Ambient Noise Spectrum Research Articles

Wind dependence of ambient noise in a biologically rich coastal area.

The wind dependence of acoustic spectrum between 100 Hz and 16 kHz is investigated for coastal biologically rich areas. The analysis of 5 months of continuous measurements run in a 10 m deep shallow water environment off Brittany (France) showed that wind dependence of spectral levels is subject to masking by biological sounds. When dealing with raw data, the wind dependence of spectral levels was not significant for frequencies where biological sounds were present (2 to 10 kHz). An algorithm developed by Kinda, Simard, Gervaise, Mars, and Fortier [J. Acoust. Soc. Am. 134(1), 77-87 (2013)] was used to automatically filter out the loud distinctive biological contribution and estimated the ambient noise spectrum. The wind dependence of ambient noise spectrum was always significant after application of this filter. A mixture model for ambient noise spectrum which accounts for the richness of the soundscape is proposed. This model revealed that wind dependence holds once the wind speed was strong enough to produce sounds higher in amplitude than the biological chorus (9 kn at 3 kHz, 11 kn at 8 kHz). For these higher wind speeds, a logarithmic affine law was adequate and its estimated parameters were compatible with previous studies (average slope 27.1 dB per decade of wind speed increase).

Singing in the city: high song frequencies are no guarantee for urban success in birds

Urbanization involves dramatic environmental alterations, which can limit survival and reproduction of organisms and contribute to loss of biodiversity. One such alteration is anthropogenic noise, which biases natural ambient noise spectra toward low frequencies where it may interfere with acoustic communication among birds. Because vocalizing at higher frequencies could prevent masking by noise, it has been hypothesized that species with higher song frequencies should be less affected by urbanization. Indeed, evidence is accumulating that urban birds often vocalize at higher frequency than nonurban birds. However, the extent to which singing frequency affects their success in cities is less clear. We tested this hypothesis with a comprehensive phylogenetic Bayesian analysis comparing song frequency of songbirds from 5 continents with 4 measures of success in urbanized environments. Tolerance to urbanization was not associated with dominant or minimum song frequencies, regardless of the metric used to quantify urban success and the intensity of the urban alterations. Although song frequency was related to habitat preferences and body size of the species, none of these factors explained the lack of association with urban success. Singing high may be beneficial for signal perception under noisy conditions, but these high frequencies are apparently no guarantee for the success of bird species in urbanized environments.

Conceptual Design of Cylindrical Hydrophone Arrays for Stabilization of Receiving Characteristics under Ocean Ambient Noise

An underwater sound surveillance system detects and tracks enemy ships in real-time using hydrophone arrays, in which seabed-mounted sensor arrays play a pivotal role. In this paper the conceptual design of seabed-mounted, cylindrical hydrophone arrays for use in shallow coastal waters is performed via finite element calculations. To stabilize the receiving characteristics under the ocean ambient noise, a technique for whitening the ambient noise spectrum using a metal baffle is proposed. Optimization of the array configuration is performed to achieve the directivity in the vertical and azimuthal directions. And the effects of the sonar dome shape and material on the structural vibration and sound scattering properties are studied. It is demonstrated that a robust hydrophone array, having a sensitivity deviation less than 4 dB over the frequency range of interest, can be obtained through the whitening of the ambient noise, the optimization of the array configuration, and the design of acoustically transparent sonar domes.

Rayleigh-Wave, Group-Velocity Tomography of the Borborema Province, NE Brazil, from Ambient Seismic Noise

Ambient seismic noise has traditionally been regarded as an unwanted perturbation that “contaminates” earthquake data. Over the last decade, however, it has been shown that consistent information about subsurface structure can be extracted from ambient seismic noise. By cross-correlation of noise simultaneously recorded at two seismic stations, the empirical Green’s function for the propagating medium between them can be reconstructed. Moreover, for periods less than 30 s the seismic spectrum of ambient noise is dominated by microseismic energy and, because microseismic energy travels mostly as surface-waves, the reconstruction of the empirical Green’s function is usually proportional to the surface-wave portion of the seismic wavefield. In this paper, we present 333 empirical Green’s functions obtained from stacked cross-correlations of one month of vertical component ambient seismic noise for different pairs of seismic stations in the Borborema Province of NE Brazil. The empirical Green’s functions show that the signal obtained is dominated by Rayleigh waves and that dispersion velocities can be measured reliably for periods between 5 and 20 s. The study includes permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region, resulting in a combined network of 34 stations separated by distances between approximately 40 and 1,287 km. Fundamental-mode group velocities were obtained for all station pairs and then tomographically inverted to produce maps of group velocity variation. For short periods (5–10 s) the tomographic maps correlate well with surface geology, with slow velocities delineating the main rift basins (Potiguar, Tucano, and Reconcavo) and fast velocities delineating the location of the Precambrian Sao Francisco craton and the Rio Grande do Norte domain. For longer periods (15–20 s) most of the velocity anomalies fade away, and only those associated with the deep Tucano basin and the Sao Francisco craton remain. The fading of the Rio Grande do Norte domain fast-velocity anomaly suggests this is a supracrustal structure rather than a lithospheric terrain, and places new constraints on the Precambrian evolution of the Borborema Province.

Offshore Marine Ambient Noise Measurement Study Based on Laser Interferometer Hydrophone

Sound pressure is an important parameter of marine background noise monitoring. A laser interferometric hydrophone is designed based on Michelson interferometry. By tracking compensation method, immunity from interference of the hydrophone is improved. Error analysis shows that the vibrating membrane is the main source of the sound pressure error, which can be eliminated to a certain extent by a vibrating membrane materials selection. Offshore data show that hydrophone pressure measurement of high precision, and with a good frequency response. By comparison with the theoretical model to some extent also verified the accuracy of data. In coastal waters, wind noise and ship radiate noise is the main noise source of marine ambient noise. Here, marine ambient noise spectrum of three wind speed were calculated, and with Kundsen curves were compared. Results show that measured data and Kundsen curve fitting is better. In addition, the measurement of vessel radiation noise is mainly concentrated in frequency range of 350~500Hz.

Evidence for an Independent 26-s Microseismic Source near the Vanuatu Islands

The 26 s peak in the ambient seismic noise spectrum is persistently excited and observed at stations globally. Using noise cross-correlation functions (NCFs), the location suggests that the source could be situated in the Gulf of Guinea and Fiji Basin. However, the Fiji Basin was proposed to be the mirror site (near antipode) of the Gulf of Guinea source instead of an independent source, assuming that the surface waves more efficiently propagate along the major-arc paths of oceanic movements. To investigate the propagation of the Rayleigh waves along continental and oceanic paths, we analyzed the surface wave data recorded from an earthquake near the Gulf of Guinea and found that Rayleigh waves travel along continental minor-arc paths more efficiently than along oceanic major-arc paths. We then located the source in the western Pacific Ocean from group velocities measured with earthquake data by using the travel time misfit in NCFs after calibration and concluded that the source is in the Vanuatu Islands. Moreover, the temporal variation of the 26 s microseismic peak observed in the western Pacific seismic stations is very different from that in stations near the Gulf of Guinea, which suggests that they are excited by independent sources. Therefore, the Vanuatu source should be an independent microseismic source. As it is close to volcanoes in the Vanuatu islands, the Pacific 26 s microseismic source might be excited by magmatic processes, which are also responsible for very-long-period volcanic tremors.

Development and drift-analysis of a modular electromagnetic induction system for shallow ground conductivity measurements

Electromagnetic induction (EMI) is used for fast near surface mapping of the electrical conductivity (EC) for a wide range of geophysical applications. Recently, enhanced methods were developed to measure depth-dependent EC by inverting quantitative multi-configuration EMI data, which increases the demand for a suitable multi-channel EMI measurement system. We have designed a novel EMI system that enables the use of modular transmitter/receiver (TX/RX) units, which are connected to a central measurement system and are optimized for flexible setups with coil separations of up to 1.0 m. Each TX/RX-unit contains a coil, which is specifically adjusted for transmitting or receiving magnetic fields. All units enable impedance measurements at the coils, which are used to simulate its electrical circuit and analyze temperature-induced drift effects. A laboratory drift analysis at 8 kHz showed that 88% of the drift in the measured data is due to the change in the electrical transmitter coil resistance. The remaining 12% is due to changes in the transmitter coil inductance and capacitance, the receiver impedance and drifts in the amplification circuit. A measurement under field conditions proved that the new EMI system is able to detect a water-filled swimming pool with 50 mS m−1, using a coil separation of 0.3 m. In addition, the system allows in-field ambient noise spectra measurements in order to select optimal low-noise measurement frequencies.

Wind-dependence of low-frequency ambient noise in the deep-ocean sound channel

In the low-frequency range (1-125 Hz), the deep-ocean ambient noise field is produced by seismic, marine life, ship traffic, and wind-dependent hydrodynamic noise mechanisms. This study focuses on the contribution of wind-related source mechanisms to the overall ambient noise field, as well as previous attempts to understand the physics of these mechanisms. The Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) hydroacoustic monitoring system has produced nearly continuous recordings of the low-frequency deep-ocean ambient noise field at sites in the Pacific, Atlantic, and Indian Oceans, each spanning several years in length. Additionally, wind speed data have been recorded at the host island of each station by the National Oceanic and Atmospheric Administration (NOAA). Correlation techniques are used with these two datasets to determine the relationship between wind speed and the sound level in different frequency bands, and to determine the prominence of wind-related noise in the combined ambient noise spectrum. Results from the three sites are compared to each other to assess the uniformity of wind-generated noise over the world's ocean basins.

Spectral probability density as a tool for marine ambient noise analysis

The empirical probability density of the power spectral density has been successfully applied as tool to assess signal variability and sensor system performance in the seismic literature. This paper presents the application of this analysis method to underwater ambient noise measurements, and demonstrates its utility in assessing the field performance of passive acoustic monitoring systems and the statistical distribution of noise levels across the frequency spectrum. Using example datasets from an autonomous passive acoustic recorder in the Moray Firth, Scotland, UK, and a cabled subsea observatory in the Strait of Georgia, British Columbia, we show how this method can reveal data limitations such as persistent tonal components and insufficient dynamic range, and phenomena such as bimodality and outliers, which may be undetected by standard analysis techniques. We then combine this approach with conventional percentiles and spectral averages, illustrating how the underlying noise level distributions influence these metrics, and propose this technique as a standard, integrative presentation of ambient noise spectra. Finally, the paper presents cumulative probability density as a method for frequency-domain characterization of chronic noise exposure in marine acoustic habitats.

Spectral probability density as a tool for ambient noise analysis

This paper presents the empirical probability density of the power spectral density as a tool to assess the field performance of passive acoustic monitoring systems and the statistical distribution of underwater noise levels across the frequency spectrum. Using example datasets, it is shown that this method can reveal limitations such as persistent tonal components and insufficient dynamic range, which may be undetected by conventional techniques. The method is then combined with spectral averages and percentiles, which illustrates how the underlying noise level distributions influence these metrics. This combined approach is proposed as a standard, integrative presentation of ambient noise spectra.

Auditory thresholds in marine vertebrates conform to natural ambient noise levels

Auditory thresholds are often displayed in a manner that reveals what is commonly called a “U-curve.” But if the threshold curves are displayed on the x axis on a true Log10 scale the profile is shaped differently. For marine mammals the shape is more like a “hockey stick.” If these curves are overlaid on the “Wenz ambient noise spectra curves” there appears to be shape conformance. This makes sense as auditory sensitivity would naturally evolve to exclude ambient environmental noise. This paper evaluates 120 legacy auditory threshold curves from 18 species of marine mammals and 60 threshold curves from 32 species of fish. The auditory threshold curves from the fish do not conform to the Wenz curves. Given that both the auditory thresholds and the Wenz curves were expressed as pressure gradient energy it is possible that the profile of the fish threshold curves express sound in either the particle velocity, or both particle velocity and pressure gradient energy. This paper extrapolates the particle velocity data from the fish threshold conditions to determine if there is some conformity to ambient noise levels in either or both the particle and pressure gradient realms.

Spatial characteristics of an Arctic ambient noise field.

During multidisciplinary experiments in the 1980s and 1990s, ambient noise characteristics were found to be related to different ice types, distance from the ice edge, tidal current, wave conditions, and ice edge eddies. Primary noise generation mechanisms are ice floe collisions, ice cracking, ridging, and breakup caused by wave interaction with sea ice, internal ice stress, and changes in air temperature. Furthermore, sound propagation conditions determined by surface reflectivity due to sea ice and ocean stratification have a strong impact on the time averaged ambient noise spectrum. Ambient noise as a way to provide information about the environment is being explored in ongoing experiments within the ACOBAR and WIFAR projects. This paper presents spatial characteristics of the low‐frequency ambient noise field from data collected simultaneously at 16 sites in the Marginal Ice Zone (Fram Strait) in October 2010. The acoustic measurements covered a 100 x 100 km2 area from open water to within the ice co...

Analysis of high-frequency wind-driven ambient noise in shallow brackish water

Ambient noise spectra in a shallow brackish water environment were found to be steeper than expected at frequencies above 10 kHz. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1-0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3 / 2 power law dependence of bubble size density on radius.

Wind-generated ambient noise in a shallow brackish water environment in the archipelago of the Gulf of Finland

Underwater ambient noise measurements were carried out in a shallow (15-20 m) brackish water in the archipelago of the Gulf of Finland for period of 1 year. The absence of traffic noise made it possible to study wind driven effects in ambient noise at lower frequencies. The ambient noise comes mostly from local sources and the propagation effects are shown to be negligible. The ambient noise develops bubble type spectral features above 100 Hz as wind speed increases. Sharp spectral declines are observed below 500 Hz, which are most likely due to resonances from oscillating bubble clouds created by breaking waves. The low frequency range of the observed declines may partly be attributed to the larger bubble size in fresh and brackish waters. In the present study the wind speed dependence factor was approximately 2.4 at 200 Hz, which is significantly higher than the typical factor of approximately 1.5 for the ocean environment. The average high-frequency spectral slope was -4.9 dB/octave which is approximately 1 dB/octave less than for typical deep water slopes. No significant seasonal effects were found in any parameter calculated from the ambient noise spectra.

Sounds of shallow water fishes pitch within the quiet window of the habitat ambient noise

The habitat ambient noise may exert an important selective pressure on frequencies used in acoustic communication by animals. A previous study demonstrated the presence of a match between the low-frequency quiet region of the stream ambient noise (termed 'quiet window') and the main frequencies used for sound production and hearing by two stream gobies (Padogobius bonelli, Gobius nigricans). The present study examines the spectral features of ambient noise in very shallow freshwater, brackish and marine habitats and correlates them to the range of dominant frequencies of sounds used by nine species of Mediterranean gobies reproducing in these environments. Ambient noise spectra of these habitats featured a low-frequency quiet window centered at 100 Hz (stream, sandy/rocky sea shore), or at 200 Hz (spring, brackish lagoon). The analysis of the ambient noise/sound spectrum relationships showed the sound frequencies matched the frequency band of the quiet window in the ambient noise typical of their own habitat. Analogous ambient noise/sound frequency relationships were observed in other shallow-water teleosts living in similar underwater environments. Conclusions may be relevant to the understanding of evolution of fish acoustic communication and hearing.

Year-round variability of ambient noise in temperate freshwater habitats and its implications for fishes

Changes in habitat acoustics over the year can potentially affect fish hearing and orientation to sound, especially in temperate climates. This is the first study where year-round changes in ambient noise in aquatic habitats were assessed. Seven different European fresh-water habitats were chosen for this study. Sound pressure level (SPL) and spectral composition of the ambient noise varied in both quiet stagnant habitats (lakes, backwaters) and in flowing habitats (streams, rivers). Linear equivalent SPL (L(Leq, 60s)) tended to be lower in stagnant habitats (means: 91.6-111.7 dB) than in flowing habitats (means: 111.2-133.4 dB). The changes in SPL were smallest in the river (means: 4.2-4.4 dB, maxima: 8.5-10.1 dB), whereas significantly higher values were measured in stagnant habitats and the stream (means: 9.9-14.9 dB, maxima: 25.1-30.9 dB). The spectral compositions of the ambient noise determined at different times of the year were highly correlated to each other at the river sites (mean cross-correlation coefficients: 0.85 and 0.94) and were weaker or not correlated at the other study sites (means: 0.24-0.76). The changes in ambient noise spectra were negatively correlated to changes in SPL, indicating that large changes in SPLs were accompanied by large changes in spectral composition and vice versa. Comparison of these ecoacoustical data with a preceding study (Amoser and Ladich in J Exp Biol 208:3533-3542, 2005) indicates that the auditory sensitivity in hearing specialists is affected by changes in ambient noise levels and spectra throughout a year and that this effect tends to be more pronounced in stagnant waters and the stream than at river sites. On the other hand, absolute noise levels result in a higher degree of masking in flowing waters.

Survey of ambient noise in aquariums.

Owning and maintaining an aquarium is a common hobby, but the ambient noise in aquariums resulting from pumps, filters, bubblers, and other equipment is not well studied. Elevated ambient noise levels have been shown to adversely affect wild fish and marine invertebrates, and anecdotal evidence suggests that this may be a problem in aquariums as well. Aquariums designed to maintain coral reefs require high water flow and pristine water conditions, which in turn require pumps and protein skimmers that contribute to underwater noise. A survey of ambient noise in both fresh and saltwater aquariums ranging in size from 5 to 550 gal was conducted. The aquariums differed in construction material, number, size and type of pumps, and presence of other equipment. The ambient noise broadband levels, spectrum levels, and time-frequency representations are compared, and general conclusions are drawn about noise characteristics of equipment and aquariums.

Characteristics and variability of ambient noise in the South China Sea basin.

A hydrophone was moored at mid depth in the South China Sea basin from November 2005 to October 2006. Operated with a 1‐min‐on and 14‐min‐off duty cycle and sampled at 1.6 kHz, the measured time series captures the spectral characteristics and variability of the ambient noise in the 0–800‐Hz band over an annual cycle. In this paper, we provide a description on the daily, monthly, and seasonal variabilities and variances in the measured noise spectrum and band levels. In order to gain insights into the predictability of the ambient noise field in this marginal sea, the interpretation of the data is facilitated with historical shipping density data and nowcast wind fields from the Navy Operational Global Atmospheric Prediction System (NOGAPS). Attention is made to the evolution of the ambient noise spectra during major storm events. Intermittent noises are also examined. The potential sources for these intermittent noises are discussed. [Research sponsored by the Office of Naval Research.]

Validating acoustic monitors for marine animals: Field experience with beaked whales and digital acoustic monitors (DMONs).

Passive acoustic monitoring (PAM) is a promising tool for improving the effectiveness of visual surveys for marine animals and for stand‐alone persistent monitoring. Nonetheless, significant data gaps make it difficult to predict PAM performance for many species. Detection functions are only beginning to be formed for some vocalizations and detectors. Statistical models for calling rates, as a function of behavior, are needed to translate acoustic detections into abundance estimates, or lack of detections into decisions about absence of animals. Finally, ambient noise levels and spectra are unavailable in most of the world’s oceans, complicating the application of results from one area to another. Although some of these gaps are being addressed by a growing group of researchers, there is an urgent need to develop standards for measuring and reporting results, and to foster the interchange of data. To exemplify the form such standards might take, and the potential pitfalls, we present results from a series of field experiments designed to assess the performance of an acoustic monitor for beaked whales. The experiments made use of a freely available reference design for an acoustic detector, called the DMON, which may provide a basis for standardizing and validating real‐time implementations.

Estimation and interpolation of underwater low frequency ambient noise spectrum using artificial neural networks

In this work, estimation of ambient noise spectrum influenced by wind speed and wave height carried out for the frequency range of 500 Hz to 5 kHz using Feed forward Neural Network (FNN) is presented. Ocean ambient noise measurements were made in the shallow waters of Bay of Bengal using a portable data acquisition system with a high sensitivity hydrophone at a depth of 5 m from the surface.100 sets of data covering a rage of wind speeds from 2.5 m/s to 8.5 m/s with approximately 15 sets of data falling within 1 m/s over the range of wind speed were used for training the FNN. The parameter wave height which contributes to the noise producing mechanism is also used for training along with wind speed. The results revealed that the proposed method is useful in the estimation and interpolation of underwater noise spectrum level and hence in simulation for the considered frequency range. These were confirmed by calculating the Mean Squared Error (MSE) between the experimental data and the simulation. As the measurements of the underwater ambient noise level are very difficult in remote oceanic regions, where conditions are often inhospitable, these studies seem to be relevant.