Low-frequency Acoustic Signals Research Articles

Azimuthal dependence of reverberation from a harmonic signal propagating in shallow water

The problem of scattering from a rough sea surface is considered for a harmonic low-frequency (200–400 Hz) acoustic signal propagating in a shallow water area. An experimental study of the azimuthal reverberation dependence is carried out on the basis of the bistatic location scheme with the use of linear phased arrays. By comparison with calculations, the formation of the frequency-angular characteristics of reverberation is investigated for various wind directions with respect to the path of acoustic signal propagation.

Optimal selection of mode signals by vertical arrays in plane-layered waveguides in the presence of undesired signals

We discuss the possibilities of forming low-frequency mode acoustic signals by vertical arrays. We present formulations and solutions for several problems of optimal formation of such signals. Results of numerical simulation of the selective excitation of mode-signals are presented with reference to practical problems related to the low-frequency low-mode acoustic tomography of inhomogeneities in shallow sea.

A Time-of-Arrival-Based Positioning Technique With Non-Line-of-Sight Mitigation Using Low-Frequency Sound

This paper presents a non-line-of-sight (NLOS) localization technique and its system for a quiet and known environment that uses first-arrival low-frequency acoustic signals. The proposed technique first measures the time-of-arrivals (ToAs) of the first-arrival low-frequency sounds. The technique then estimates the location of a target by minimizing the differences between the measured ToAs and the corresponding ToAs estimated from the known map. The strength of the proposed technique is that the positioning accuracy is not corrupted by any small or known obstacle. The performance of the technique was investigated by its comparison to other techniques in both simulations and an experiment. The numerical results show successful localization of a target in various situations, demonstrating the efficacy of the technique and system as a feasible solution for NLOS mitigation.

Application of complex acoustic signals in the long-range navigation of underwater objects

The creation of modern technical means of research and exploration of the ocean based on submersibles of different designations is a priority field of science and technology development. Creation of intellectual autonomous unmanned submersibles (AUS), which provide multidisciplinary wide-scale study of sea basins and the ocean bottom and new basic knowledge in oceanography, marine geology, biology, and power engineering, is a pressing issue [1]. Significant progress in the development of electric power systems promoted the creation of an AUS with an operating coverage range of hundreds of kilometers. It has become reasonable to equip them with systems of transmitting and receiving low-frequency acoustic signals with a propagation range not smaller than the operating range of the submersibles as means for remote control of the operation and mutual maneuvering during their group operation. Up to the present, the development of such systems has been hampered by a lack of technical solutions for the measurement of the time of signal propagation from a sound source to a receiver over a distance of hundreds of kilometers with the required accuracy. In this work, we present the results of experimental studies of the propagation of low-frequency broadband pulse signals in an underwater sound channel (USC) using state-of-the-art technologies for synchronizing the receiving and transmitting channels. This allowed us to acquire unique data on the stability of acoustic wave propagation velocity in the USC for developing the technology of measurement of the distance between the sound source and the receiver. The main objective of the experiment was to discriminate the part of the signal propagating closest to the USC axis and to measure the time of its propagation from the source to the receiver. In order to do this, the method developed by the authors for sounding the sea medium by complex phase-manipulated signals was used. This technology allows us to distinguish and identify the arrivals of acoustic energy over different beam trajectories [2, 3]. The experiment was carried out in August 2006 in the Sea of Japan. Figure 1 shows the schematic geometry and methodological peculiarities of the experiment. According to the commands of the receiving vessel, complex signals (M-sequences, 511 symbols, and 4 periods of the carrier frequency per symbol) were transmitted with an interval of 5 min at a frequency of 600 Hz by the sound source stationary located near the bottom at a depth of 40 m and a distance of 450 m from the coast. The receiving vessel was represented by a yacht, which was used to deploy the radio hydroacoustic buoy with a hydrophone. The yacht was maneuvering under sail in the zone of reliable recording of radio signals. The hydrophone was lowered approximately to the USC axis, whose location was found from the measurement of the vertical distribution of sound velocity using the hydrological profiler from the yacht (Fig. 2). Signals were received at six points of the path at a distance from the transmitter ranging from 55 km to 368 km (Table 1). The buoy with the hydrophone was drifting in this process, and the coordinates were measured from the yacht using a GPS navigator when the yacht was passing close to the buoy. The signal information and marks of the common time system were recorded on a PC. The systems of common time based on thermally stabilized generators were included into the transmitting and receiving systems and were started before the beginning of the experiment. This allowed us to measure the times of signal propagation between the corresponding points with an accuracy not less than 10 ‐8 s. Processing of the information consisted in the calculation of the cross-correlation function between the received signals and the mask of the transmitted signal preliminary recorded by the PC. From two to four arrivals of acoustic energy, which propagated over different beam trajectories, were recorded in the pulse characteristics obtained using this method. The latest and maximal (in amplitude) arrival was identified as the one that passed near the USC axis, because it propagated near the minimum of sound velocity along the shortest path

Perception of Low-Frequency Acoustic Signals by a Harbour Porpoise (<I>Phocoena phocoena</I>) in the Presence of Simulated Offshore Wind Turbine Noise

Using auditory evoked potential (AEP) methods, a study was conducted on a harbour porpoise (Phocoena phocoena) at the Dolfinarium Harderwijk in The Netherlands. The study measured the audible range of wind turbine sounds and their potential masking effects on the acoustic perception of the animal. AEPs were evoked with two types of acoustic stimuli: (1) click-type signals and (2) amplitude-modulated signals. The masking noise resembling the underwater sound emissions of an operational wind turbine was simulated. At first, the animal’s hearing threshold was measured at frequencies between 0.7 and 16 kHz. Subsequently, these measurements were repeated at frequencies between 0.7 and 2.8 kHz in the presence of two different levels of masking noise. The resulting data show a masking effect of the simulated wind turbine sound at 128 dB re 1 µPa at 0.7, 1.0, and 2.0 kHz. This masking effect varied between 4.8 and 7.3 dB at those frequencies. No significant masking was measured at a masking level of 115 dB re 1 µPa. The available data indicate that the potential masking effect would be limited to short ranges in the open sea, but limitations exist to this conclusion and all estimates are based on existing turbine types, not taking into account future developments of larger and potentially noisier turbine types.

Physiological condition monitors utilizing very low frequency acoustic signals

Physiological condition monitors utilizing very low frequency acoustic signals and signals indicative of body orientation are disclosed. The physiological condition monitors comprise a sensor that is capable of detecting low frequency acoustic signals in the frequency range of one tenth Hertz to thirty Hertz. The sensor comprises a chamber having portions that form a cavity and a low frequency microphone placed within the cavity. An alternate embodiment of the invention comprises a chamber having portions that form a resonant cavity, a microphone mounted in the resonant cavity, and a membrane that covers the resonant cavity. Low frequency acoustic signals that are incident on the membrane cause the membrane to move and amplify the acoustic signals within the resonant cavity. The sensor provides information concerning physiological conditions, such as respiration and cardiac activity. The sensor in a physiological condition monitor does not need to be directly coupled to the skin of the person being monitored. The physiological condition monitor simultaneously provides information concerning cardiac activity, and respiration activity, and the movement and position orientation of the monitored person's body.

Sensor and method for detecting very low frequency acoustic signals

Sensor and method for detecting very low frequency acoustic signals

Low-Frequency Acoustic Tomography of a Shallow Sea by Low-Mode Pulses

Possibilities for the observation of randomly distributed and spatially localized inhomogeneities in a shallow sea by the method of low-frequency low-mode pulsed diffraction tomography are discussed. Results of computer simulations and experimental studies of the emission and reception of low-frequency low-mode acoustic signals in a shallow sea are presented.

Testing low/very low frequency acoustic sources for basin-wide propagation in the Indian Ocean

Low/very low frequency acoustic signals were transmitted to distant receivers in the Indian Ocean. The aim was to test methods for characterizing the hydroacoustic capability of the International Monitoring System (IMS) that discriminates for nuclear tests in the region. Several acoustic sources were deployed between Seychelles and Fremantle, Australia, and the IMS receivers comprised a network of hydrophones off Diego Garcia and Australia. Two of the three acoustic sources tested produced basin-scale propagation of impulsive signals. Single glass spheres imploded within the sound channel produced a clear signal at frequencies above ∼40 Hz, at ranges of hundreds to a thousand kilometers. Five-sphere glass implosions were recorded at ranges up to 4400 km. Near-sea surface shots from a large airgun array were recorded in several cases at ranges of hundreds to thousands of kilometers, the frequency of the highest signal-to-noise ratio arrivals varied within the 5–100 Hz band. High background noise level was a key factor at IMS stations that did not detect the airgun signals in the 5–15 Hz band. In a few cases, details of bathymetric features that are not well represented in the digital elevation model contributed to unexpected variation in relative signal levels between IMS stations.

Reviews Of Acoustical Patents

The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.

Geoacoustic inversion of broad-band acoustic data in shallow water on a single hydrophone

The inversion of broad-band low-frequency acoustic signals received on sparse arrays can lead to robust and efficient estimations of sea-bed properties. This paper describes a shallow-water geoacoustic inversion scheme based on the use of a model-based matched-impulse response on a single hydrophone. Results from the INTIMATE'96 experiment on the Portuguese shelf break are reviewed. In order to minimize the effects of strong time variability due to internal tides, only the time-stable waterborne bottom-surface reflected arrivals are exploited. A quasi-linear inversion algorithm is first applied to refine the geometry of the experiment. Then, inversion of bottom parameters is performed with an objective function that only makes use of the bottom-surface reflected arrivals' amplitudes. The experimental results show that broad-band transmissions (300-800 Hz) received on a single hydrophone, combined with the use of a simple eigenray code, are sufficient to correctly resolve geometrical parameters and bottom features. The analysis of the reflection coefficients both on simulated and real data helps to understand the validity of the inverted parameters and to derive the basis of an equivalent medium concept for geoacoustic inversion based on a "through-the-sensor" approach.

Apparatus and method for detecting very low frequency acoustic signals

There is disclosed an apparatus and method for detecting very low frequency acoustic signals. The apparatus comprises a sensor that is capable of detecting low frequency acoustic signals in the frequency range of one tenth Hertz to thirty Hertz. The sensor comprises a chamber having portions that form a cavity and a low frequency microphone placed within the cavity. An alternate embodiment of the invention comprises a chamber having portions that form a resonant cavity, a low frequency microphone placed within the resonant cavity, and a membrane that covers the resonant cavity. Low frequency acoustic signals that are incident on the membrane cause the membrane to move and amplify the acoustic signals within the resonant cavity. The sensor provides information concerning physiological conditions, such as respiration and cardiac activity. The sensor in a physiological condition monitor does not need to be directly coupled to the skin of the person being monitored.

Sensor and method for detecting very low frequency acoustic signals

There is disclosed a sensor and method for detecting very low frequency acoustic signals. The sensor is capable of detecting low frequency acoustic signals in the frequency range of one tenth Hertz to thirty Hertz. The sensor comprises a chamber having portions that form a cavity and a low frequency microphone placed within the cavity. An alternate embodiment of the invention comprises a chamber having portions that form a resonant cavity, a low frequency microphone placed within the resonant cavity, and a membrane that covers the resonant cavity. Low frequency acoustic signals that are incident on the membrane cause the membrane to move and amplify the acoustic signals within the resonant cavity.

A low-cost dynamic range-finding device based on amplitude-modulated continuous ultrasonic wave

A practical low-cost acoustic range-finding method is presented, which provides a high-speed and high-accuracy solution for distance measurement. It is based on the linear phase delay of the received signal, with respect to the transmitted signal caused by the distance between the transmitter and the receiver. A continuous ultrasonic signal amplitude modulated by a low-frequency acoustic signal is used. The accuracy can be up to 2 mm within 1.5 m and the dynamic data update rate can be up to 150 Hz. The design principles, hardware implementation, experimental results, and performance analysis are included.

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.

Acoustic noise and temperature monitoring of the Crater Lake of Mount Ruapehu Volcano

Acoustic signals in Ruapehu Crater Lake, which are now being telemetered via a satellite transmission system, show promise as a possible precursor of increased volcanic activity from Ruapehu. The start of a recent period of rapid heating of Crater Lake was preceded by low-frequency (2 Hz) acoustic signals. These accompanied similar frequency seismic signals, but seemed to be produced independently. Audio-frequency (350–3000 Hz) acoustic noise also showed a very clear peak shortly before the lake temperature started to rise.

The effects of the acoustic thermometry of ocean climate signals on dolphins and small whales

The acoustic thermometry of ocean climate (ATOC) program of Scripps Institute of Oceanography and the Applied Physics Laboratory, University of Washington, will broadcast a low-frequency 75-Hz phase-modulated acoustic signal over ocean basins in order to study ocean temperatures on a global scale. One of the major concerns is the possible effect of the ATOC signal on marine life, especially on dolphins and whales. In order to address this issue, the hearing sensitivity of a false killer whale (Pseudorcacrassidens) and a Risso’s dolphin (Grampusgriseus) to the ATOC sound was measured behaviorally. A staircase procedure with the signal levels being changed in 1-dB steps was used to measure the animals’ threshold to the actual ATOC coded signal. The results will be used to examine how the ATOC sound will affect the hearing capability of these small cetaceans. The relative effect of the ATOC sound will also be compared with effects from other sources of noise such as aircraft, ocean going ships, pleasure craft, and other whales.

Acoustic characterisation of thoracic body tissues in the audible frequency range

A previous study by Jones and Thomas [2] suggests that data relating to the physiological condition within the thoracic cavity may be obtainable utilising low frequency acoustic signals applied to the mouth and detected on the chest wall. In order to evaluate the contribution to the mouth to chest wall frequency response of the separate elements within the thorax, and to estimate the effect on this response when the lung physiology changes, an acoustic model of the thorax is required. To aid the development of this model, experiments have been carried out in order to establish the frequency dependence of the acoustic attenuation and speed of propagation through thoracic tissue samples in the audible frequency range 20–500 Hz. Samples from the porcine family were used due to their physical similarity to those of humans and their being obtainable within a short time of death.

A simplified method using wind speed for the estimation of low-frequency acoustic signal under the contamination of wind noise.

This paper describes a new and simplified method for estimating exclusively objective acoustic signal levels where the observation of the signals with microphones is inevitably disturbed by wind-induced noise in the outdoor measurement of low-frequency sounds. The estimation method in the frequency domain is discussed using the information on wind speed observed in the vicinity of a microphone, mainly from an experimental viewpoint. Concretely, the power spectrum of the wind noise and that of the observed sound (composed of objective signal and wind noise) are first expressed in the form of probability distribution conditioned by the wind speed. Next, a simplified method is shown for estimating the objective signal through a spectrum estimation procedure employing the conditional means of these power spectra. The method is supported by the fundamental additive property of power spectra based on the statistical independence of the signal and the noise. The effectiveness of the proposed method is finally discussed and shown through the measurement of an objective signal of band noise with the wind speed under breezy conditions.

A Comparison of CBL, RBT, and PET Logs in a Test Well With Induced Channels

Summary The introduction of new ultrasonic and ratio logging techniques has dramatically transformed cement evaluation. Previous methods [cement bond logs (CBL's)] that used a low-frequency (appx. 20 × 103-cycle/sec [appx. 20-kHz]) acoustic signal and a single-transmitter/dual-receiver combination suffered from inherent measurement problems. To eliminate several of the CBL problems a new dual-transmitter/multireceiver acoustic measurement tool, the ratio bond tool (RBT), has been introduced. The RBT measures attenuation directly through ratiometric techniques, is self-calibrating, and is much less sensitive to tool centralization. For cement evaluation across "fast formation" intervals, a 1-ft [0.33-m] spaced transmitter/receiver signal amplitude is measured. While techniques such as those implemented with the RBT tool improve on the CBL response, there are still inherent problems that could not be eliminated, such as sensitivity to microannuli and low sensitivity to local channels. An improved tool, the pulse echo tool (PET), uses ultrasonic frequency (appx. 500 X 103 cycles/sec [appx. 500 kHz]) to measure cement/casing bond and casing ID and wall thickness. The use of a double helix array of eight transducers, each independently measuring casing bond, ID, and wall thickness, allows detection of local channels in the cement and a detailed analysis of casing conditions. The radial measurement principle reduces the effect of formation signal and allows the attenuation of the resonant wave even in the presence of a small microannulus. A "logging well" has been constructed by the U.S. Environmental Protection Agency (EPA) to evaluate downhole tool response to cement channels. This test well was constructed with numerous sizes and weights of casings and has specially constructed flaws on the casing circumference to produce channels of variable lengths and widths. A detailed analysis of the ability of the CBL, RBT, and PET logs to detect these channels is presented. Introduction Cement evaluation logging tools are designed specifically to evaluate casing annular conditions and are constructed to make measurements that can be related to cement quantity, compressive strength, and bonding to pipe and formation, with the determination of cement isolation being the major objective of the log response. For oil and gas production purposes, fluid or gas migration in the annulus is economically undesirable, but in underground injection wells, it is imperative that there is no significant fluid movement into an underground source of drinking water through vertical channels adjacent to the wellbore. EPA underground injection control regulations require all Class II injection wells to demonstrate mechanical integrity before operations begin and at least once every 5 years thereafter. In July 1981, a research project was founded by the EPA to establish guidelines for mechanical integrity verification. As part of this project, a special logging well was constructed to determine present industry capability to evaluate cement bond between the cement/casing and cement/formation in injection wells. Service companies were invited to run their cement evaluation logging tools and to provide a complete interpretation of the condition of the cement before leaving the job site. The logging tools used in the initial test were of two basic generations: the first was the CBL with a low-frequency acoustic signal and a single-transmitter/dual-receiver combination, and the second was an ultrasonic tool with eight transducers in a helical array. On the basis of the test results, logging procedures for determining mechanical integrity in injection wells were established. Since the EPA test, new ratiometric logging techniques have been introduced that eliminate many of the inherent CBL problems. Calibrated CBL, RBT, and PET logs have been run on the logging well, and a detailed analysis of the three log responses to channels is presented. The results of this analysis should contribute to a better understanding of the ability of currently available technology to evaluate the cement integrity of the cased wellbore properly.