizes sound data and acoustic characteristics.

Assessing the effect of zoo closure on the soundscape using multiple acoustic indicators

The zoo soundscape has important implications for animal welfare, management, and conservation. However, despite its importance, the zoo soundscape is yet to be examined in depth. Consistent human presence can influence the zoo soundscape. However, it is difficult to determine the specific impact of human presence, as visitors are usually present during the day when animals are active. The COVID-19 lockdown in 2020 provided a unique opportunity to study zoo soundscapes in the absence of visitors. The main aim of this study was to compare the sound environment across three zoo aviaries during the 2020 closure period to a comparable period in 2019 in which the zoo was open. We examined broad band frequency measures of sound pressure levels, sound pressure levels in defined frequency bands, and ecoacoustic indices (the Acoustic Complexity Index and Normalized Difference Soundscape Index) to describe the zoo soundscape. Ecoacoustic indices have not, to our knowledge, previously been used in the zoo setting, although they may provide a useful metric to assess zoo soundscapes. Therefore, we used this natural experiment to explore how successful these measures may be in assessing sound in zoo environments. We found that, during the zoo closure period, the overall sound pressure levels were lower (by 4.4 – 6.4 dB(Z) depending on aviary), and this effect was particularly pronounced in the lower frequency bands. The proportion of sound energy at low frequencies was also lower during the zoo closure period in two of the three aviaries. We argue that NDSI could be a useful index for determining the impact of human presence in zoos, although further information on how it is influenced by additional factors, such as human speech, would be beneficial. The use of multiple indices to assess the sound environment can provide additional information beyond traditional measures of sound levels in zoos, such as frequencies where sound energy is concentrated and characteristics of the soundscape, which could be used to better target management and mitigation.

Discovering ecoacoustic codes in beehives: First evidence and perspectives

The sounds present inside a beehive originate from the overlap of honeybee buzzes with external sounds. They reveal patterns that support the hypothesis that the sonic context of the beehive may be utilized by honeybees as a source of ecoacoustic codes for communication and the coordination of social activity. Patterns were observed in a data series of acoustic files sampled at a frequency of 48 kHz during the period May–July 2023 in a beehive of Apis mellifera ligustica (Spinola, 1806). The acoustic information was extracted using the acoustic complexity index (ACItf) algorithm applied to a fast Fourier transform matrix. Data series, aggregated in 1368 min × 512 frequency bins × 61 days, were tentatively classified according to three temporal classes of aggregation (eight, six, and four clusters, respectively) using the hierarchical K-means clustering algorithm. The clusters obtained at these three resolutions were considered potential ecoacoustic codes (PECs) belonging to each minute of the data series.The number of discontinuities along the 24-h PEC sequence, the coefficient of variation of the number of PECs at daily and seasonal scales, and the PEC sample entropy confirmed a patterned distribution of PECs across the 24 h, modulated at a monthly scale. A significant correlation was found between these indices and the daily average wind speed, and temperature. Honeybee buzz is an informative medium used by honeybees to develop survival strategies.

The soundscape method and eco-acoustic indicators in the analysis of the acoustic environment of the mountain national parks in Poland and Sweden

Soundscape is one of the essential environmental elements affecting people’s experience in nature spaces. In the complex mechanism of landscape and soundscape perception, the acoustic characteristics of the environment are the primary influencing factor. The aim of the paper is the analysis of soundscape of two mountain national parks—Tatra National Park (TNP) in Poland and Sarek National Park (SNP) in Sweden. The first park is very frequently visited by tourists (around 4 million tourists a year) while the second park has an attendance of around a few thousand tourists. The paper presents the results of the analysis of acoustic measurements and ambisonic recordings made in the Koscieliska Valley (TNP) and the Rapa Valey (SNP, Swedish: Rapadalen) using classical method and the soundscape method. In addition, psychoacoustic parameters and soundecology indicators such as loudness, sharpness or roughness, ACI (acoustic complexity index), NDSI (normalized difference soundscape index), BIO (bioacoustic index), ADI (acoustic diversity index), and AEI (acoustic evenness index) were calculated.