Long-range Acoustic Communication Research Articles

Ice-tethered acoustic buoys for real-time active and passive monitoring in the Western Arctic

Real-time monitoring of channel impulse response functions in the Beaufort Lens was performed by five Ice-Tethered Acoustic Buoys (ITABs) during March-September 2021. Each ITAB had an eight-element vertical line array and a 900-1000 Hz acoustic source, providing a reciprocal acoustic transmission of up to a 300 km range. Acoustic navigation of one ITAB was successfully tested using the acoustic travel-time from the other ITABs. In addition, a strong single arrival through the Beaufort Lens was observed on each ITAB. Simulations using the measured Q-functions [T. C. Yang, IEEE J. Ocean. Eng. 30(4), 865–880 (2005)] showed a reliable long-range acoustic communication through the Beaufort lens within the 900–1000 Hz frequency band. Finally, a passive ITAB was deployed in March 2022 to measure third-octave under/in-ice ambient noise in the Western Arctic. Both experiments showed that ITABs can be used for under-ice acoustic navigation and communication as well as real-time ambient noise measurements in the Western Arctic. [Work supported by the ONR.]

Long-Range Underwater Communication Based on Time Reversal Processing Using Various Diversity Methods.

Time reversal processing (TRP) exploits signal diversity methods, namely, spatial, temporal, beam, and frequency, to mitigate the distortion caused by multipath time delay. Using the same experimental data, this study compares the performance of communication utilizing TRP based on various diversity methods. In October 2018, the biomimetic long-range acoustic communication experiment 2018 (BLAC18) was conducted in the East Sea, east of Pohang, Korea. During the experiment, communication signals modulated by binary phase-shift keying were transmitted over a range of 60 km, and a vertical line array of 16 elements (with an aperture of ∼42 m) was utilized. The BLAC18 analysis showed that the performance of each diversity method depends on the order of diversity. When the order of diversity was one, the beam diversity method with the beamformed signal yielded the best performance. For the maximum order of diversity, however, the spatial diversity method delivered the best performance, owing to the high channel variability and large number of receivers.

Long-Range Acoustic Communication Using Differential Chirp Spread Spectrum

Here, we propose a new modulation method using chirp spread spectrum (CSS) modulation to indicate the result of long-range acoustic communication (LRAC). CSS modulation had outstanding matched filter characteristics even though the channel was complex. The performance of the matched filter depends on the time–bandwidth product. We studied the method of using the same modulation method while increasing the amount of the time–bandwidth product. When differential encoding is applied, the de-modulation is made using the difference between the current symbol and the previous symbol. If the matched filter is applied using both the current and the previous symbol, such as the use of two symbols, the amount of the time–bandwidth product can be doubled, and this method can make the output of the matched filter larger. The proposed method was verified in lake and sea experiments, in which the experimental environment was analyzed and compared with the result using the channel impulse response (CIR) of the lake and sea. The lake experiment was conducted over a distance of about 100–300 m between the transmitter and receiver and at a depth of ~40 m. As a result of the communication, the conventional method’s bit error rate (BER) was 1.22×10−1, but the proposed method’s BER was 1.98×10−2. The sea experiment was conducted over a distance of ~90 km and at a depth of ~1 km, and the conventional method BER in this experiment was 1.83×10−4, while the proposed method’s BER was 0.

Using playbacks to monitor and investigate the behaviour of wild maned wolves

ABSTRACT Maned wolves are difficult to observe in the wild because of their low densities and their cryptic and crepuscular-nocturnal habits. Exploring their long-range acoustic communication may offer an efficient alternative to study the species. Here we evaluated the applicability of playbacks to study maned wolves in the wild and compare the results with 20 nights of passive recordings on the same area and month during the previous year. We obtained vocal responses on 3 of 6 nights tested, including responses involving two animals and an approach after an interactive playback. Although we conducted 3–6 playback sessions each day at different times, we only obtained vocal responses during sessions between 17:00 and 19:40. During our passive recordings we detected on average 0.86 roar-bark sequences per recorder per night, mostly during the first half of the night. Vocal activity – responses and spontaneous roar-bark sequences – during playback nights was nearly 4 times greater than during the passive recordings. We conclude that playbacks stimulate maned wolves to emit roar-barks and that this method is applicable to test hypotheses about maned wolf behaviour and aid in their monitoring.

Preliminary estimates of year-round acoustic communications potential in the Canada Basin

Long-range acoustic communication networks, which operate over hundreds of kilometers, are important enablers for persistent surveillance in the Arctic. Acoustic communication must take place over such long ranges due to difficulty in deploying communication nodes in such an inhospitable environment. Signal transmission over long ranges is challenged by distortion caused by refraction and multipath addition, so the propagation medium should be understood in order to optimize node placement and estimate the network performance. The Arctic is a unique acoustic propagation environment due to the presence of an ice-covered surface and oceanographic variability around the shelf breaks. To better understand the current state of Arctic propagation, acoustic signals were transmitted during the year-long Canada Basin Acoustic Propagation Experiment (CANAPE). In this talk, signals from the CANAPE data are used to assess the feasibility of year-round, long-range acoustic communications in the Canada Basin by examining several metrics. These metrics include: the signal-to-noise ratio (SNR) of the received signals, time-spreading of the signals due to multipath arrivals, and signal coherence. This talk will focus on the seasonality of these metrics in order to assess how the performance of acoustic communication systems may be altered by temporal variability of the Arctic acoustic propagation characteristics.

The efficiency of territory mapping, point-based censusing, and point-counting methods in censusing and monitoring a bird species with long-range acoustic communication – the Corncrake Crex crex

Capsule A comparison of various methods of counting a Corncrake population revealed that the accuracy of the methods depends on population density, which could have important ramifications for a monitoring scheme.Aims We examined the accuracy of three counting methods – territory mapping, point-based censusing, and point counting – in evaluating the population size and spatial distribution of the Corncrake, a species that uses long-range acoustic communication.Methods We performed single-visit counts within ten study plots (1 km2). Each study plot was censused in a single night using all counting methods. With territory mapping, we approached each calling male and recorded his position with a Global Positioning System (GPS) receiver. In point-based censusing, we estimated the distance and direction to calling males from five points in each study plot, while with the point-counting method, we only recorded the number of males within each of four distance categories, which estimated the distance from the same five counting points in each study plot. To estimate males’ density within the study plot using the last method, we used only the data obtained from the central point of each study plot (middle-point counting approach).Results We found significant differences among the three methods in the number and distribution of males counted within the study plots. The middle-point counting approach consistently underestimated population size regardless of the number of males present, while the point-based census approach overestimated population size when a small number of males were present within a study plot, but underestimated it when males were numerous.Conclusions Our study showed that the accuracy of counting methods may depend on bird density. Moreover, point-based censusing may prove quite inaccurate in pinpointing the locations of calling males. When censusing species with long-range acoustic communication, it is challenging to estimate distance to calling birds, which means that the standard point-counting method should be applied carefully.

Long-Range Double-Differentially Coded Spread-Spectrum Acoustic Communications With a Towed Array

This paper presents a novel double-differentially coded spread-spectrum (DD-SS) system design for long-range acoustic communications (LRAC) between a moving source and a towed horizontal line array (HLA). The proposed design relies on two techniques that have been historically developed for radio-frequency terrestrial wireless communications, namely, direct sequence spread spectrum and double-differential coding. In DD-SS, direct sequence spread spectrum is employed to: 1) increase the signal-to-noise ratio; 2) suppress multipath interference; and 3) support data multiplexing, while double-differential coding makes reliable symbol recovery possible without explicit phase and Doppler tracking/correction. Thanks to the two techniques together with traditional beamforming, the DD-SS system is shown capable of effectively dealing with many challenging issues posed by LRAC with a towed HLA, without using complicated receiver processing. During the 2010 Long-Range Acoustic Communication Experiment (LRAC10) in deep waters, the DD-SS system was tested by using a moving source at a speed of 2-3 kn at 75-m depth and a 64-element HLA towed at 3.5 kn at a depth of 200 m. Excellent uncoded error performance (less than 4% bit error rate) is demonstrated at a data rate of 6.4 b/s for a bandwidth of 200 Hz and at a range of 550 km.

Acoustic communication in deep water exploiting multiple beams with a horizontal array

A recent experiment showed that coherent long-range acoustic communication (200-300 Hz) is feasible in deep water over a ∼550-km range between a source and a horizontal towed array (∼100 m aperture). The low input signal-to-noise ratio (SNR) (e.g., -10 dB) at an element level required conventional beamforming to enhance the SNR of the communication signals for subsequent channel equalization and decoding. This paper demonstrates that with a larger aperture (∼200 m), multiple adjacent beams can be exploited to further improve the communication performance, achieving an almost error-free data rate of 100 bits/s for QPSK modulation at ∼550 km range.

Diversity combining for long-range acoustic communication in deep water

A recent experiment showed that coherent long-range acoustic communication is feasible in deep water over a ∼550 km range between a source towed slowly at ∼75 m depth and a horizontal line array towed at 3.5 knots at ∼200 m depth. This letter further demonstrates that diversity combining mitigates channel fading and increases the output signal-to-noise ratio. Using sparse channel-estimate-based equalization, three transmissions are combined successfully to decode a 40 Hz bandwidth (230-270 Hz) 8 phase-shift-keying communication signal, achieving an effective data rate of 17 bits/s at ∼550 km range.

Long-range time reversal communication in deep water: Experimental results

In December 2011 a long-range acoustic communication experiment was conducted in deep water, west of Izu-Ogasawara Islands, Japan. The experiment involved a stationary source (450-550 Hz) and an 18-element vertical array (102-m aperture), both deployed at around the sound channel axis. Initial analysis of data demonstrates that a data rate of 400 bits/s can be achieved over ~600-km range in deep water using 16 quadrature amplitude modulation and passive time reversal equalization.

Long-range multi-carrier acoustic communication in deep water using a towed horizontal array

During a recent long-range acoustic communication experiment carried out in deep water, multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) communication signals were transmitted with a 50 Hz bandwidth (225-275 Hz) at various source-receiver ranges from 100 to 700 km. The experiment consisted of two mobile components: (1) a source towed slowly at a speed of 2-3 knots at ∼75 m depth and (2) a horizontal line array towed at 3.5 knots at a depth of ∼200 m. In addition to beamforming, an interleaver gain is exploited to compensate for low signal-to-noise ratio at the expense of data rate while providing diversity in the frequency domain. Error-free performance is shown at effective data rates of 15 and 7.5 bits/s at ranges of 550 km and 700 km, respectively, by combining interleaved repetitions with low-density parity-check coding after beamforming, demonstrating the feasibility of multi-carrier OFDM communications in deep water using a towed horizontal array.

Diversity combining for long-range acoustic communication in deep water using a towed array

A recent experiment showed that coherent long-range acoustic communication is feasible in deep water over a ~550-km range between a source towed slowly at 75-m depth and a horizontal line array towed at 3.5 knots at 200-m depth. This paper further demonstrates that diversity combining mitigates channel fading and increases the output SNR. Using sparse channel-estimate-based equalization, three transmissions are combined successfully to decode a 40-Hz bandwidth (230-270 Hz) 8-PSK (phase-shift-keying) communication signal, achieving an effective data rate of 17 bits/s at ~550 km range.

Long-range acoustic communication in deep water with a shallow source and horizontal towed array.

In September 2010 a long-range acoustic communication (LRAC10) experiment was carried out in deep water off the Southern California Coast. The experiment involved two mobile components: (1) a source towed slowly at a speed of 2–3 knots at 75-m depth and (2) a horizontal line array towed at 3.5 knots at a depth of 200-m. Phase-coherent communication sequences were transmitted in the 200–300 Hz frequency band at various ranges. Initial analysis of the LRAC10 data demonstrates that an information rate of 50 bits/s can be achieved over a 550-km range using QPSK modulation and error-correction coding combined with beamforming.

Long-range acoustic communication in deep water using a towed array

In September 2010 a long-range acoustic communication (LRAC10) experiment was carried out in deep water off the Southern California Coast. The experiment involved two mobile components: (1) a source towed slowly at a speed of 2-3 knots at ∼75-m depth and (2) a horizontal line array towed at 3.5 knots at a depth of ∼200 m. Phase-coherent communication sequences were transmitted in the frequency band of 200-300 Hz at various ranges (100-700 km). Initial analysis of the LRAC10 data demonstrates that an information rate of 50 bits/s can be achieved over ∼550-km range using quadrature-phase shift-keying (QPSK) modulation and error-correction coding combined with beamforming.

Alternate tactics in male bladder grasshoppers Bullacris membracioides (Orthoptera: Pneumoridae)

Summary Bladder grasshoppers (Orthoptera, Pneumoridae) are a unique group of nocturnal, specialized herbivores, endemic to coastal regions of Southern Africa. Adult males are characterized by an inflated abdomen, which develops at the final molt, and a signaling system capable of producing high intensity calls detectable by conspecifics at distances of up to 2 km. Female response to male calls leads to reciprocal duetting and pair formation. We have found an alternative form to the dominant inflated male tactic in three pneumorid species. These alternate males are incapable of flight and lack the air-filled abdominal bladder used in long-range acoustic communication; but may be found in copula with mature females in the field. Here we address the issue of neoteny/paedogenesis in the alternate male by comparing the morphology of inflated and uninflated adult male Bullacris membracioides with that of ultimate and penultimate nymphal instars, to determine whether these forms follow a common developmental trajectory. We then compare the behavioral responsiveness of inflated and noninflated morphs to conspecific advertisement calls, to ascertain whether alternate males have the potential to actively exploit the mate location system of duetting pairs. Morphometric analyses indicate that alternate males are a distinctly divergent form from the primary male developmental tactic. We conclude that males following the alternate tactic eavesdrop on the duets of inflated males and females to exploit the acoustic mate location system. This satellite tactic has potential impact on several aspects of pneumorid biology contributing to biasing sexual selection, sensory drive, and ultimately speciation events.

High-frequency sound transmission in natural habitats: implications for the evolution of insect acoustic communication

Transmission and reception of high-frequency sound in the natural environment of bushcrickets (Tettigonia viridissima L.) was studied using the activity of an identified neuron in the insect's auditory pathway as a “biological microphone”. Different positions of the receiver within the habitat were simulated by systematic variation of the distance from a loudspeaker and the height above the ground. Attenuation and filtering properties of the habitat were investigated with pure-tone frequencies between 5 and 40 kHz. Sound attenuation in excess of the attenuation due to geometrical spreading alone increased with increasing frequency, distance between sender and receiver, and decreasing height within the vegetation (Figs. 2–4). The data also confirm the existence of two kinds of excess attenuation. The amount of amplitude fluctuations in the sound signals was investigated by analysing the variability of the neuronal responses at a given receiver position. Variability increased with decreasing bandwidth of a noise signal at some distance from the loadspeaker. The variability in the responses to pure tones increased with both increasing frequency and distance from the source (Fig. 7). In the selected habitat, the temporal pattern of the natural calling song of male T. viridissima was very reliably reflected in the activity of the recorded neuron up to a distance of 30 m at the top of the vegetation, and 15–20 m near ground level (Figs. 5, 8). The maximum hearing distance in response to the calling song was about 40 m. Environmental constraints on long-range acoustic communication in the habitat are discussed in relation to possible adaptations of both the signal structure and the behavior of the insects.

Associations of Song Properties with Habitats for Territorial Oscine Birds of Eastern North America

To investigate adaptations for long-range acoustic communication in birds, I analyzed associations between broad categories of habitats and properties of territorial songs for eastern North American oscines. From published recordings. I obtained three frequency properties (maximal, minimal, and dominant) and three temporal properties of songs (presence of sidebands, presence of buzzes, minimal period of repeated elements). Sidebands and buzzes indicated rapid amplitude modulation of a carrier frequency. Habitats occupied by territorial males were classified into six categories (broad-leaved or mixed forest, coniferous forest, parkland or forest edge, shrubland, grassland, and marshes). Frequencies in songs correlated strongly with body size, which varied among habitats. Analysis of covariance and phylogenetic regression, after controlling for body size, revealed an association of maximal but not dominant or minimal frequencies with habitat. In contrast, the temporal properties of song were all strongly associated with habitat, even within phylogenetic groupings. These results suggest that the temporal properties of songs of many oscines have evolved to reduce the effects of reverberation in forested habitats. Exceptional species might have retained features of song subject to degradation to permit listeners to judge distances to singers. In addition, adaptations for acoustic communication in different habitats might include differences in the perception of songs.

Hearing and communication in blue monkeys ( Cercopithecus mitis)

Hearing and vocal communication in blue monkeys ( Cercopithecus mitis) was studied within an ecological context. Field measurements of the acoustical characteristics of the blue monkey's natural habitat were conducted in the Kibale forest (Uganda) and in Kakamega forest (Kenya). Measurements of background noise levels indicated that vocal communication pitched in the 100–1000-Hz frequency band would be relatively unimpeded by disruptive background noises. Furthermore, measurements of the propagation rate of audio signals indicated that calls pitched in the 125–200-Hz region penetrated the forest with minimal decrement in amplitude. Tests of the blue monkey's acoustic sensitivyty and range of hearing were conducted in the laboratory with standard audiometric procedures. Hearing in the blue monkey was characterized by a U-shaped function, with maximum sensitivity of about 5 dB SPL spanning a four-octave range from 1 to 16 kHz. The hearing of blue monkeys was superior to human hearing for tones below 500 Hz and above 8 kHz in frequency. A comparative analysis of primate hearing indicated that the blue monkey was approximately 18 dB more sensitive to low-frequency tones than the comparably sized, semi-terrestrial rhesus monkey ( Macaca mulatta). Furthermore, blue monkeys exhibit phonatory specializations for vocal production in this relatively unused, low-frequency band of 125–200 Hz. These specializations for low-frequency vocal production and low-frequency hearing collectively act to increase the effective distance of long-range acoustic communication in the forest canopy.

Reverberations and Amplitude Fluctuations in the Propagation of Sound in a Forest: Implications for Animal Communication

Effective communication requires that the receiver not only detect the presence of a signal but also discriminate significant variations in signals. Consequently, both attenuation and degradation of the structure of acoustic signals during transmission will limit the range of communication. In this study we document two primary sources of degradation of acoustic signals during propagation through natural environments, irregular amplitude fluctuations and reverberations. Amplitude fluctuations arise especially from atmospheric turbulence, while reverberations also result from scattering surfaces, such as vegetation. Both primarily mask information coded in amplitude modulation of the signal and repetitive frequency modulation, like the trills in the songs of many passerine birds. Irregular amplitude fluctuations primarily mask low frequencies of amplitude modulation in signals. Atmospheric turbulence from wind is the primary determinant of the intensity of irregular amplitude fluctuations, although amplitude fluctuations also increase with carrier frequency and range. In contrast, reverberations depend primarily on carrier frequency and range. Reverberations are least at intermediate frequencies (2-8 kHz). At lower frequencies reverberations in sound transmission near the ground often take the form of discrete echoes, probably from canopy foliage or from the change in acoustic impedance between air in the canopy and overlying air masses. At higher frequencies reverberations usually consist of a steady decay in acoustic energy. Consequently, in contrast to irregular amplitude fluctuations, reverberations primarily mask high rates of amplitude modulation and repetitive frequency modulation in acoustic signals. Intermediate frequencies (2-8 kHz) are most suitable for long-range acoustic communication, because irregular amplitude fluctuations, reverberations, and attenuation increase with carrier frequency, while reverberations and attenuation from ground interference increase at low frequencies. The great majority of animals that engage in long-range acoustic communication use this middle range of frequencies. Perhaps because of the increase in reverberations at low carrier frequencies, the songs of rufous-sided towhees show a correlation between trill rate and the minimum frequency in trills. To minimize the effects of amplitude fluctuations and reverberations on long-range acoustic communication, signals should encode information either in frequency modulation or in repetitive amplitude modulation that allow enough redundancy or signal averaging to permit recognition of signals by receivers. Because reverberations are more severe in environments with many scattering surfaces, long-range acoustic communication in forests, as opposed to open environments, should avoid rapid amplitude modulation or repetitive frequency modulation. Among North Carolina passerine birds, species that breed in forests tend to avoid rapid repetition rates at any given frequency in their long-distance songs. The directionality of both the broadcast and reception of acoustic signals will influence the effects of scattering on reverberations and attenuation of acoustic signals. In scattering environments, the optimal directionality of sound production and reception will require compromises.