Acoustic Entropy Research Articles

Acoustic Entropy of the Materials in the Course of Degradation

We report experimental observations on the evolution of acoustic entropy in the course of cyclic loading as degradation occurs due to fatigue. The measured entropy is a result of the materials’ microstructural changes that occur as degradation due to cyclic mechanical loading. Experimental results demonstrate that maximum acoustic entropy emanating from materials during the course of degradation remains similar. Experiments are shown for two different types of materials: Aluminum 6061 (a metallic alloy) and glass/epoxy (a composite laminate). The evolution of the acoustic entropy demonstrates a persistent trend over the course of degradation.

Ecoacoustic indices as proxies for biodiversity on temperate reefs

Summary Diversity measurement techniques can present logistical and financial obstacles to conservation efforts. Ecoacoustics has recently emerged as a promising solution to these issues, providing a mechanism for measuring diversity using acoustic indices, which have proven to be beneficial in terrestrial habitats. This study investigates the application of acoustic measures as a tool for quick and effective marine diversity monitoring via direct, in situ comparison of ecoacoustics indices with species assemblage diversity measures from temperate rocky reefs. Acoustic recordings and visual surveys of reef fish abundance were collected at nine sites in north‐eastern New Zealand. Three ecoacoustic indices originally developed for terrestrial use were then compared to three species assemblage diversity measures and compared using Pearson correlations. Additionally, four criteria for successful ecoacoustic indices were developed and tested as a means of standardizing future evaluation and use of acoustic indices: (i) positive correlations between species diversity and ecoacoustic indices in relevant frequency ranges, (ii) robustness to changes in spectral resolution, (iii) robustness to the presence of natural noise interference (i.e. wind) and (iv) robustness to the presence of anthropogenic noise. Acoustic Complexity Index (ACI) was significantly correlated with Pielou's Evenness (J′) and Shannon's index (H′). Neither Acoustic Richness (AR) nor ACI was impacted by changes in spectral resolution, but values of the Acoustic Entropy Index (H) increased significantly between fast Fourier transformation (FFT) sizes 512 and 1024. H was consistently positively correlated with both H′ and estimated number of species (S) above a spectral resolution of c. 140·6 Hz (FFT size 1024). Wind did not affect any of the acoustic indices. As anthropogenic noise was included in these investigations, both ACI and H were considered robust to its presence. While AR failed to meet all four criteria for a successful ecoacoustic indices, both ACI and H appeared to be appropriate for use on temperate reefs. In a time of accelerated global diversity loss, these two ecoacoustic indices show strong potential for use as efficient, non‐invasive marine diversity measures.

Connecting soundscape to landscape: Which acoustic index best describes landscape configuration?

Soundscape assessment has been proposed as a remote ecological monitoring tool for measuring biodiversity, but few studies have examined how soundscape patterns vary with landscape configuration and condition. The goal of our study was to examine a suite of published acoustic indices to determine whether they provide comparable results relative to varying levels of landscape fragmentation and ecological condition in nineteen forest sites in eastern Australia. Our comparison of six acoustic indices according to time of day revealed that two indices, the acoustic complexity and the bioacoustic index, presented a similar pattern that was linked to avian song intensity, but was not related to landscape and biodiversity attributes. The diversity indices, acoustic entropy and acoustic diversity, and the normalized difference soundscape index revealed high nighttime sound, as well as a dawn and dusk chorus. These indices appear to be sensitive to nocturnal biodiversity which is abundant at night in warm, subtropical environments. We argue that there is need to better understand temporal partitioning of the soundscape by specific taxonomic groups, and this should involve integrated research on amphibians, insects and birds during a 24h cycle. The three indices that best connected the soundscape with landscape characteristics, ecological condition and bird species richness were acoustic entropy, acoustic evenness and the normalized difference soundscape index. This study has demonstrated that remote soundscape assessment can be implemented as an ecological monitoring tool in fragmented Australian forest landscapes. However, further investigation should be dedicated to refining and/or combining existing acoustic indices and also to determine if these indices are appropriate in other landscapes and for other survey purposes.

Assessing marine ecosystem acoustic diversity across ocean basins

Concurrent with the elevation of the concern over the state of sound in the ocean, advances in terrestrial acoustic monitoring techniques have produced concepts and tools that may be applicable to the underwater world. Several index values that convey information related to acoustic diversity with a single numeric measurement made from acoustic recordings have been proposed for rapidly assessing community biodiversity. Here we apply the acoustic biodiversity index method to low frequency recordings made from three different ocean basins to assess its appropriateness for characterizing species richness in the marine environment. Initial results indicated that raw acoustic entropy (H) values did not correspond to biological patterns identified from individual signal detections and classification. Noise from seismic airgun activity masked the weaker biological signals and confounded the entropy calculation. A simple background removal technique that subtracted an average complex spectrum characteristic of seismic exploration signals from the average spectra of each analysis period that contained seismic signals was applied to compensate for salient seismic airgun signals present in all locations. The noise compensated (HN) entropy index was more reflective of biological patterns and holds promise for the use of rapid acoustic biodiversity in the marine environment as an indicator of habitat biodiversity and health.

Assessing the cross platform performance of marine mammal indicators between two collocated acoustic recorders

Equipment and deployment strategies for remote passive acoustic sensing of marine environments must balance memory capacity, power requirements, sampling rate, duty-cycle, deployment duration, instrument size, and environmental concerns. The impact of different parameters on the data and applicability of the data to the specific questions being asked should be considered before deployment. Here we explore the effect of recording and detection parameters on marine mammal acoustic data across two platforms. Daily classifications of marine mammal vocalizations from two passive acoustic monitors with different subsampling parameters, an AURAL and a Passive Aquatic Listener (PAL), collocated in the Bering Sea were compared. The AURAL subsampled on a pre-set schedule, whereas the PAL sampled via an adaptive protocol. Detected signals of interest were manually classified in each dataset independently. The daily classification rates of vocalizations were similar. Detections from the higher duty-cycle but lower sample rate AURAL were limited to species and vocalizations with energy below 4kHz precluding detection of echolocation signals. Temporal coverage from the PAL audio files was limited by the adaptive sub-sampling protocol. A method for classifying ribbon (Histriophoca fasciata) and bearded seal (Erignathus barbatus) vocalizations from the sparse spectral time histories of the PAL was developed. Although application of the acoustic entropy as a rapid assessment of biodiversity was not reflective of the number of species detected, acoustic entropy was robust to changes in sample rate and window length.

Long-term passive acoustic monitoring of nearshore ecosystems in the Northwestern Hawaiian Islands

Monitoring the changing state of marine habitats in remote areas is, in most cases, a challenging task due to limited and/or infrequent opportunities to make direct observations. Passive acoustic monitoring is sometimes the best means of establishing long-term biological trends in such areas. Since 2006, an effort has been underway to monitor the neashore ecosystems of the Northwestern Hawaiian Islands (NWHI) using a network of Ecological Acoustic Recorders. A wide range of acoustic signals are being monitored to infer biological trends and to gauge the relative stability of the ecosystem. Among the variables measured are the acoustic activity of snapping shrimp, the incidence of cetaceans and the extent of spectral and temporal partitioning of the acoustic space by different taxa, measured as the “acoustic entropy” of the habitat. Multiyear time series of the different measures provide baseline levels of biological activity at each location and also reveal periods of anomaly. Observed trends are then exa...

Signal detection as a function of relative acoustic entropy.

Natural sounds are far from random. Instead, they possess structure that reflects physical constraints on sound-producing objects and events. Listeners likely exploit such structure (predictability and redundancy) when perceiving complex natural sounds. The present experiments investigate listeners’ ability to discover and use structure in novel sounds to identify a repeating target sound against a competing background of sounds. Complementary signal-processing strategies independently varied acoustic entropy (complement to redundancy) in frequency or time. Novel sounds were generated by stretching the entropy in a restricted frequency band (instantaneous frequency dilation) or temporal slice (phase vocoding) of pink (1/f) noise across the bandwidth or duration, respectively, of an unstretched noise sample. Listeners matched a probe sound to a target sound (1%, 3.2%, or 10% entropy of pink noise) that repeats amidst distractor sounds (1%, 10%, or 100% entropy) at 0-dB SNR. In separate experiments varying entropy in frequency or time, identification of target sounds depended critically on the difference in entropy across targets and distractors. Listeners were more proficient identifying lower-entropy targets against higher-entropy distractors but not lower-entropy distractors. Similarly, higher-entropy targets were better identified amidst lower-entropy distractors. Potential implications for cochlear implant processing will be discussed. [Work supported by NIDCD.]

Rapid Acoustic Survey for Biodiversity Appraisal

Biodiversity assessment remains one of the most difficult challenges encountered by ecologists and conservation biologists. This task is becoming even more urgent with the current increase of habitat loss. Many methods–from rapid biodiversity assessments (RBA) to all-taxa biodiversity inventories (ATBI)–have been developed for decades to estimate local species richness. However, these methods are costly and invasive. Several animals–birds, mammals, amphibians, fishes and arthropods–produce sounds when moving, communicating or sensing their environment. Here we propose a new concept and method to describe biodiversity. We suggest to forego species or morphospecies identification used by ATBI and RBA respectively but rather to tackle the problem at another evolutionary unit, the community level. We also propose that a part of diversity can be estimated and compared through a rapid acoustic analysis of the sound produced by animal communities. We produced α and β diversity indexes that we first tested with 540 simulated acoustic communities. The α index, which measures acoustic entropy, shows a logarithmic correlation with the number of species within the acoustic community. The β index, which estimates both temporal and spectral dissimilarities, is linearly linked to the number of unshared species between acoustic communities. We then applied both indexes to two closely spaced Tanzanian dry lowland coastal forests. Indexes reveal for this small sample a lower acoustic diversity for the most disturbed forest and acoustic dissimilarities between the two forests suggest that degradation could have significantly decreased and modified community composition. Our results demonstrate for the first time that an indicator of biological diversity can be reliably obtained in a non-invasive way and with a limited sampling effort. This new approach may facilitate the appraisal of animal diversity at large spatial and temporal scales.