Underwater Acoustic Intensity Research Articles

Underwater Acoustic Intensity Analysis using Noise Assisted-MEMD with Varying Distances

With current developments, underwater communication using acoustic signals is widely used. Many things need to be prepared to support a reliable underwater communication system, such as taking measurements in a test tank to find out the correct measurement configuration. Underwater acoustic intensity measurements, which are detailed in this paper, are performed in the test tank using distance variation schemes. Measurements were made at various distances of 4, 10, 20, and 50 meters from the signal source. The hydrophone that was used has a sensitivity of -180 dB re 1V/µPa. The hydrophone was placed at a depth of 2 meters below the surface of the water in the test tank, which divided the test tank depth in half to ensure that reflections from the bottom and the surface were kept to a minimum. However, the problem is that there are noisy signals at different frequencies. This paper proposes a method using Noise Assisted - Multivariate Empirical Mode Decomposition (NA-MEMD) to decompose the signal and then calculate the sound intensity. The result shows that an increase in the distance between the transmitter and receiver, also causes a change in the intensity with an average change of 0.467 dB/meter. It is concluded that the NA-MEMD approach was shown to be successful in decomposing the intended signal from the noise to equalize the quality of the signal received at different distances, and the correlation between intensity value and change in distance is resilient, with a correlation value of 0.98, indicating a very strong correlation.

Underwater acoustic intensity characterization of anechoic panels in laboratory tank

Acoustic vector intensity contains more information about sound propagation than standard pressure measurements. This fact motivates the characterization of sound propagation in an acrylic rectangular water tank with dimensions of 3.6 × 1.2 m2 and a maximum depth of 0.91 m, contained in the Underwater Acoustics Lab at Brigham Young University. With the acquisition of an M20-040 Particle Motion Sensor from GeoSpectrum Technologies Inc, comparisons are made between the p-p and the p-u methods of calculating intensity as a function of frequency and source-receiver range. In addition, the decrease in intensity is quantified when Apltile SF5048 anechoic panels made by Precision Acoustics are used to line the side of the tank. This work helps characterize the impact of panels on sound propagation in the tank. The continuation of research on acoustic vector intensity will also allow for future research collaboration with biologists at BYU to make important measurements relating to the effects of sound intensity on the auditory responses of fish.

The exponential decay of underwater acoustic intensity with increasing altitude of low-frequency sound from a Robinson R44 helicopter

A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16 m) and deep (>100 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture. In the shallow water experiments the array was configured as a horizontal line (HLA) 0.5m above the seabed; whereas in deep water the array was suspended from the surface in a vertical line (VLA). An in-air microphone was located immediately above the surface. In this paper the power spectral density as a function of helicopter altitude will be reported from measurements on a single hydrophone. The main rotor blades of the helicopter produce low-frequency harmonics, the lowest frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. Between heights of 30 m to 600 m above the sea surface the underwater intensity was found to decay exponentially with increasing helicopter altitude. Interpretation of the observed low-frequency sound signatures is being facilitated with a numerical three-layer (atmosphere-ocean-sediment) acoustic propagation model in which the source may be moving or stationary. [Research supported by ONR, NAVAIR, and SIO.]

Quantifying hurricane destructive power, wind speed, and air‐sea material exchange with natural undersea sound

Passive ocean acoustic measurements may provide a safe and inexpensive means of accurately quantifying the destructive power of a hurricane. This is demonstrated by correlating the underwater sound intensity of Hurricane Gert with meteorological data acquired by aircraft transects and satellite surveillance. The intensity of low frequency underwater sound measured directly below the hurricane is found to be approximately proportional to the cube of the local wind speed, or the wind power. It is shown that passive underwater acoustic intensity measurements may be used to estimate wind speed and quantify the destructive power of a hurricane with an accuracy similar to that of aircraft measurements. The empirical relationship between wind speed and noise intensity may also be used to quantify sea‐salt and gas exchange rates between the ocean and atmosphere, and the impact of underwater ambient noise on marine life and sonar system performance.

Near-field acoustic intensity measurements using an accelerometer-based underwater intensity vector sensor

An accelerometer-based underwater acoustic intensity vector sensor is used to measure the acoustic nearfield of a single spherical source, and a pair of sources that vibrate in or out of phase with each other. The intensity sensor consists of co-located pressure and inertial sensors within a neutrally buoyant probe body. The design of this probe has been published previously. The measurements were performed in a large tank at a frequency of 5 kHz for two sources of different sizes, corresponding to ka values of 0.7 and 1.2 respectively, where k is acoustic wavenumber and a is the source radius. By way of validation, the acoustic intensity field from two closely spaced, interacting spherical radiators is predicted using the exact theory of the translational addition theorem for spherical wave functions. The predictions using this theory compare favorably well with the measured intensity field. Beam pattern and calibration data obtained for the intensity sensor suggest that underwater acoustic intensity generated by simple and complex sources can be measured to an accuracy of ±1 dB provided that ka is less than approximately 0.2.

Ocean acoustic hurricane classification

Theoretical and empirical evidence are combined to show that underwater acoustic sensing techniques may be valuable for measuring the wind speed and determining the destructive power of a hurricane. This is done by first developing a model for the acoustic intensity and mutual intensity in an ocean waveguide due to a hurricane and then determining the relationship between local wind speed and underwater acoustic intensity. From this it is shown that it should be feasible to accurately measure the local wind speed and classify the destructive power of a hurricane if its eye wall passes directly over a single underwater acoustic sensor. The potential advantages and disadvantages of the proposed acoustic method are weighed against those of currently employed techniques.

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.

CLASSIFYING TROPICAL CYCLONES WITH UNDERWATER SOUND

Theoretical and empirical evidence are synthesized to show that underwater acoustic sensing techniques may provide a valuable tool for measuring the wind speed and determining the destructive power of a hurricane.This is done by first developing a model for the acoustic intensity and mutual intensity in an ocean waveguide due to a hurricane and then determining the relationship between local wind speed and underwater acoustic intensity. From this it is shown that it should be feasible to accurately measure the local wind speed and classify the destructive power of a hurricane with a single underwater acoustic sensor.The potential advantages and disadvantages of the proposed acoustic method are weighed against those of currently employed techniques

Flow-induced noise on a pressure-acceleration underwater acoustic intensity probe

The results of an experiment to determine the effect of flow-induced noise on a pressure-acceleration intensity probe are presented. The sensor is a bluff body that conforms to the geometry of a right circular cylinder having a diameter of 10.16 cm and an aspect ratio of unity. A flexural mode hydrophone is used to measure the acoustic pressure and a flexural mode accelerometer is used to measure the acoustic particle acceleration. The hydrophone has a nominal sensitivity of about −185 dB re: 1 V/uPa and the accelerometer has an in-water acoustic sensitivity of nearly 1 V/g. The bandwidth of the device covers the 10-Hz to 1-kHz frequency range and the principle axis of sensitivity is coincident with the axis of the cylinder. The sensor was towed in cross-flow at speeds ranging from 0.1 to 0.5 cm/s and data were collected over the 10-Hz to 100-Hz frequency range. Of particular interest is the decomposition of the intensity spectrum into real and reactive parts along with an assessment of any filtering that can be accomplished using the intensity technique to measure far-field sound in the presence of near-field noise. [Work supported by various grants from ONR and NAVAIR.]

Underwater acoustic intensity measurements using a pressure/particle-acceleration intensity vector sensor

The acoustic field of simple sources is investigated by measuring the simultaneous, colocated acoustic pressure and particle acceleration with a neutrally buoyant underwater acoustic intensity vector sensor. This sensor consists of a piezoceramic hollow-cylinder pressure transducer, and a pair of miniature accelerometers mounted inside of the cylinder. A free-field calibration is performed under the ice of a flooded quarry, while in a laboratory test tank, the 2-D acoustic intensity field is mapped for various acoustic source configurations. These include a single spherical source and a pair of spherical sources that vibrate in or out of phase with each other. A sinusoidal signal of various frequencies between 2 and 20 kHz is gated in all of these experiments in order to suppress the effects of environmental reflections. The receiving beam pattern of the intensity sensor is also measured and reported. An analytical model for the interaction of two closely spaced spherical radiators is developed, and predictions from it compare well with the measured intensity field. Some limitations in the experiments, and future work are discussed. [Work supported by ONR, Code 321SS.]

Effect of shallow water internal waves on ocean acoustic striation patterns

Contour plots of underwater acoustic intensity, mapped in range and frequency, often exhibit striations. It has been claimed that a scalar parameter ‘beta’, defined in terms of the slope of the striations, is invariant to the details of the acoustic waveguide. In shallow water, the canonical value is β=1. In the present paper, the waveguide invariant is modelled as a distribution rather than a scalar. The effects of shallow water internal waves on the distribution are studied by numerical simulation. Realizations of time-evolving shallow water internal wave fields are synthesized and acoustic propagation simulated using the parabolic equation method. The waveguide invariant distribution is tracked as the internal wave field evolves in time. Both random background internal waves and more event-like solitary internal waves are considered.

Development of a velocity gradient underwater acoustic intensity sensor

A neutrally buoyant, underwater acoustic intensity probe is constructed and tested. This sensor measures the acoustic particle velocity at two closely spaced locations, hence it is denoted a “u-u” intensity probe. A new theoretical derivation infers the acoustic pressure from this one-dimensional velocity gradient, permitting the computation of one component of acoustic intensity. A calibration device, which produces a planar standing-wave field, is constructed and tested. In this calibrator, the performance of the u-u intensity probe compares favorably to that of an acoustic intensity probe which measures both pressure and velocity directly.

Pressure-particle acceleration underwater acoustic intensity sensor

An underwater acoustic intensity sensor designed to measure one component of the acoustic intensity vector is discussed and evaluated experimentally. This sensor consists of a pressure transducer in the form of a piezoceramic hollow cylinder, and an accelerometer mounted inside of the cylinder. This is a pressure–acoustic particle acceleration type of intensity probe, and is henceforth denoted as a ‘‘p-u-dot intensity probe.’’ The probe body has syntactic foam endcaps that are adjusted in thickness to allow the entire assembly to be neutrally buoyant. It is coated with polyurethane for waterproofing while maintaining acoustical transparency in water. Each of the two transducers making up the intensity sensor is calibrated individually in both air and water. The integrated intensity probe is calibrated in a water-filled plane-wave tube. It is shown that this neutrally buoyant underwater acoustic p-u-dot probe measures one component of sound intensity that is in very close agreement to the predicted value using slow-waveguide acoustic theory. Some limitations in the experiments and future works are discussed. [Work supported by ONR, Code 321 MURI in Acoustic Transduction: Materials and Devices.]

Comparative study: Flow-induced noise on underwater acoustic pressure, particle velocity, and intensity sensors

A neutrally buoyant underwater acoustic p-u intensity probe contains discrete sensors for direct measurement of acoustic pressure and particle velocity [J. A. McConnell and G. C. Lauchle, J. Acoust. Soc. Am. 103, 2755(A) (1998)]. Intensity is calculated from the cross spectrum between these two quantities. Thus a single probe can be categorized as a compound sensor capable of independently measuring acoustic pressure, particle velocity, and intensity. Using this probe, a comparative study concerning the measurement of all three quantities in the presence of mean flow and an independent sound source was performed. Bias errors, which result from the flow-induced noise generated by towing the sensor (e.g., a cylindrical body) in cross flow at low speeds through a channel of water, are presented. Other parameters of interest include analysis of experimental pressure–pressure and pressure–velocity correlation functions. The effect of body geometry (e.g., variation in aspect ratio) on the measured bias errors and correlation functions is also presented. [Work supported by the Office of Naval Research, the Naval Submarine League, and the Acoustics Technology Branch of the Naval Air Warfare Center—Aircraft Division, via the DoD Small Business Innovation Research Program.]

Bias error due to flow-induced noise on a velocity gradient underwater acoustic intensity sensor

A neutrally buoyant underwater acoustic u-u intensity probe employs discrete sensors for the direct measurement of acoustic particle velocity at two locations. Active intensity is calculated from the cross spectrum between the two velocity sensors [K. J. Bastyr and G. C. Lauchle, J. Acoust. Soc. Am. 103, 2755(A) (1998)]. Under quiescent conditions the sensor has no bias error; however, in the presence of mean flow, this is no longer true. A derivation of the bias error that results from performing acoustic intensity measurements in the presence of mean flow is presented. Results show that the bias error due to the turbulent boundary layer generated by fluid flow over the cylindrical body that encapsulates the sensors is proportional to the cross spectrum of the turbulent velocities measured by the sensors. This theoretical derivation is quantified with preliminary experimental data obtained by towing the sensor at low speeds through an open channel of water. [Work supported by Office of Naval Research, Multi-University Research Initiative on Acoustic Transduction, Materials, and Devices.]

Development of a compact suspension system for a neutrally buoyant underwater acoustic intensity probe

Practical use of neutrally buoyant underwater acoustic intensity probes requires a suspension system to position them in some preferred orientation with respect to an acoustic field of interest. However, since these probes typically contain a moving coil geophone to facilitate particle velocity measurements, the design of such suspension systems is not trivial. On one hand, theory mandates that the dynamics of the geophone will not be adversely affected by the suspension as long as the mass-spring system created by the probe and the elastic members has a low-resonance frequency and high quality factor relative to that of the geophone. However, by meeting these two design criteria, wave effects (e.g., flexural resonances) in the elastic members can become a limiting factor in the performance of the device, particularly at high frequencies. This paper presents theoretical and experimental results of an effort to design a compact suspension system that embodies the aforementioned design considerations, but has relatively low susceptibilty to flexural resonances. [Work supported by the Acoustics Technology Branch of the Naval Air Warfare Center—Aircraft Division, via the DoD Small Business Innovation Research Program.]

Calibration of a neutrally buoyant p-u intensity probe

Theory and test methods are presented for calibrating an underwater acoustic intensity probe containing two pressure hydrophones and one moving coil velocity sensor. The intensity probe is classified as a p-u probe and is neutrally buoyant in fresh water. In contrast to p-p and u-u probes, it determines intensity via direct measurement of the acoustic pressure and particle velocity. Specifically, intensity is computed by adjusting the time-averaged rms cross spectrum between the transducer output voltages by an appropriate calibration curve. Because intensity is a vector quantity, both a magnitude and phase calibration is required. This is accomplished by a straightforward technique involving a single reference hydrophone and a priori knowledge of the acoustic field where the calibration is performed. A simple experiment was conducted using a standard-wave tube to verify the effectiveness of the calibration technique. In this experiment, the pressure and velocity sensors were calibrated, then an intensity measurement was made. Results show that the calibration technique is highly effective. [Work supported by the Office of Naval Research and the Naval Air Warfare Center Aircraft Division via the DoD Small Business Innovation Research Program.]

Calibration and testing of a neutrally buoyant u-u intensity probe

The theory of operation and the methodology for calibration of an underwater acoustic intensity probe are explained. The intensity probe which is classified as a u-u probe, consists of two moving coil velocity sensors mounted separately in neutrally buoyant, collinearly oriented bodies. This two-velocity sensor probe is used to determine intensity in a manner analogous to that of pressure gradient hydrophones. The acoustic intensity is shown to be the imaginary part of the cross spectrum measured between the output voltages of the velocity sensors, scaled by a complex, frequency-dependent calibration constant. Due to the vector nature of intensity, these complex calibration constants contain magnitude and phase information. To establish the calibration constant, an experiment was conducted that employed the u-u intensity probe and a reference pressure transducer in a standing wave tube. The intensity in the tube is known, and measurements made of this field with the u-u probe are in agreement with the prediction. [Work supported by the Office of Naval Research, Multi University Research Initiative on Acoustic Transduction, Materials, and Devices.]

The anatomy of underwater rain noise

When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as ‘‘minuscule’’ drops (D≤0.8 mm), ‘‘small’’ drops (0.8 mm≤D≤1.1 mm), ‘‘mid-size’’ drops (1.1≤D≤2.2 mm), and ‘‘large’’ drops (D≥2.2 mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a ‘‘type I’’ damped microbubble oscillating at frequencies near 15 kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, ‘‘type II,’’ ‘‘primary’’ oscillating microbubble of characteristic frequency 2 to 10 kHz depending on the drop diameter. Also, large drops often generate weaker sounds from ‘‘secondary’’ bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the air–water interface is calculated from the probability of bubble formation during a heavy rainfall.

Particle velocity detection using a thin membrane

Underwater acoustic intensity measurements require the detection of both the acoustic pressure and the fluid particle velocity. The measurement of the acoustic pressure is easily obtained using conventional (pressure‐type) hydrophones. The particle velocity may be obtained by measuring the flexural response of an appropriate membrane (constrained by an inertial structure) to the fluid particle velocity. Several thin (50–300 μm) membranes were constructed from materials which closely match (impedance, density) the acoustic medium. The use of low shear speed materials allowed for the construction of small (< 10 cm) membranes which are resonant (fundamental mode) to low‐frequency fluid oscillations. The response of such membranes to the acoustic particle velocity and pressure is reported in the 0.02 to 0.5 ka range.