Transmission characteristics of primate vocalizations: implications for acoustic analyses

Acoustic analyses have become a staple method in field studies of animal vocal communication, with nearly all investigations using computer-based approaches to extract specific features from sounds. Various algorithms can be used to extract acoustic variables that may then be related to variables such as individual identity, context or reproductive state. Habitat structure and recording conditions, however, have strong effects on the acoustic structure of sound signals. The purpose of this study was to identify which acoustic parameters reliably describe features of propagated sounds. We conducted broadcast experiments and examined the influence of habitat type, transmission height, and re-recording distance on the validity (deviation from the original sound) and reliability (variation within identical recording conditions) of acoustic features of different primate call types. Validity and reliability varied independently of each other in relation to habitat, transmission height, and re-recording distance, and depended strongly on the call type. The smallest deviations from the original sounds were obtained by a visually-controlled calculation of the fundamental frequency. Start- and end parameters of a sound were most susceptible to degradation in the environment. Because the recording conditions can have appreciable effects on acoustic parameters, it is advisable to validate the extraction method of acoustic variables from recordings over longer distances before using them in acoustic analyses.     

Comparison of manual and automated methods for identifying target sounds in audio recordings of Pileated, Pale-billed, and putative Ivory-billed woodpeckers

ABSTRACT.   Although offering many benefits over manual recording and survey techniques for avian field studies, automated sound recording systems produce large datasets that must be carefully examined to locate sounds of interest. We compared two methods for locating target sounds in continuous sound recordings: (1) a manual method using computer software to provide a visual representation of the recording as a sound spectrogram and (2) an automated method using sound analysis software preprogrammed to identify specific target sounds. For both methods, we examined the time required to process a 24-h recording, scanning accuracy, and scanning comprehensiveness using four different target sounds of Pileated Woodpeckers ( Dryocopus pileatus ), Pale-billed Woodpeckers ( Campephilus guatemalensis ), and putative Ivory-billed Woodpeckers ( Campehilus principalis ). We collected recordings from the bottomland forests of Florida and the Neotropical dry forests of Costa Rica, and compared manual versus automated cross-correlation scanning techniques. The automated scanning method required less time to process sound recordings, but made more false positive identifications and was less comprehensive than the manual method, identifying significantly fewer target sounds. Although the automated scanning method offers a fast and economic alternative to traditional manual efforts, our results indicate that manual scanning is best for studies requiring an accurate account of temporal patterns in call frequency and for those involving birds with low vocalization rates.     

Comparative bioacoustics: a roadmap for quantifying and comparing animal sounds across diverse taxa

Animals produce a wide array of sounds with highly variable acoustic structures. It is possible to understand the causes and consequences of this variation across taxa with phylogenetic comparative analyses. Acoustic and evolutionary analyses are rapidly increasing in sophistication such that choosing appropriate acoustic and evolutionary approaches is increasingly difficult. However, the correct choice of analysis can have profound effects on output and evolutionary inferences. Here, we identify and address some of the challenges for this growing field by providing a roadmap for quantifying and comparing sound in a phylogenetic context for researchers with a broad range of scientific backgrounds. Sound, as a continuous, multidimensional trait can be particularly challenging to measure because it can be hard to identify variables that can be compared across taxa and it is also no small feat to process and analyse the resulting high-dimensional acoustic data using approaches that are appropriate for subsequent evolutionary analysis. Additionally, terminological inconsistencies and the role of learning in the development of acoustic traits need to be considered. Phylogenetic comparative analyses also have their own sets of caveats to consider. We provide a set of recommendations for delimiting acoustic signals into discrete, comparable acoustic units. We also present a three-stage workflow for extracting relevant acoustic data, including options for multivariate analyses and dimensionality reduction that is compatible with phylogenetic comparative analysis. We then summarize available phylogenetic comparative approaches and how they have been used in comparative bioacoustics, and address the limitations of comparative analyses with behavioural data. Lastly, we recommend how to apply these methods to acoustic data across a range of study systems. In this way, we provide an integrated framework to aid in quantitative analysis of cross-taxa variation in animal sounds for comparative phylogenetic analysis. In addition, we advocate the standardization of acoustic terminology across disciplines and taxa, adoption of automated methods for acoustic feature extraction, and establishment of strong data archival practices for acoustic recordings and data analyses. Combining such practices with our proposed workflow will greatly advance the reproducibility, biological interpretation, and longevity of comparative bioacoustic studies.     

Applications of bioacoustics in animal ecology

In communication animals use a full range of signals: acoustic, visual, chemical, electrical and tactile. The processes involved in how and why animals communicate have long held veritable fascination for scientists. A branch of science concerned with the production of sound and its effects on living organisms is bioacoustics.The main purpose of the present study is to raise and discuss some issues related to the relationship between animals, their sounds and ecology, including presentation of methods of analysis of sound recordings. A better understanding of the relationship between the studied animals will allow for development of a better framework for future research, as well as a better grasp of interactions between different organisms, including humans. The paper discusses the significance of acoustic research in animal ecology and its possible applications in the future. The author also summarizes previous research in the field of sound communication of various animal species.The paper proves that vocalizations of every acoustically communicating animal are threatened by climate change. For marine animals, the source of changes in vocalization abilities is ocean acidification and increased ambient noise, which can affect communication and foraging behavior. For terrestrial animals, changes in precipitation and temperature may result in modifications of the sounds emitted, as well as certain modifications to the auditory system. Together with changes in species distribution due to environmental parameters, cumulatively these factors can cause changes in the entire landscape of acoustics ecosystems. Thanks to acoustic biomonitoring, we can understand how the sounds of entire habitats and acoustic ecosystems will change in response to the changing climate and how it will affect bioacoustics on a global scale.     

Mysterious bio-duck sound attributed to the Antarctic minke whale (Balaenoptera bonaerensis)

For decades, the bio-duck sound has been recorded in the Southern Ocean, but the animal producing it has remained a mystery. Heard mainly during austral winter in the Southern Ocean, this ubiquitous sound has been recorded in Antarctic waters and contemporaneously off the Australian west coast. Here, we present conclusive evidence that the bio-duck sound is produced by Antarctic minke whales (Balaenoptera bonaerensis). We analysed data from multi-sensor acoustic recording tags that included intense bio-duck sounds as well as singular downsweeps that have previously been attributed to this species. This finding allows the interpretation of a wealth of long-term acoustic recordings for this previously acoustically concealed species, which will improve our understanding of the distribution, abundance and behaviour of Antarctic minke whales. This is critical information for a species that inhabits a difficult to access sea-ice environment that is changing rapidly in some regions and has been the subject of contentious lethal sampling efforts and ongoing international legal action.     

RECENT BIOACOUSTIC PUBLICATIONS (mainly 1993)

ABSTRACT

Some acoustic signals produced by small insects are very low in amplitude and attenuate rapidly with distance. To achieve high quality recordings with such signals, the use of specialised microphones or of sound insulation chambers is necessary. This paper presents a simple and efficient method for the recording of acoustic signals emitted by small sources. Its principle is based upon the use of two simultaneous digital recordings from two microphones: one records the ambient noise while the other records the ambient noise plus the signal to analyse. Both these recordings are converted into digital files and then a simple subtraction between the two isolates the signal with a good signal-to-noise ratio. With this method of background noise removal, the recording of low amplitude sounds in an uninsulated room with common microphones becomes possible. We have applied this method to the study of 12 complete courtships of Drosophila melanogaster and particularly to the analysis of pulse sounds produced by the male in presence of a female. The study focuses mainly on the rhythm of production of pulse trains over the course of the courtship.     

Automated identification of field-recorded songs of four British grasshoppers using bioacoustic signal recognition

Recognition of Orthoptera species by means of their song is widely used in field work but requires expertise. It is now possible to develop computer-based systems to achieve the same task with a number of advantages including continuous long term unattended operation and automatic species logging. The system described here achieves automated discrimination between different species by utilizing a novel time domain signal coding technique and an artificial neural network. The system has previously been shown to recognize 25 species of British Orthoptera with 99% accuracy for good quality sounds. This paper tests the system on field recordings of four species of grasshopper in northern England in 2002 and shows that it is capable of not only correctly recognizing the target species under a range of acoustic conditions but also of recognizing other sounds such as birds and man-made sounds. Recognition accuracies for the four species of typically 70-100% are obtained for field recordings with varying sound intensities and background signals.     

Six steps towards operationalising freshwater ecoacoustic monitoring

1. Applications in bioacoustics and its sister discipline ecoacoustics have increased exponentially over the last decade. However, despite knowledge about aquatic bioacoustics dating back to the times of Aristotle and a vast amount of background literature to draw upon, freshwater applications of ecoacoustics have been lagging to date.
2. In this special issue, we present nine studies that deal with underwater acoustics, plus three acoustic studies on water-dependent birds and frogs. Topics include automatic detection of freshwater organisms by their calls, quantifying habitat change by analysing entire soundscapes, and detecting change in behaviour when organisms are exposed to noise.
3. We identify six major challenges and review progress through this special issue. Challenges include characterisation of sounds, accessibility of archived sounds as well as improving automated analysis methods. Study design considerations include characterisation analysis challenges of spatial and temporal variation. The final key challenge is the so far largely understudied link between ecological condition and underwater sound.
4. We hope that this special issue will raise awareness about underwater soundscapes as a survey tool. With a diverse array of field and analysis tools, this issue can act as a manual for future monitoring applications that will hopefully foster further advances in the field.

     

Orientation to distant sounds by foraging big brown bats (Eptesicus fuscus)

Big brown bats (Eptesicus fuscus) detect and orient toward relatively low-frequency sounds produced by chorusing frogs or groups of stridulating insects. This response occurs at distances of at least 600 m. The bats are also attracted to a broadcasted recording of the sounds. If recently fed, they do not orient to these sounds. Bats that fly toward the most intense sound field locate insects at a significantly greater rate than those that choose another direction. This suggests that the use of these long-distance acoustic cues may be important for locating concentrations of flying insects.     

SILIC: A cross database framework for automatically extracting robust biodiversity information from soundscape recordings based on object detection and a tiny training dataset

• 1.Passive acoustic monitoring (PAM) offers many advantages comparing with other survey methods and gains an increasing use in terrestrial ecology, but the massive effort needed to extract species information from a large number of recordings limits its application. The convolutional neural network (CNN) has been demonstrated with its high performance and effectiveness in identifying sound sources automatically. However, requiring a large amount of training data still constitutes a challenge.
• 2.Object detection is used to detect multiple objects in photos or videos and is effective at detecting small objects in a complex context, such as animal sounds in a spectrogram and shows the opportunity to build a good performance model with a small training dataset. Therefore, we developed the Sound Identification and Labeling Intelligence for Creatures (SILIC), which integrates online animal sound databases, PAM databases and an object detection-based model, for extracting information on the sounds of multiple species from complex soundscape recordings.
• 3.We used the sounds of six owl species in Taiwan to demonstrate the effectiveness, efficiency and application potential of the SILIC framework. Using only 786 sound labels in 133 recordings, our model successfully identified the species' sounds from the recordings collected at five PAM stations, with a macro-average AUC of 0.89 and a mAP of 0.83. The model also provided the time and frequency information, such as the duration and bandwidth, of the sounds.
• 4.To our best knowledge, this is the first time that the object detection algorithm has been used to identify sounds of multiple wildlife species. With an online sound-labeling platform embedded and a novel data preprocessing approach (i.e., rainbow mapping) applied, the SILIC shows its good performance and high efficiency in identifying wildlife sounds and extracting robust species, time and frequency information from a massive amount of soundscape recordings based on a tiny training dataset acquired from existing animal sound databases. The SILIC can help expand the application of PAM as a tool to evaluate the state of and detect the change in biodiversity by, for example, providing high temporal resolution and continuous information on species presence across a monitoring network.

     

Are minidisc recorders adequate for the study of respiratory sounds?

Digital audio tape (DAT) recorders have become the de facto gold standard recording devices for lung sounds. Sound recorded on DAT is compact-disk (CD) quality with adequate sensitivity from below 20 Hz to above 20 KHz. However, DAT recorders have drawbacks. Although small, they are relatively heavy, the recording mechanism is complex and delicate, and finding one desired track out of many is inconvenient. A more recent development in portable recording devices is the minidisc (MD) recorder. These recorders are widely available, inexpensive, small and light, rugged, mechanically simple, and record digital data in tracks that may be named and accessed directly. Minidiscs hold as much recorded sound as a compact disk but in about 1/5 of the recordable area. The data compression is achieved by use of a technique known as adaptive transform acoustic coding for minidisc (ATRAC). This coding technique makes decisions about what components of the sound would not be heard by a human listener and discards the digital information that represents these sounds. Most of this compression takes place on sounds above 5.5 KHz. As the intended use of these recorders is the storage and reproduction of music, it is unknown whether ATRAC will discard or distort significant portions of typical lung sound signals. We determined the suitability of MD recorders for respiratory sound research by comparing a variety of normal and pathologic lung sounds that were digitized directly into a computer and also after recording by a DAT recorder and 2 different MD recorders (Sharp and Sony). We found that the frequency spectra and waveforms of respiratory sounds were not distorted in any important way by recording on the two MD recorders tested.     

Diel changes in the movement patterns of Ganges River dolphins monitored using stationed stereo acoustic data loggers

We monitored the underwater movements of Ganges River dolphins using stationed stereo acoustic data loggers. We estimated these movements using changes in the relative angle of the sound source direction (trajectory). Of the total acoustic recordings (66 h), 26.2% contained trajectories of dolphins, and 78.6% of these trajectories involved single animals, suggesting that dolphins tended to swim alone and were localized near the monitoring station. The observed trajectories were categorized as follows: staying type characterized by small changes in the sound source direction, moving type A (moving in the same direction), and moving type B (moving up and down the stream during recording). The average interpulse intervals of sounds in moving types A and B were significantly shorter than that of the staying type, suggesting that dolphins produce the former types of trajectories to echolocate across shorter distances during movement. The frequency of occurrence of moving type A increased during the night, whereas that of type B increased in the late afternoon and that of the staying type increased during the daytime. These results indicate that dolphins moving at night tended to use short‐range echolocation, whereas during the day, they remained in relatively small areas and used long‐range sonar.     

Comparison of first and second heart sounds after mechanical heart valve replacement

In this article, the spectral features of first heart sounds (S1) and second heart sounds (S2), which comprise the mechanical heart valve sounds obtained after aortic valve replacement (AVR) and mitral valve replacement (MVR), are compared to find out the effect of mechanical heart valve replacement and recording area on S1 and S2. For this aim, the Welch method and the autoregressive (AR) method are applied on the S1 and S2 taken from 66 recordings of 8 patients with AVR and 98 recordings from 11 patients with MVR, thereby yielding power spectrum of the heart sounds. Three features relating to frequency of heart sounds and three features relating to energy of heart sounds are obtained. Results show that in comparison to natural heart valves, mechanical heart valves contain higher frequency components and energy, and energy and frequency components do not show common behaviour for either AVR or MVR depending on the recording areas. Aside from the frequency content and energy of the sound generated by mechanical heart valves being affected by the structure of the lungs–thorax and the recording areas, the pressure across the valve incurred during AVR or MVR is a significant factor in determining the frequency and energy levels of the valve sound produced. Though studies on native heart sounds as a non-invasive diagnostic method has been done for many years, it is observed that studies on mechanical heart valves sounds are limited. The results of this paper will contribute to other studies on using a non-invasive method for assessing the mechanical heart valve sounds.     

Baseline data for automated acoustic monitoring of Orthoptera in a Mediterranean landscape,the Hymettos,Greece

Acoustic emissions of animals serve communicative purposes and most often contain species-specific and individual information exploitable to listeners, rendering bioacoustics a valuable tool for biodiversity monitoring. Recording bioacoustic signals allows reproducible species identification. There is a great need for increased use and further development of automated animal sound recording and identification to improve monitoring efficiency and accuracy for the benefit of conservation. Greece, with its high number of endemic species, represents a hotspot for European Biodiversity, including Orthopteran insects. Songs of many Orthoptera might be employed for the inventorying and monitoring of individual species and communities. We assessed the regional spatio-temporal composition of Orthoptera species at the Hymettos near Athens, which is a Natura 2000 site under constant threat due to the surrounding megacity. Within the framework of the EU Life Plus funded AmiBio project, we documented the Orthopteran species’ habitat characteristics, their co-occurrence and phenology. We found, in total, 20 species with seven to ten Orthoptera at locations characterised by diverse vegetation patterns of perennial herbs and bushes. For the purposes of implementation of an automated remote monitoring scheme, we identified sampling sites with high Orthopteran diversity, allowing the monitoring of all singing Orthoptera within single localities. By analysing sound depositories and adding recordings from new sample individuals, we established a song library as prerequisites for future automatic song detection. Based on our results, acoustic recording units have been placed at remote sites at the Hymettos. We discuss recommendations for further studies to fully employ the potential of automated acoustic monitoring of Orthoptera. A reliable assessment of singing Orthoptera needs recording units covering ultrasound. Due to high attenuation and absorbance by the vegetation, particularly of the high frequencies characterising Orthopteran songs, positioning of microphones at sites is critical: the microphone sensor network has to be an order of magnitude denser than for monitoring birds.     

Estimating animal population density using passive acoustics

Reliable estimation of the size or density of wild animal populations is very important for effective wildlife management, conservation and ecology. Currently, the most widely used methods for obtaining such estimates involve either sighting animals from transect lines or some form of capture‐recapture on marked or uniquely identifiable individuals. However, many species are difficult to sight, and cannot be easily marked or recaptured. Some of these species produce readily identifiable sounds, providing an opportunity to use passive acoustic data to estimate animal density. In addition, even for species for which other visually based methods are feasible, passive acoustic methods offer the potential for greater detection ranges in some environments (e.g. underwater or in dense forest), and hence potentially better precision. Automated data collection means that surveys can take place at times and in places where it would be too expensive or dangerous to send human observers. Here, we present an overview of animal density estimation using passive acoustic data, a relatively new and fast‐developing field. We review the types of data and methodological approaches currently available to researchers and we provide a framework for acoustics‐based density estimation, illustrated with examples from real‐world case studies. We mention moving sensor platforms (e.g. towed acoustics), but then focus on methods involving sensors at fixed locations, particularly hydrophones to survey marine mammals, as acoustic‐based density estimation research to date has been concentrated in this area. Primary among these are methods based on distance sampling and spatially explicit capture‐recapture. The methods are also applicable to other aquatic and terrestrial sound‐producing taxa. We conclude that, despite being in its infancy, density estimation based on passive acoustic data likely will become an important method for surveying a number of diverse taxa, such as sea mammals, fish, birds, amphibians, and insects, especially in situations where inferences are required over long periods of time. There is considerable work ahead, with several potentially fruitful research areas, including the development of (i) hardware and software for data acquisition, (ii) efficient, calibrated, automated detection and classification systems, and (iii) statistical approaches optimized for this application. Further, survey design will need to be developed, and research is needed on the acoustic behaviour of target species. Fundamental research on vocalization rates and group sizes, and the relation between these and other factors such as season or behaviour state, is critical. Evaluation of the methods under known density scenarios will be important for empirically validating the approaches presented here.     

饲养状态真鲂(鱼弗)发声的昼夜与季节变化

本文研究了受水温和光周期等自然变化影响的饲养状态真鲂鮄发声的昼夜与季节变化,并研究了实验鱼活动(鱼类游泳次数)的日变化。声音信号的昼夜节律记录发声活动的日常水平(摄食之外的时期),但是每月变化的记录(季节性式型)则在摄食期间进行,因为摄食时声音信号增加,而日常发声活动较不频繁。实验鱼包括雌雄两性,且未达性成熟。真鲂鮄在白天发声多一些,也更活跃。声音为阵发式的,较不频繁(平均值=0.04发声/min每鱼每天)。最少的发声活动出现在晚上,凌晨和黄昏居中(声音的阵发更频繁,但是声音更少),最多的发声活动出现在白天(声音的阵发更频繁,并且含更多数目的声音)。竞争摄食时声音信号的比率不呈现季节性变化(平均值=3.98发声/min每鱼),与温度也不相关,显示出竞争摄食时声音的发出以最大比率进行。敲击声和呼噜声的某些声学特征与温度相关,特别是在较高的温度下呼噜声的节拍间隔急剧下降。敲击声和呼噜声的声音参数中的季节性变化,多数可以解释为发声肌肉和中央声音控制回路的温度效应。     

Vocal Repertoire and Its Behavioral Contexts in the Pied Tamarin, <Emphasis Type="Italic">Saguinus bicolor</Emphasis>

The study of animal sound signals can be useful in assisting conservation strategies. Understanding the vocal repertoires of endangered species and the behavioral contexts in which they are given is relevant for monitoring protocols, such as those based on automated sound recordings. The pied tamarin (Saguinus bicolor) is at risk of extinction because of deforestation and urban growth in its restricted geographic range. Between 2012 and 2015 we studied the vocal repertoire of the species and the contexts in which different signals are emitted. We made focal recordings of eight free-living groups, two rescued individuals, and one temporarily captive group of pied tamarins in Manaus, central Brazilian Amazonia. From the 766 sounds analyzed we identified 12 distinct signals within the range of 2–11 kHz. Most signals were emitted during resting or locomotion. Less frequently emitted signals were associated with intergroup agonistic interactions, foraging, and infant-exclusive vocalizations. These results increased the known vocal repertoire of the pied tamarin providing more reliable baseline data for monitoring the species by means of automated or focal sound recordings.     

The use of on‐animal acoustical recording devices for studying animal behavior

Audio recordings made from free‐ranging animals can be used to investigate aspects of physiology, behavior, and ecology through acoustic signal processing. On‐animal acoustical monitoring applications allow continuous remote data collection, and can serve to address questions across temporal and spatial scales. We report on the design of an inexpensive collar‐mounted recording device and present data on the activity budget of wild mule deer (Odocoileus hemionus) derived from these devices applied for a 2‐week period. Over 3300 h of acoustical recordings were collected from 10 deer on their winter range in a natural gas extraction field in northwestern Colorado. Analysis of a subset of the data indicated deer spent approximately 33.5% of their time browsing, 20.8% of their time processing food through mastication, and nearly 38.3% of their time digesting through rumination, with marked differences in diel patterning of these activities. Systematic auditory vigilance was a salient activity when masticating, and these data offer options for quantifying wildlife responses to varying listening conditions and predation risk. These results (validated using direct observation) demonstrate that acoustical monitoring is a viable and accurate method for characterizing individual time budgets and behaviors of ungulates, and may provide new insight into the ways external forces affect wildlife behavior.     

Automated birdsong recognition in complex acoustic environments: a review

Conservationists are increasingly using autonomous acoustic recorders to determine the presence/absence and the abundance of bird species. Unlike humans, these recorders can be left in the field for extensive periods of time in any habitat. Although data acquisition is automated, manual processing of recordings is labour intensive, tedious, and prone to bias due to observer variations. Hence automated birdsong recognition is an efficient alternative. However, only few ecologists and conservationists utilise the existing birdsong recognisers to process unattended field recordings because the software calibration time is exceptionally high and requires considerable knowledge in signal processing and underlying systems, making the tools less user‐friendly. Even allowing for these difficulties, getting accurate results is exceedingly hard. In this review we examine the state‐of‐the‐art, summarising and discussing the methods currently available for each of the essential parts of a birdsong recogniser, and also available software. The key reasons behind poor automated recognition are that field recordings are very noisy, calls from birds that are a long way from the recorder can be faint or corrupted, and there are overlapping calls from many different birds. In addition, there can be large numbers of different species calling in one recording, and therefore the method has to scale to large numbers of species, or at least avoid misclassifying another species as one of particular interest. We found that these areas of importance, particularly the question of noise reduction, are amongst the least researched. In cases where accurate recognition of individual species is essential, such as in conservation work, we suggest that specialised (species‐specific) methods of passive acoustic monitoring are required. We also believe that it is important that comparable measures, and datasets, are used to enable methods to be compared.     

Population Differentiation in a Complex Bird Sound: A Comparison of Three Bioacoustical Analysis Procedures

We examined three bioacoustical analysis methods for comparing complex sounds among different populations. We chose the D‐syllable of the chick‐a‐dee call of the black‐capped chickadee (Poecile atricapilla) because it is a broadband sound representative of a class of vocalizations, common in many animals, that resists simple subjective classification for comparative studies. We examined the properties of the D‐syllable in field‐recorded samples from three different populations. The first method of data extraction sampled the amplitude values of a spectrum obtained in a single fast Fourier transform (SFFT) taken at the midpoint of each D‐syllable using multi‐speech software. The second method employed spectrogram cross‐correlation (SPCC) to obtain a matrix of similarity values between D‐syllables in the samples using canary software. The third method calculated similarity values obtained from the evaluation of four acoustic features of the D‐syllables derived from multi‐taper spectral analysis (MTSA) using sound analysis software. Following data extraction by these three techniques, we used multivariate statistical procedures to reduce the data for examination of differences among populations and to represent in scatter‐plots the patterns of clustering of the sounds. We found that the SFFT in the middle of the D‐syllable provided the poorest population discrimination following statistical processing, the SPCC method produced the next clearest population separation, and the MTSA method resulted in the most distinct separation of the three populations of D‐syllables. In carrying out these comparisons, we discovered that the characteristic environmental noise of a recording area can influence the signal properties of broadband sounds being compared by automated procedures, and could lead to faulty conclusions unless appropriate care is taken to mitigate the noise in which the signals of interest are embedded. Consequently we re‐analyzed our data following noise reduction and found less discrete population separation overall. However, the methods of SPCC and MTSA retained the ability to separate populations, with MTSA providing the sharpest discrimination among groups.