A comparison of three noise reduction procedures applied to bird vocal signals

ABSTRACT.   Recordings of avian vocal signals in natural habitats include ambient noise. Often this background noise corrupts across all frequencies and is of substantial amplitude. Reducing this ambient noise to prepare vocal signals for playback stimuli or to remove habitat-specific noise signatures prior to analyzing a signal's acoustic characteristics can be useful. We conducted experimental evaluations of three noise reduction procedures to determine their effectiveness. We embedded two bird vocalizations ("clean" signals) in four kinds of natural noise, resulting in eight noise-signal combinations. We then applied three noise reduction procedures (Noise Profile, Band Pass, and Noise Estimate) to each of the embedded signals and compared the recovered signals to the original (clean) signals. Noise Profile filtering was effective in reducing noise and returning fairly high-quality signals from even severe levels of masking noise. The other two noise reduction procedures did not perform as well. For the two most corrupting maskers, however, Noise Profile filtering also altered the signal properties by reducing signal amplitude at those frequencies containing high levels of noise. Apart from this loss of amplitude, the quantitative features of the filtered signals were similar to those of the original model sounds. We conclude that Noise Profile filtering produces good results for cases where noise is approximately constant over the signal duration and the signal intensity exceeds noise intensity over the frequencies of interest.     

Effective pre-processing of long term noisy audio recordings: An aid to clinical monitoring

Nowadays, great attention is devoted to minimizing the discomfort caused by connection of patients to sensors for long-term monitoring of physiological parameters. Hence, the need for contact-less monitoring systems is increasingly recognized in clinical investigation. To this aim, audio signals recorded by ambient microphones are an appealing and increasing field of research: in the biomedical field, application of contact-less audio recording of long duration may concern obstructive apnoea syndrome, preterm newborns in Intensive Care Units, daily monitoring in occupational dysphonia, speech therapy, Parkinson and Alzheimer disease, monitoring of psychiatric and autistic subjects, etc. However, a significant amount of ambient noise is inevitably included in the records.Especially in the case of recordings that take a long time, manual extraction of clinically useful information from a whole record is a time-consuming operator-dependent task, the length of a whole recording (even several hours) being prohibitive both for perceptual analysis made by listening to it and for visual inspection of signal patterns. Moreover, objective measures of signal characteristics may serve clinicians as a common ground for diagnosis. Hence, automatic methods are needed to speed up and objectify the analysis task.The present work describes a new, automatic, fast and reliable method for extracting “voiced candidates” from audio recordings of long duration for both clinical and home applications.To demonstrate its effectiveness, the method is compared to existing software tools commonly used in biomedical applications using synthetic signals.     

Your attention please: increasing ambient noise levels elicits a change in communication behaviour in humpback whales (Megaptera novaeangliae)

High background noise is an important obstacle in successful signal detection and perception of an intended acoustic signal. To overcome this problem, many animals modify their acoustic signal by increasing the repetition rate, duration, amplitude or frequency range of the signal. An alternative method to ensure successful signal reception, yet to be tested in animals, involves the use of two different types of signal, where one signal type may enhance the other in periods of high background noise. Humpback whale communication signals comprise two different types: vocal signals, and surface-generated signals such as ‘breaching’ or ‘pectoral slapping’. We found that humpback whales gradually switched from primarily vocal to primarily surface-generated communication in increasing wind speeds and background noise levels, though kept both signal types in their repertoire. Vocal signals have the advantage of having higher information content but may have the disadvantage of loosing this information in a noisy environment. Surface-generated sounds have energy distributed over a greater frequency range and may be less likely to become confused in periods of high wind-generated noise but have less information content when compared with vocal sounds. Therefore, surface-generated sounds may improve detection or enhance the perception of vocal signals in a noisy environment.     

ACOUSTIC COMMUNICATION AND MATING BEHAVIOUR IN WATER BUGS OF THE GENUS MICRONECTA

ABSTRACT

The role of male sounds in attracting females and in mating was investigated in the three most common species of Micronecta found in ponds 60 km NE of Melbourne, Australia: M. concordia, M. tasmanica and M. robusta. In playback experiments using recorded male signals, females were attracted to signals of conspecific males, in preference to signals of heterospecific sympatric males. Studies of mating behaviour, using video recording, showed that signals were obligatory for mating. These findings strongly suggest that acoustic signalling is important in reproductive isolation in Micronecta. Comparisons between matings and rejected matings showed that, within each species, copulation only occurred following a certain minimum number of acoustic signals with more pulse-trains than the mean for the species. Low amplitude sounds (after signals) were also important in courtship, immediately preceding mating. No sounds occurred during copulation.     

Acoustic behavior of the damselfish Dascyllus albisella: behavioral and geographic variation

Behavioral and geographic variation in animal communication has been well-studied in insects, frogs, birds, and mammals, but little is known about variation in fishes. We used underwater audio-video recordings of the behavior and associated sounds produced by the domino damselfish, Dascyllus albisella, at Johnston Atoll and Hawaii, which are separated by 1000 km, to study behavioral and geographic variation in communication sounds. Males produced pulsed sounds during the courtship behavior known as the signal jump, visiting by females (during pseudospawning), mating, aggression to heterospecifics and conspecifics, and nest preparation. Females made only aggressive sounds. The following features of the sounds were measured: number of pulses, pulse rate, pulse duration, inter-pulse interval, dominant frequency, and frequency envelope. The only difference between visiting and mating sounds was a small difference in pulse duration. Two types of aggressive sounds were produced, pops and chirps. Pops contained only one or two pulses and were more commonly made towards heterospecifics than conspecifics. Aggressive chirps had between 3–11 pulses and were made most often towards conspecifics. The pulse rate of aggressive chirps was faster than signal jump sounds. The only difference in signal jump sounds made by males from Johnston Atoll and Hawaii, was a small difference in pulse duration, which was likely due to differences in the depths of the recording environment and not in the sounds produced.     

Acoustic localization of terrestrial wildlife: Current practices and future opportunities

Autonomous acoustic recorders are an increasingly popular method for low‐disturbance, large‐scale monitoring of sound‐producing animals, such as birds, anurans, bats, and other mammals. A specialized use of autonomous recording units (ARUs) is acoustic localization, in which a vocalizing animal is located spatially, usually by quantifying the time delay of arrival of its sound at an array of time‐synchronized microphones. To describe trends in the literature, identify considerations for field biologists who wish to use these systems, and suggest advancements that will improve the field of acoustic localization, we comprehensively review published applications of wildlife localization in terrestrial environments. We describe the wide variety of methods used to complete the five steps of acoustic localization: (1) define the research question, (2) obtain or build a time‐synchronizing microphone array, (3) deploy the array to record sounds in the field, (4) process recordings captured in the field, and (5) determine animal location using position estimation algorithms. We find eight general purposes in ecology and animal behavior for localization systems: assessing individual animals' positions or movements, localizing multiple individuals simultaneously to study their interactions, determining animals' individual identities, quantifying sound amplitude or directionality, selecting subsets of sounds for further acoustic analysis, calculating species abundance, inferring territory boundaries or habitat use, and separating animal sounds from background noise to improve species classification. We find that the labor‐intensive steps of processing recordings and estimating animal positions have not yet been automated. In the near future, we expect that increased availability of recording hardware, development of automated and open‐source localization software, and improvement of automated sound classification algorithms will broaden the use of acoustic localization. With these three advances, ecologists will be better able to embrace acoustic localization, enabling low‐disturbance, large‐scale collection of animal position data.     

Seismic communication between the burrows of kangaroo rats, Dipodomys spectabilis

Banner-tailed kangaroo rats, Dipodomys spectabilis, footdrum to produce substrate-borne and airborne acoustic energy. Previous studies show that they communicate territorial ownership via airborne footdrumming signals. The research reported here used simulated footdrum patterns generated by an artificial `thumper' to address the question of whether kangaroo rats communicate through seismic components of these acoustic signals. With microphones suspended in sealed burrows, we found that airborne sounds were attenuated by approximately 40 dB as they passed through the burrow wall into the burrow chamber. The substrate-borne vibrations from the thumper yielded sound approximately 40 dB greater in peak amplitude than the attenuated airborne sound. Thus, 99.9% of the peak power of the thumper was transmitted directly through the substrate into the burrow. The rats in sealed burrows timed their responses to playbacks of footdrums from the thumper and a loudspeaker so they did not initiate a drumming sequence during either the seismic or airborne signals. When these signals were masked by loud noise, the rats continued to drum to the seismic signal but drummed randomly during the airborne playback. These results suggest that the sealed burrow provides a quiet place in which D. spectabilis can listen for substrate-borne communications from conspecifics. Accepted: 13 May 1997     

Microphone Handling Noise: Measurements of Perceptual Threshold and Effects on Audio Quality

A psychoacoustic experiment was carried out to test the effects of microphone handling noise on perceived audio quality. Handling noise is a problem affecting both amateurs using their smartphones and cameras, as well as professionals using separate microphones and digital recorders. The noises used for the tests were measured from a variety of devices, including smartphones, laptops and handheld microphones. The signal features that characterise these noises are analysed and presented. The sounds include various types of transient, impact noises created by tapping or knocking devices, as well as more sustained sounds caused by rubbing. During the perceptual tests, listeners auditioned speech podcasts and were asked to rate the degradation of any unwanted sounds they heard. A representative design test methodology was developed that tried to encourage everyday rather than analytical listening. Signal-to-noise ratio (SNR) of the handling noise events was shown to be the best predictor of quality degradation. Other factors such as noise type or background noise in the listening environment did not significantly affect quality ratings. Podcast, microphone type and reproduction equipment were found to be significant but only to a small extent. A model allowing the prediction of degradation from the SNR is presented. The SNR threshold at which 50% of subjects noticed handling noise was found to be 4.2 ± 0.6 dBA. The results from this work are important for the understanding of our perception of impact sound and resonant noises in recordings, and will inform the future development of an automated predictor of quality for handling noise.     

Sounds produced by the cichlid fish Metriaclima zebra allow reliable estimation of size and provide information on individual identity

Sounds produced by male cichlids Metriaclima zebra during aggressive interactions were recorded to conduct a detailed analysis and to search for potential individual acoustic signatures. Fish from two different size groups (small and large individuals) were analysed. The two groups were significantly different for all acoustic variables considered; six of seven features demonstrated a significant interindividual variability and most of them were correlated with the size of the emitter. A cross-validated and permuted discriminant function analysis (pDFA) separated the two groups and correctly classified around 50% of the sounds to the correct individuals. Acoustic features that best distinguished among males were the instantaneous frequency of sounds and the modulation of pulse amplitude. These results suggest that acoustic signals could bear information about individual identity. The long-term stability of this signature is likely to be weak since the signature of a growing individual may change over time.     

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.     

Acoustic communication in two freshwater gobies: the relationship between ambient noise,hearing thresholds and sound spectrum

Two freshwater gobies Padogobius martensii and Gobius nigricans live in shallow (5-70 cm) stony streams, and males of both species produce courtship sounds. A previous study demonstrated high noise levels near waterfalls, a quiet window in the noise around 100 Hz at noisy locations, and extremely short-range propagation of noise and goby signals. To investigate the relationship of this acoustic environment to communication, we determined audiograms for both species and measured parameters of courtship sounds produced in the streams. We also deflated the swimbladder in P. martensii to determine its effect on frequency utilization in sound production and hearing. Both species are maximally sensitive at 100 Hz and produce low-frequency sounds with main energy from 70 to 100-150 Hz. Swimbladder deflation does not affect auditory threshold or dominant frequency of courtship sounds and has no or minor effects on sound amplitude. Therefore, both species utilize frequencies for hearing and sound production that fall within the low-frequency quiet region, and the equivalent relationship between auditory sensitivity and maximum ambient noise levels in both species further suggests that ambient noise shapes hearing sensitivity.     

Regulation of vocal amplitude by the blue-throated hummingbird, Lampornis clemenciae

Animals that rely on vocal communication must broadcast sound so that a perceptible signal is transmitted over an appropriate distance. We found that male blue-throated hummingbirds modified the amplitude of their vocalizations in response to both naturally occurring and experimenter-controlled changes in ambient noise levels. This phenomenon is known as the Lombard effect and may increase the efficiency of acoustic signalling. This study demonstrates the effect under natural field conditions and documents the first hummingbird species (Apodiformes: Trochilidae) to show this behaviour. We measured sound pressure levels (SPLs) of Serial Chip territorial advertisement calls across a natural range of ambient noise, primarily due to creeks within male territories. We found a significant correlation between the amplitude of Serial Chips and the amplitude of background noise. To test this relationship, we broadcast recordings of creek noise at high and low amplitudes while target individuals were producing Serial Chip vocalizations. We measured vocal SPLs before and during the playback. Individuals responded to changes in playback creek noise by changing the amplitude of Serial Chip production. We also measured transmission properties of Serial Chip calls through natural habitat to calculate the approximate amplitude of vocalizations at the position of the calling bird. We suggest that amplitude regulation of vocalizations contibutes to signal transmission distance along with the established relationships between singing behaviour, acoustic structure and habitat. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.     

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 (H_N) 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.     

Adaptations to the acoustic environment by the squirrelfishes Myripristis violaceus and M. Pralinius

Three sounds naturally produced by squirrelfish of the genus Myripristis were recorded and analyzed sonographically. Captive Myripristis violaceus responded acoustically and behaviorally to playbacks of calls by conspecifics. Acoustic characteristics (velocity and pressure levels, and their attenuation as a function of frequency and distance) of grunt sounds, produced by hand held fish, were determined for M. violaceus and M. pralinius. Background noise components were analyzed for four different environments of these fish. Background noise and grunt sounds had high velocity levels, relative to pressure levels, expected in acoustic near fields, but attenuated at rates characteristic of acoustic far fields. Electrophysiological recordings from the lateral line organs of M. violaceus indicated that the lateral line system is directionally sensitive to a vector component (e.g., displacement or velocity) of the sound field, and is capable of mediating the observed behavioral responses.     

Songbird tap dancing produces non-vocal sounds*

Vocalizations have been elucidated in previous songbird studies, whereas less attention has been paid to non-vocal sounds. In the blue-capped cordon-bleu (Uraeginthus cyanocephalus), both sexes perform courtship displays that are accompanied by singing and distinct body movements (i.e. dance). Our previous study revealed that their courtship bobbing includes multiple rapid steps. This behaviour is quite similar to human tap dancing, because it can function as both visual and acoustic signals. To examine the acoustic signal value of such steps, we tested if their high-speed step movements produce non-vocal sounds that have amplitudes similar to vocal sounds. We found that step behaviour affected step sound amplitude. Additionally, the dancing step sounds were substantially louder than feet movement sounds in a non-courtship context, and the amplitude range overlapped with that of song notes. These results support the idea that in addition to song cordon-bleus produce acoustic signals with their feet.     

Overview on the Diversity of Sounds Produced by Clownfishes (Pomacentridae): Importance of Acoustic Signals in Their Peculiar Way of Life

Background

Clownfishes (Pomacentridae) are brightly colored coral reef fishes well known for their mutualistic symbiosis with tropical sea anemones. These fishes live in social groups in which there is a size-based dominance hierarchy. In this structure where sex is socially controlled, agonistic interactions are numerous and serve to maintain size differences between individuals adjacent in rank. Clownfishes are also prolific callers whose sounds seem to play an important role in the social hierarchy. Here, we aim to review and to synthesize the diversity of sounds produced by clownfishes in order to emphasize the importance of acoustic signals in their way of life.

Methodology/Principal Findings

Recording the different acoustic behaviors indicated that sounds are divided into two main categories: aggressive sounds produced in conjunction with threat postures (charge and chase), and submissive sounds always emitted when fish exhibited head shaking movements (i.e. a submissive posture). Both types of sounds showed size-related intraspecific variation in dominant frequency and pulse duration: smaller individuals produce higher frequency and shorter duration pulses than larger ones, and inversely. Consequently, these sonic features might be useful cues for individual recognition within the group. This observation is of significant importance due to the size-based hierarchy in clownfish group. On the other hand, no acoustic signal was associated with the different reproductive activities.

Conclusions/Significance

Unlike other pomacentrids, sounds are not produced for mate attraction in clownfishes but to reach and to defend the competition for breeding status, which explains why constraints are not important enough for promoting call diversification in this group.     

Noise affects porpoise click detections – the magnitude of the effect depends on logger type and detection filter settings

Automatic click detectors and full-bandwidth sound recorders are widely used in passive acoustic monitoring of small cetaceans. Detection of these signals depends on a variety of factors, including signal to noise ratio. Passive acoustic monitoring is often used to study impact of underwater noise on small cetaceans, but as detection probability is affected by changes in signal to noise ratio, variable noise levels may affect conclusions drawn from these experiments. Therefore, we examine how different detectors and filters perform in varying ocean noise conditions. C-PODs and full-bandwidth recorders (Wildlife Acoustics, SM2M+) were deployed at two stations in an environment with fluctuating ambient noise for 42 days. Noise level and harbour porpoise (Phocoena phocoena) click trains simultaneously recorded on both loggers were compared. Overall, we found that porpoise click detections by the algorithm used to analyse full-band recorder data (Pamguard) paralleled detections by the C-POD. However, Pamguard detected significantly more clicks than the C-POD. A decrease in detections was seen for both loggers with increasing noise in the band 20 –160 kHz, in particular for levels above 100 dB re 1μPa rms. We also found that the Pamguard detection function changed the least over varying noise conditions when compared to the C-POD detectors. This study sheds light on the fact that inference of animal presence/absence or density that are based on echolocation cues (here, Porpoise Positive Minutes) shall account for the acoustic environments where probability of detecting signals may be affected by variability in ambient noise levels.     

URBAN NOISE PREDICTS SONG FREQUENCY IN NORTHERN CARDINALS AND AMERICAN ROBINS

ABSTRACT

We examined the extent to which acoustic noise in urban environments influences song characteristics and singing behaviour of Northern Cardinals Cardinalis cardinalis and American Robins Turdus migratorius. We predicted that, in response to loud noise, birds would improve signal transmission by (1) increasing singing rate and (2) adjusting song characteristics such as pitch and length. From May—July 2006, 42 cardinals and 53 robins were recorded in forests located within four acoustic environments in central Ohio: rural, residential, commercial, and highway. Following each recording, we measured ambient noise level and recorded information describing location, weather, habitat, and conspecific presence within 75 m. As predicted, frequency range was positively correlated with noise level for both species, but neither song length nor rate was related to noise level for either species. These data support the idea that anthropogenic noise influences avian singing behaviour and acts as a selective force in urban areas.     

Airborne vs. radio-transmitted vocalizations in two primates: a technical report

Elucidating the structure and function of joint vocal displays (e.g. duet, chorus) recorded with a conventional microphone has proved difficult in some animals owing to the complex acoustic properties of the combined signal, a problem reminiscent of multi-speaker conversations in humans. Towards this goal, we set out to simultaneously compare air-transmitted (AT) with radio-transmitted (RT) vocalizations in one pair of humans and one pair of captive Bolivian grey titi monkeys (Plecturocebus donacophilus) all equipped with an accelerometer – or vibration transducer – closely apposed to the larynx. First, we observed no crosstalk between the two radio transmitters when subjects produced vocalizations at the same time close to each other. Second, compared with AT acoustic recordings, sound segmentation and pitch tracking of the RT signal was more accurate, particularly in a noisy and reverberating environment. Third, RT signals were less noisy than AT signals and displayed more stable amplitude regardless of distance, orientation and environment of the animal. The microphone outperformed the accelerometer with respect to sound spectral bandwidth and speech intelligibility: the sounds of RT speech were more attenuated and dampened as compared to AT speech. Importantly, we show that vocal telemetry allows reliable separation of the subjects’ voices during production of joint vocalizations, which has great potential for future applications of this technique with free-ranging animals.