Transient Sound Sources Research Articles

Experimental progress of noise measurements in reverberation tanks

The methods and means for measuring the radiation characteristics of underwater sound sources have been developed under the conditions of reverberant tanks. From the perspective of experimental research, this paper introduces the methods of measuring the radiated sound power of steady-state sound sources and transient sound sources in reverberation tanks. The experimental subjects include the typical transducer under steady-state excitation, the cylindrical shell under steady-state excitation, the transient CW pulse sound signal and the underwater transient spark sound. The measurement principles, experimental system and measurement accuracy involved in each experiment are also listed. The experimental results show that the reverberation tank is an effective experimental device for measuring the acoustic characteristics of underwater sound sources, which has the advantages of simple experimental conditions, high measurement efficiency and strong tolerance for different types of sound sources.The methods and means for measuring the radiation characteristics of underwater sound sources have been developed under the conditions of reverberant tanks. From the perspective of experimental research, this paper introduces the methods of measuring the radiated sound power of steady-state sound sources and transient sound sources in reverberation tanks. The experimental subjects include the typical transducer under steady-state excitation, the cylindrical shell under steady-state excitation, the transient CW pulse sound signal and the underwater transient spark sound. The measurement principles, experimental system and measurement accuracy involved in each experiment are also listed. The experimental results show that the reverberation tank is an effective experimental device for measuring the acoustic characteristics of underwater sound sources, which has the advantages of simple experimental conditions, high measurement efficiency and strong tolerance for different types of sound sources.

Discrimination of chronic and transient sound sources in marine soundscapes

Soundscapes are comprised of chronic and transient sounds that overlap each other in time and frequency. These sound sources can be of anthropogenic, geological or biological origin. We automatically discriminated and classified sources with an unsupervised learning strategy based on clustering and subsequent training of a neural net. The parameter space was comprised spectral features and variation of these features over time. We document how different time and frequency binning influenced the outcome of the machine learning for the different sources. This analysis is based on multi-month data collected within several US National Marine Sanctuaries as part of the SanctSound project (https://sanctuaries.noaa.gov/science/monitoring/sound/). The project aims to characterize soundscapes within sanctuaries including assessing the influence of anthropogenic noise on marine life in their boundaries. We are testing whether the automated process helps to gain comparable results across the very different sanctuary habitats located in temperate to subtropical waters.

Experimental study on measurement of transient sound in a reverberation tank

As a new application of reverberation tanks, the experimental study on measuring the sound energy level of transient sound sources in a reverberation tank was proposed in this paper. According to the law of energy conservation, the transient sound energy obtained by sound intensity integration in time domain, which is the same with sound energy density spectrum integration in frequency domain. By utilizing the invariant sound field correction factors, the transient sound source level could be obtained in a reverberation tank, and both the reverberant measurement methods in frequency domain and that in time domain were established. The impulsive sound and the spark sound experiments were carried out in UATL (Underwater Acoustics Technology Lab, Harbin Engineering University, China) to confirm the validity of the proposed method. Experiments results showed that the sound source level of the transient sounds could be precisely measured in reverberation tanks, and compared to free field measurement, the deviation was less than 1 dB.

Method for the measurement of the underwater transient sound characteristics in a reverberation tank

A method to evaluate the sound power of transient sound sources in reverberation tanks is proposed and tested. The method is based on the steady state sound field characteristics in closed space, which are analyzed by normal-wave theory. To eliminate the interference caused by the boundary of the sound field, the volume integral of the spatial mean-square pressure in local space is measured and the sound power density of the non-anechoic is then obtained. Using the relationship between the sound field in the enclosed space and that of the free field, the sound power of the sound source can be obtained using the measured spatial mean-square pressure. The sound power of an impulsive sound generated by a spherical sound source is measured in a reverberation tank and the results are compared with the sound power measured in anechoic pool to verify the accuracy of the proposed method. The proposed method can be used to measure the radiated sound power of different types of transient sound sources in reverberation tanks.

Characteristics of the under-ice soundscape in the southern Beaufort Sea during Ice Exercise 2016

Ice Camp SARGO was the remote hub of operations for the multi-national naval operation Ice Exercise 2016 (ICEX-16), held in March of that year. Over a three-day period in early March, continuous recordings of the under-ice soundscape were collected with receivers deployed at various depths through first-year ice in the vicinity of the ice camp as it drifted westward across the Beaufort Sea approximately 175 nm north of Prudhoe Bay, Alaska. A significant reduction in the strength of easterly winds resulted in deceleration of the ice sheet during the period of observation, inducing notable ice cracking and ridging events near the camp. Ice sheet movement slowed from about 0.5 knots early in the test to virtually coming to a halt near the end of the recording period. Sounds from naturally occurring and anthropogenic sources in the 10-Hz to 10-kHz band detected in the recordings were analyzed in connection to the origins of the sound and correlated to varying environment conditions including wind speed and ice motion. Results show a wide variety of persistent and transient sound sources contribute to the total soundscape at this remote location. Comparisons to other soundscape observations in the region are discussed.

Method for underwater impulsive sound characteristic measurement in non-anechoic pool

A method for measuring the sound power of impulsive sound sources in non-anechoic pool was proposed in this paper, which can also be applied to measure the sound power of other transient sound sources. The characteristic of sound field in enclosed space were presented based on normal-wave theory. By calculating the volume integral of square pressure within local sound field, the interference caused by boundary was eliminated and the steady sound power density of the enclosed space was obtained. The sound power of the source can then be obtained by introducing the sound field transmission relationship from enclosed space to free field. The method was extended to measure the sound power of transient sound sources in non-anechoic pool. The sound power of impulse sound sources was measured by using the method proposed in this paper. The effectiveness of the proposed method was proved by comparing the sound power measured in anechoic pool and non-anechoic pool.

How often is human auditory detection in natural environments limited by our absolute hearing thresholds?

The National Park Service (NPS) has collected long-term sound level measurements from more than 800 sites, using equipment that measures 1 second, 1/3rd octave band levels. One-third octave bands approximate the critical bands for the human auditory system, and the initial motivation for collecting these data was to predict the levels at which incoming aircraft noise would be audible. Here, we will compare the nominal human threshold of hearing—expressed in 1/3rd octave bands—with an appropriate summary of the background or residual sound level of each environment. Several procedures have been recommended for estimating the residual sound level, or the sound level that remains after energy from nearby and transient sound sources is removed. The merits of alternative procedures for estimating residual sound levels will be assessed. The NPS data show that human hearing has evolved to take advantage of the quietest conditions that occur in all but the very quietest environments, across a substantial portion of the audible spectrum. For species whose hearing is up to 10 dB less sensitive than humans, they will be operating in a masked hearing regime in the majority of locations and hours.

Direction-finding techniques for a small-aperture hydrophone array

A volumetric array of hydrophones was coupled to a long-term, autonomous acoustic recorder and deployed ~130 km offshore of Southern California to the seafloor (~1300 m depth) to track continuous-wave (CW) and transient underwater sound sources. The array was composed of four, wide-band, omnidirectional hydrophones closely-spaced ~1 m apart. Sampling was continuous at 100 KSamples/s for each hydrophone over a period of more than two months. To track CW sound sources, conventional and adaptive beamforming techniques were implemented. The array’s beam pattern characteristics as a function of frequency (10—1000 Hz) were investigated by simulating the arrival of plane waves from directions of interest. Beamforming techniques were opportunistically applied to nearby (<5 km) transiting commercial ships with automated identification system (AIS) transmitted locations. Discrepancies were within the physical dimension of the transiting ships. Source levels for each of the transiting ships were estimated at the ships’ various aspects to characterize the directionality of underwater ship noise. In addition, a cross-correlation based direction-finding technique for transient sounds was developed and implemented for short duration (<1 ms), frequency-modulated echolocation clicks emitted by deep-diving beaked whales.

Transient sounds in marine seismology

In marine seismology two types of transient sound sources are used. Controlled sources such as airguns or explosions are used in seismic reflection and refraction surveys to study geological structure below the seafloor. Naturally occurring earthquakes are also observed and they are used to study tectonic and regional and whole earth structure problems. In both cases three‐component particle motion, either velocity or acceleration, is measured on or within the seafloor. However controlled source studies work in the VLF band (1.0–100 Hz) and earthquake studies generally run in the lower ULF band (0.001–5 Hz). A major advantage of transient sources versus harmonic sources is that multi‐pathing can be distinguished by the time separation of phases. In fact, many schemes for inferring geological structure from transient seismic data are based solely on the arrival times of certain phases. Also time spread (coda or signal‐generated noise) can be observed directly as reverberation after the coherent arrivals. The polarization of the separated phases can be used to study propagation effects. Examples of quantitative data of both controlled and earthquake sources will be shown and discussed.

The application of a normal mode perturbation method to assess the effects of sound-speed fluctuations on the propagation of narrow- and broadband acoustic energy in the sea

Fluctuations in the intensity of an acoustic signal are a common feature of propagation of sound in the sea. The fluctuations are a manifestation of the temporal variability of the index of refraction (sound velocity) of the medium which, in turn, is induced by a variety of random and deterministic ocean processes covering a wide spectrum of temporal and spatial scales. In this paper, a recently developed high-order perturbation method is described and applied to the problem of sound propagation in the sea. The basis functions used in the calculations are derived from sound-speed profiles obtained in an acoustic propagation experiment conducted in a shallow-water region of the Mediterranean. Each profile is treated as a perturbation from a mean profile, the latter obtained by depth-averaging the profiles. At particular source frequencies, calculations of modal functions and acoustic transmission loss were compared for the mean and several perturbed profiles. These initial discrete-frequency results were presented at the Scintillations Conference (University of Washington, Seattle, Washington, August 1992). Here, the method is extended to include the response of broadband, transient sound sources and sources at several water depths. The broadband (time-domain) approach enables one to readily determine the optimum frequencies of the shallow-water duct and, in addition, to assess the effect of sound-speed fluctuations on the optimum frequencies. These results, along with the earlier ones, confirm the significant effects on acoustic transmission of seemingly minor variations in sound speed and, moreover, demonstrate the efficacy of the new perturbation method in handling such problems.

Definition and measurement of sound energy level of a transient sound source.

Concerning stationary sound sources, sound power level which describes the sound power radiated by a sound source is clearly defined. For its measuring methods, the sound pressure methods using free field, hemi-free field and diffuse field have been established, and they have been standardized in the international and national standards. Further, the method of sound power measurement using the sound intensity technique has become popular. On the other hand, concerning transient sound sources such as impulsive and intermittent sound sources, the way of describing and measuring their acoustic outputs has not been established. In this paper, therefore, “sound energy level” which represents the total sound energy radiated by a single event of a transient sound source is first defined as contrasted with the sound power level. Subsequently, its measuring methods by two kinds of sound pressure method and sound intensity method are investigated theoretically and experimentally on referring to the methods of sound power level measurement.

Sound energy calculation of transient sound sources by the radiation efficiency method

In this paper theoretical arguments are presented to show that the radiation efficiency method can be used to calculate the sound energy radiated by transient sound sources. Radiation efficiency method results are shown to be strictly applicable to transients as well as steady state. An example is presented to illustrate that for machines of even relatively high damping the radiation efficiency for a transient can be approximated by its value at the ringing frequency, thus permitting simple calculation of acoustic energy output from time-domain data alone.