Pressure Spectral Density Research Articles

Controllable acoustic deterrent based on the warning signals generated by nonel detonators

Acoustic deterrents are a practical strategy to mitigate the impact of underwater noise on marine mammals. However, their safety and effectiveness are still debatable. This study proposes a controllable acoustic deterrence method to protect marine mammals threatened by underwater blasting noise. The method creates strong-randomness warning signals using nonel detonators and establishes an escape time for animals protected. Combining the BELLHOP ray-based acoustic model with the marine environmental parameters and animals' auditory characteristics, we built a prediction model to establish a link between the acoustic fields and the adjustable source parameters, and provide a Risk zone and Deterrent zone for animals. The simulation and experimental results demonstrated that the root mean squared error between the simulated and measured sound pressure spectral density levels did not exceed 4.5 dB and the coefficient of determination remained at approximately 0.8, indicating that the new deterrent is an effective method with good controllable performances.

Measurements of open-water arctic ocean noise directionality and transport velocity.

Measurements from bottom-mounted acoustic vector sensors, deployed seasonally between 2008 and 2014 on the shallow Beaufort Sea shelf along the Alaskan North Slope, are used to estimate the ambient sound pressure power spectral density, acoustic transport velocity of energy, and dominant azimuth between 25 and 450 Hz. Even during ice-free conditions, this region has unusual acoustic features when compared against other U.S. coastal regions. Two distinct regimes exist in the diffuse ambient noise environment: one with high pressure spectral density levels but low directionality, and another with lower spectral density levels but high directionality. The transition between the two states, which is invisible in traditional spectrograms, occurs between 73 and 79 dB re 1 μPa2/Hz at 100 Hz, with the transition region occurring at lower spectral levels at higher frequencies. Across a wide bandwidth, the high-directionality ambient noise consistently arrives from geographical azimuths between 0° and 30° from true north over multiple years and locations, with a seasonal interquartile range of 40° at low frequencies and high transport velocities. The long-term stability of this directional regime, which is believed to arise from the dominance of wind-driven sources along an east-west coastline, makes it an important feature of arctic ambient sound.

The characterisation of underwater noise at facilities holding marine mammals

AbstractA collaborative effort was undertaken to delineate underwater noise levels within holding enclosures at marine mammal facilities. Ambient noise levels were measured under normal operating conditions in the enclosures of 14 participating facilities. Facility habitats varied from ocean environments to fully enclosed pools. The means and standard errors of the noise pressure spectral densities measured across all pools were similar to those measured in natural coastal environments with relatively low presence of anthropogenic noise. Highest levels of noise in land-based pools were generally at frequencies < 2 kHz and primarily due to the operation of water treatment/filtration systems. Noise levels in land-based pools were comparable to or lower than semi-natural and natural systems at higher frequencies because of the presence of biological noise sources in these systems (eg snapping shrimp [Alpheus spp]). For odontocete enclosures, the whales themselves were often the greatest source of sound at frequencies where the whales have their best hearing (~40-100 kHz). The potential for facility ambient noise to acoustically mask odontocete communication signals and echolocation clicks appears to be low. In general, when noise was elevated it was at frequencies outside the typical frequency ranges of whistles and echolocation clicks, and where odontocetes have poor hearing sensitivity. Occasional noise issues were found; it is therefore recommended that facilities periodically assess enclosure noise conditions to optimise animal management and welfare.

Modelling, measurement and optimization of self-noise of hydrophone with preamplifier

Self-noise is an important specification of hydrophone with preamplifier. This paper presents the modelling, measurement and optimization of the self-noise of hydrophone with preamplifier. Through analyzing various noise sources of hydrophone and its preamplifier, an effective self-noise model of hydrophone with preamplifier was established, and the self-noise was mainly affected by the hydrophone quality factor, preamplifier noise and thermal noise of preamplifier input resistance. In order to measure the self-noise of hydrophone with preamplifier, a measurement system was constructed and used for measuring the self-noise of a RHSA30 hydrophone which manufactured by Hangzhou Applied Acoustics Research Institute (HAARI) and has a typical sensitivity of -173 dB. After optimization of the self-noise of RHSA30 hydrophone, the measured equivalent noise pressure spectral density level was close to 30 dB at the frequency of 1000 Hz. The measurement results proved that the preamplifier current noise which mainly coming from the Operational Amplifier played a leading role at low frequencies of hydrophone with preamplifier. And some basic principles for self-noise optimization of hydrophone with preamplifier were given at the end.

Measurement and characterisation of radiated underwater sound from a 3.6 MW monopile wind turbine

This paper describes underwater sound pressure measurements obtained in close proximity (∼50 m) to two individual wind turbines, over a 21-day period, capturing the full range of turbine operating conditions. The sound radiated into the water was characterised by a number of tonal components, which are thought to primarily originate from the gearbox for the bandwidth measured. The main signal associated with the turbine operation had a mean-square sound pressure spectral density level which peaked at 126 dB re 1 μPa2 Hz-1 at 162 Hz. Other tonal components were also present, notably at frequencies between about 20 and 330 Hz, albeit at lower amplitudes. The measured sound characteristics, both in terms of frequency and amplitude, were shown to vary with wind speed. The sound pressure level increased with wind speed up to an average value of 128 dB re 1 μPa at a wind speed of about 10 ms-1, and then showed a general decrease. Overall, differences in the mean-square sound pressure spectral density level of over 20 dB were observed across the operational envelope of the turbine.

Impact of the chosen turbulent flow empirical model on the prediction of sound radiation and vibration by aircraft panels

A precise definition of the turbulent boundary layer excitation is required to accurately predict the sound radiation and surface vibration levels, produced by an aircraft panel excited turbulent flow during flight. Hence, any existing inaccuracy on turbulent boundary layer excitation models leads to an inaccurate prediction of the panel response. A number of empirical models have been developed over the years to provide the turbulent boundary layer wall pressure spectral density. However, different empirical models provide dissimilar predictions for the wall pressure spectral density. The objective of the present study is to investigate and quantify the impact of the chosen empirical model on the predicted radiated sound power, and on the predicted panel surface acceleration levels. This study provides a novel approach and a detailed analysis on the use of different turbulent boundary layer wall pressure empirical models, and impact on mathematical predictions. Closed-form mathematical relationships are developed, and recommendations are provided for the level of deviation and uncertainty associated to different models, in relation to a baseline model, both for panel surface acceleration and radiated sound power.

Flow-noise and turbulence in two tidal channels.

Flow-noise resulting from oceanic turbulence and interactions with pressure-sensitive transducers can interfere with ambient noise measurements. This noise source is particularly important in low-frequency measurements (f < 100 Hz) and in highly turbulent environments such as tidal channels. This work presents measurements made in the Chacao Channel, Chile, and in Admiralty Inlet, Puget Sound, WA. In both environments, peak currents exceed 3 m/s and pressure spectral densities attributed to flow-noise are observed at frequencies up to 500 Hz. At 20 Hz, flow-noise exceeds mean slack noise levels by more than 50 dB. Two semi-empirical flow-noise models are developed and applied to predict flow-noise at frequencies from 20 to 500 Hz using measurements of current velocity and turbulence. The first model directly applies mean velocity and turbulence spectra while the second model relies on scaling arguments that relate turbulent dissipation to the mean velocity. Both models, based on prior formulations for infrasonic (f < 20 Hz) flow-noise, agree well with observations in Chacao Channel. In Admiralty Inlet, good agreement is shown only with the model that applies mean velocity and turbulence spectra, as the measured turbulence violates the scaling assumption in the second model.

On the origins of ambient biological sounds in shallow water tropical ecosystems

Although discovered more than 60 years ago, the origins of much ambient underwater biological noise remain unclear. Snapping shrimp sounds dominate some environments but elsewhere the shallow-water biological sound field is often heterogeneous. Here we show that dominant components of underwater ambient noise recorded on coral reefs around five islands in the central Pacific may be caused by the interaction of hard-shelled benthic macro-organisms with the substrate. Recordings show a consistent, nightly 4.7 to 6.9 dB increase in estimated pressure spectral density level in the 11 to 17 kHz band with a spectral peak centered between 14 to 15 kHz. Underwater time-lapse photography reveals a marked night-time increase in benthic invertebrate activity at most locations, temporally consistent with the increase in pressure spectral density level. Intensity-filtered recordings of an example species, the hermit crab Clibanarius diugeti, in quiet aquarium conditions reveal that transient sounds produced by interaction between the crustacean’s carapace, shell, and coral substrate are spectrally consistent with the central Pacific recordings. Passive acoustic monitoring of such ambient noise may be useful as a complementary ecological survey technique to SCUBA-based visual observations, which are typically poor in estimating the abundance and diversity of cryptobenthic organisms.

Auditory masking of a 10 kHz tone with environmental, comodulated, and Gaussian noise in bottlenose dolphins (Tursiops truncatus)

The pattern of auditory masking derived from Gaussian noise is often cited and used to predict the detrimental effects of masking noise on marine mammals. However, environmental noise (both anthropogenic and natural) may not always be Gaussian distributed. Some noise sources are highly structured with complex amplitude fluctuations that extend across frequency regions, which are often termed comodulated noise. Recent evidence with bottlenose dolphins using comodulated noise demonstrated a significant release from masking compared to Gaussian maskers of the same bandwidth and pressure spectral density level, a result known as comodulation masking release. The present study demonstrates a pattern of masking where both temporally fluctuating comodulated noise and environmental noise produce lower masked thresholds compared to Gaussian noise of the same spectral density level and bandwidth. Furthermore, a threshold reduction or "masking release" occurred when the environmental noise bandwidth increased beyond a critical band. These results provide further evidence that conventional models of auditory masking using Gaussian maskers (i.e., the power spectrum model) do not fully describe the masking effects that occur in realistic environments.

Sensitivity analysis of coupled structural–acoustic systems subjected to stochastic excitation

A development for computing the acoustic pressure spectral density and its sensitivity of coupled structural–acoustic systems subjected to stochastic excitation is presented. Previous work in the area of structural–acoustic response considered systems subject to deterministic excitations. The response computation depends on the excitation; therefore, new methods are developed to account for stochastic excitation. The structural–acoustic response is calculated using finite element methods and stochastic analysis techniques. An accurate and highly efficient algorithm series for structural stationary random response analysis, pseudo excitation method (PEM) is used. Numerical examples are given to demonstrate the effectiveness of the methods and the program.

Structural/Acoustic Sensitivity Analysis of a Structure Subjected to Stochastic Excitation

A development for computing the structural/acoustic sensitivity of systems subjected to stochastic excitation is presented. Previous work in the area of structural/acoustic sensitivity analysis considered systems subject to deterministic excitations. The sensitivity computation depends on the excitation; therefore, new algorithms are developed to account for stochastic excitation, The structural/acoustic sensitivity is calculated using finite element methods, boundary element methods, and stochastic analysis techniques. Structural/acoustic sensitivities for the radiated sound pressure spectral density and the radiated sound power are computed. The sensitivity values are calculated with respect to a set of sizing structural design variables. The random pressure distribution exerted on the surface of the structure by turbulent boundary-layer flow is considered as excitation. Two classic structural models, a simply supported plate and a stiffened simply supported plate, are analyzed to demonstrate and validate the algorithms. Structural/acoustic sensitivity results are compared successfully to data computed by finite difference analyses.

Turbulent Noise Generated from A Multiple-Disk Fan.

We proposed a theory for estimating the spectral density distribution and the overall sound pressure level of the turbulent noise radiated from a multiple-disk fan. In the theory, experimental data of correlations of the longitudinal and the lateral correlation length as well as the pressure spectral densities were used. The effects of the casing on the radiated noise were clarified by experiments by comparing the spectral density distributions with and without casing. The validity of theoretically obtained formula was examined with respect to the effects of parameters such as the gap between two disks, the rotational speed, the number of disks, the thickness of disks, the radius of an impeller and the flow rate. The agreement between the theoretical and the experimental results was generally satisfactory.

Features of the turbulent boundary layer pressure field on an elastic surface

In any approach to calculation of the wall pressure spectrum of a TBL based on an inhomogeneous (Lighthill) wave equation, singularities of the spectral density arise along various lines or surfaces in (k,ω) space. These lines of surfaces correspond to the acoustic or structural dispersion relations, and the singularities may be controlled by such mechanisms as surface finiteness, dissipation in the structure or fluid, and surface compliance. This paper will examine the way in which these various mechanisms control the singularities in different regimes of the spatial and dynamical parameters involved, and will give scaling laws for the pressure spectral densities at the critical conditions in (k,ω) space. [Work supported by ONR, Code 425.]

Dynamic pressure distribution on large circular cylinders caused by wind generated random waves

A technique has been developed to evaluate the dynamic pressures on cylindrical structures due to irregular waves based on McCamy and Fuchs linear diffraction theory and principle of superposition. Experiments have been conducted in Hydraulic Engineering Laboratory, Indian Institute of Technology, Madras to examine the dynamic pressures excited by irregular waves on circular cylinders of large diameter. The pressures around the cylinder at representative locations were measured and compared with calculated pressures. The spectral density functions computed from pressure time series measured in the laboratory agree well with the calculated pressure spectral densities. The agreement is good and encouraging.

Prototype model of diesel engine vibration and noise generation

Insight into how combustion‐pressure time variations generate external noise results from analysis of a simplified model of a single cylinder four‐cycle engine. The crankcase is a long rectangular “thin‐walled” box that vibrates as a cantilever beam. Oscillations are excited by forces exerted on the main bearings, these forces being transmitted from the combustion cavity through the piston and connecting rod. The oil pan is a thin plate whose oscillations are driven by displacements along its perimeter, these displacements resulting from the crankcase beam oscillation. The engine head is assumed rigid. Damping results from the sliding of the piston relative to the block. Analysis leads to a detailed description of the outer surface vibrations. Sound generation is bracketed by computations based on (1) the multipole matched‐asymptotic‐expansion model (low frequencies) and (2) the geometrical theory of diffraction (high frequencies). The result relates ratio of pressure spectral densities (external to internal) to engine structure dimensions, mass distribution, elastic modulus, piston‐cylinder dashpot constant, and engine speed.

Low wavenumber wall pressure measurements using a rectangular membrane as a spatial filter

The response of a rectangular membrane to a convecting random pressure field is interpreted to reveal the inherent wavenumber filtering characteristics of the device. After experimental determination of its resonant response characteristics, one such membrane is used to measure the low wavenumber components of the wall pressure fluctuations beneath a plane turbulent boundary layer. The measurements were made at wavenumbers far below the convective region ( k 1 = ω/ U c ) but above the acoustic region ( k 1 ⩽ ω/ c o ). Possible contamination by acoustic and convective ridge effects is considered. The low wavenumber measurements are also compared with values of the wavenumber-frequency pressure spectral density Φ p( k,ω) obtained by Fourier transforming cross-spectral density data obtained by Blake in the same wind tunnel. From this comparison it is seen that this transformation of Blake's data (based on a Corcos model) grossly overestimates the magnitude of Φ p( k,ω) in the low-wavenumber region. The measured values of Φ p( k, c o) are about 36 dB down from convective ridge levels at the same frequency. The data are also compared with earlier results obtained by Jameson. In a similar frequency range the current data levels are approximately 10 dB higher than those of Jameson.

Rational Function Representation of Flap Noise Spectra including Correction for Reflection Effects

A rational function is presented for the acoustic spectra generated by deflection of engine exhaust jets for under-the-wing and over-the-wing versions of externally blown flaps. The functional representation is intended to provide a means for compact storage of data and for data analysis. The expressions are based on Fourier transform functions for the Strouhal normalized pressure spectral density, and on a correction for reflection effects based on the TV-independent-source model of P. Thomas extended by use of a reflected ray transfer function. Curve fit comparisons are presented for blown flap data taken from turbofan engine tests and from largescale cold-flow model tests. Application of the rational function to scrubbing noise theory is also indicated.

Random pressure-induced vibration of pin-ended cylindrical rods and tubes

The theory is developed for the linear elastic analysis of the response of pin-ended rods and tubes in a stationary fluid medium, to random stationary homogeneous surface pressure fluctuations. A limited number of numerical results are given, comparing the results of simple beam theory with the full elastic analysis, with regard to the prediction of response spectra and mean square values of radial and tangential surface displacements and differential axial surface strains, when the local pressure spectral density is either band-limited white noise or Gaussian.

Measurements of the Low-Wavenumber Region of the Turbulent Boundary-Layer Wall Pressure Spectrum

Measurements of the low-wavenumber region, (ωc&amp;lt;k&amp;lt;ωUc), of the TBL wall pressure spectrum have been performed using a thin rectangular steel plate clamped at its edges and located in a wall of an open jet flow of air. The magnitudes of oscillation of individual plate modes were monitored with an accelerometer for a range of air speeds from 21 to 112 ft/sec. We concluded that the response of the plate was controlled by the low-wavenumber region since the contributions from the acoustic noise in the room and the convected pressure field scattering at the plate boundary were too small to account for the observed levels. A knowledge of the modal decay time allowed us to infer the value of the pressure spectral density. The speed dependence was about U∞3.

Wall Pressure Fluctuations at Smooth and Rough Boundaries under Turbulent Boundary Layers with Positive and Negative Freestream Pressure Gradients

Turbulent boundary layers were fully developed over either smooth or rough walls. Reynolds numbers based upon boundary layer thickness and freestream velocity are on the order 104. Fluctuating pressures at the boundary were investigated within a local region, where a freestream pressure gradient was applied. The flow reequilibrated to the negative pressure gradient and was self-preserving, in the region of measurement. With a positive pressure gradient, the flow tended toward separation with increasing downstream distance and was not self-preserving. Two flush-mounted microphones provided pressure spectral densities and time-autocorrelations at single points on the boundary, as well as cross spectra and cross correlations from pairs of points. The influence on wall pressure of roughness is explained in terms of its effects on the mean velocity profile. Fluctuating pressure models, which incorporate a unique dispersion relation c(f) (the phase velocity c is obtained from the cross-spectral density) are found to be inapplicable to flows with a positive freestream pressure gradient. [Research supported by NSRDC.]