Transmission Loss Variation Research Articles

Experimental Investigation on Transmission of Sound in To Metallic Enclosures During Static Firing Tests Of Solid Rocket Motor

Satellite launch vehicle during lift-off generates intense dynamic environment mainly the acoustic noise, which in turn acts on the nearby launch pad systems. Reliable operation of these launch pad ground systems needs to be ensured by safeguarding against these dynamic loads with adequate protection measures. For future launch vehicle programs where the launch turnaround time is a crucial aspect, sensitive systems like ground check-out electronic packages are planned to be placed adjacent to the launch vehicle mounting pedestal on the launch pad to ensure minimal time for preparation. The enclosures housing these electronic packages needs to be configured to minimize the transmission of acoustic levels being imposed from the lift-off induced noise. In the present work, an experimental investigation is carried out on acoustic transmission characteristics of metallic enclosures of different material and thickness during the static firing tests of solid rocket motors. Acoustics generated from the supersonic jet noise has a wide frequency band from 100 to 10,000 Hz with sound power peaking near 1,000 Hz. The electronic components and packages, which are generally vulnerable to the high frequency noise, need to be protected from this lift-off induced acoustics. The enclosures are placed in the far field of this jet noise and the transmission loss measurements are obtained. Variation of transmission loss with material and thickness of the enclosures are studied with specific attention to the internal acoustic modes and the coincidence phenomena. Subsequently, the enclosures treated with acoustic blankets of different thickness are investigated to verify its influence on internal acoustic modes and also the additional reduction in transmission of sound in the dominant frequency band of supersonic jet noise.

Acoustic propagation uncertainty in internal wave environments using an ocean-acoustic joint model

An ocean-acoustic joint model is developed for research of acoustic propagation uncertainty in internal wave environments. The internal waves are numerically produced by tidal forcing over a continental slope using an ocean model. Three parameters (i.e., internal wave, source depth, and water depth) contribute to the dynamic waveguide environments, and result in stochastic sound fields. The sensitivity of the transmission loss (TL) to environment parameters, statistical characteristics of the TL variation, and the associated physical mechanisms are investigated by the Sobol sensitivity analysis method, the Monte Carlo sampling, and the coupled normal mode theory, respectively. The results show that the TL is most sensitive to the source depth in the near field, resulted from the initial amplitudes of higher-order modes; while in middle and far fields, the internal waves are responsible for more than 80% of the total acoustic propagation contribution. In addition, the standard deviation of the TL in the near field and the shallow layer is smaller than those in the middle and far fields and the deep layer.

Predicting Acoustic Variability: Pragmatic Considerations for Selecting a Stochastic or Deterministic Approach

Sonar performance prediction models require environmental inputs, including bottom depth and composition, sound-speed profile (SSP), and wave or wind conditions. Temporal and spatial variation and measurement uncertainties in environmental inputs result in uncertain model outputs. Ideally, acoustic fluctuations could be predicted from input parameter variability using a suitable modeling framework. A data set consisting of 460 SSPs and acoustic transmission loss (TL) measurements between 300 and 900 Hz was acquired during GLISTEN15, a 2015 Mediterranean Sea trial led by North Atlantic Treaty Organization's Centre for Maritime Research and Experimentation. The TL was measured in 5-min sampling periods at fixed source-to-receiver ranges of 0.5-10 km over two separate days. A gradual change in TL as large as 15 dB over 5 min was observed at some receivers and frequencies. Predicted TL variability was compared to measured TL variability by using the SSPs as inputs to an acoustic propagation model. Purely stochastic modeling either over- or underpredicted variability, depending on the temporal and spatial separation of the SSPs used as inputs. In this article, an internal wave model derived from the SSP time series was used to demonstrate the possible effect of internal waves propagating perpendicular to the source-receiver direction. The best agreement between modeled and measured variability was obtained for internal waves of 1536-s period and 0.8-1.6-m amplitude. In this way, an approach that included a physically plausible model of internal wave dynamics grounded in a few relevant in situ measurements provided good predictions of observed TL variability and a reasonable explanation of its origin.

The Oceanographic Sensitivity of the Acoustic Channel in Shallow Water

This article analyzes the effects of key physical oceanographic conditions on underwater propagation in a shallow water environment. Signals at 2 kHz were transmitted and received over ranges of 1-10 km, and the variability in the sound-speed profile, bathymetry, position of the instruments, and sea surface roughness was measured and the uncertainty on each parameter was estimated. The acoustic channel characteristics, including the transmission loss, delay spread, and coherence time, were calculated between a moored five-element vertical line array and a vessel deployed source. The measurement results were modeled using a Bellhop's ray tracing algorithm. The sensitivity of the simulator output was evaluated as a function of the environmental conditions, and the observed variability in the transmission loss was reproduced by varying the input parameters to the model within the observed and estimated bounds. The relative importance of the physical properties of the environment in terms of their impact on the acoustic channel is determined through a comparison of modeled and measured transmission loss variability.

Arctic acoustic transmission loss variability due to ice cover during the year 2016–2017

Over the course of one year, underwater acoustic transmissions were made in the deep-water of the Beaufort Sea as part of the Canada Basin Acoustic Propagation Experiment (CANAPE). The transmissions were monitored at a variety of ranges (100–300 km)and the low-frequency signals (200–300 Hz) showed a strong sensitivity to the evolving ice cover. As the ice cover grew from absent in the mid-September, to its thickest and roughest in late April, the transmission loss grew also. The experimental setup included the ability to beamform so that the arrival pattern could be easily resolved into early individual ray-paths and late near-surface arrivals. This capability was used to estimate the TL variation with the angle of interaction with the ice and with the number of ice reflections. The thickness of the ice cover and roughness was monitored at 6 mooring sites. This allows one to compare the measured losses to the expected loss from the ice-free state. The amount of excess loss can then be estimated via the method of small perturbations for example.

Machine learning methods for estimating probability density functions of transmission loss: Robustness to source frequency and depth

Predicted values of transmission loss (TL) in ocean environments are sensitive to environmental uncertainties. The resulting predicted-TL uncertainty can be quantified via the probability density function (PDF) of TL. Monte Carlo methods can determine the PDF of TL but typically require thousands of field calculations, making them inappropriate for real-time applications. Thus, a variety of alternative techniques based on polynomial chaos, field shifting, modal propagation in ocean waveguides, and spatial variations of TL near the point(s) of interest have been proposed. Recently, an approach to estimating the PDF of TL based on nominal TL, ocean environmental parameters, and machine learning was found to have a success rate of 95% with constant source depth (91 m) and frequency (100 Hz) when tested on 657,775 receiver locations within 100 randomly selected ocean environments. This presentation describes an extension of this approach and its success predicting the PDF of TL for different source depths and frequencies for ranges up to 100 km. This increase in the size of the parameter space furthers the need for a sophisticated method of choosing training examples. Such a method is proposed, and its performance is compared to that of prior techniques. [Work supported by ONR.]

High sensitivity temperature sensor based on singlemode-no-core-singlemode fibre structure and alcohol

A high sensitivity temperature sensor based on a singlemode-no-core-singlemode (SNCS) fibre structure and surrounded with alcohol within a silica capillary is described. In this investigation, no-core fibre (NCF) is used as the multimode waveguide and alcohol is chosen as the temperature sensitive medium. By packaging the alcohol solution with a short length of NCF enclosed within a silica capillary, the surrounding temperature can be detected by monitoring the variations of transmission loss at a specific wavelength. The theoretical analysis predicts this temperature sensor can provide high sensitivity, and the experimental results support this. The maximum temperature sensitivity of the sample is 0.49 dB/°C with a potential temperature resolution of 0.02 °C at the operating wavelength of 1545.9 nm. In addition, the repeatability and response time of the sensor of this investigation are investigated experimentally.

Estimating the probability density function of transmission loss in an uncertain ocean using machine learning

Predicted values of transmission loss (TL) in ocean environments are sensitive to environmental uncertainties. The resulting predicted-TL uncertainty can be quantified via the probability density function (PDF) of TL. Monte Carlo methods can determine the PDF of TL but typically require thousands of field calculations, making them inappropriate for real-time applications. Thus, a variety of alternative techniques based on polynomial chaos, field shifting, modal propagation in ocean waveguides, and spatial variations of TL near the point(s) of interest have been proposed. This presentation describes an innovative approach to estimating the PDF of TL based on nominal TL, ocean environmental parameters, and machine learning. This approach has two main challenges. First, appropriate representations must be found for ground-truth PDFs of TL generated from Monte Carlo calculations so that a neural network can be constructed to predict each parameter of the estimated PDF of TL. Four such representations are considered here. And second, a framework must be developed to generate training data for the neural networks. A proposed framework that predicts candidate environments’ training utility without computing any PDFs of TL is described. The performance of this approach is analyzed and compared to that of prior techniques. [Sponsored by ONR.]

Dependence of magnetic and microwave loss on evolving microstructure in yttrium iron garnet

The parallel magnetic and microwave loss dependence on microstructural evolutions in several polycrystalline yttrium iron garnet samples were studied in detail, focusing on the attendant occurrence of their relationships. In this study, polycrystalline YIG samples were synthesized by employing the mechanical alloying technique and sintering toroidal compacts at temperatures from 600 to 1400 °C. The samples were characterized for their evolution in crystalline phases, structure, microstructure, magnetic hysteresis parameters, microwave losses and electrical resistivity. The results showed an increasing tendency of the saturation magnetization with grain size, which is attributed to crystallinity increase in the grains. The M–H hysteresis loop results showed a transition from disordered-to-ordered magnetism which belongs to different magnetically dominant stages of formation. The starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped loops seems to be dependent on crystallinity, phase purity and a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. An increasing trend of transmission loss with grain size may be attributed to increment of loss contribution from hysteresis and domain wall resonance of the samples. The changes in crystallinity and microstructure, and the associated processes of microwave resonance and relaxation due to domain wall movements and damping of spin rotation contributes to the variations in transmission loss and ferromagnetic linewidth of the samples. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicates improved phase purity and compositional stoichiometry.

Characterizing Transmission Loss Variability During the Target and Reverberation Experiment 2013

A significant driver of uncertainty in sonar performance is the variability in underwater acoustical propagation caused by environmental fluctuations and uncertainty in the position of sources, targets, and receivers. A set of echo-repeat experiments was conducted during the Target and Reverberation Experiment 2013 (TREX13), a sea trial that took place in April to May 2013 in the Gulf of Mexico near Panama City, FL, USA. The variability in measured transmission loss (TL) was characterized using two different methods: Variability with respect to a mean observed TL, and variability with respect to modeled TL. Both one-way and quasi-reciprocal two-way TL measurements at 2250 and 7500 Hz were analyzed to characterize the variability at timescales ranging from less than one second to several days, with the results indicating that the acoustic propagation fluctuates stochastically on all these time scales. The results of statistical tests suggest that the TL variability can be treated as Gaussian fluctuations about a central TL obtained from an acoustic propagation model, with standard deviations of 5 dB over timescales up to one day, or 10 dB over timescales from one to six days.

Impact of a deformed extreme ultraviolet pellicle in terms of the critical dimension uniformity

The usage of an extreme ultraviolet (EUV) pellicle is regarded as a potential solution for defect control because it can protect the mask from airborne debris. However, some obstacles disrupt realistic application of the pellicle, such as its structural weakness, the risk of thermal damage, and so on. For these reasons, flawless fabrication of the pellicle is impossible. We discuss the influence of a deformed pellicle in terms of the nonuniform intensity distribution and the critical dimension (CD) uniformity. When we consider a 16-nm periodic pattern with dipole illumination, a transmission difference (max-min) of 0.7% causes CD uniformity of 0.1 nm. The deflection of the aerial image caused by gravity is small enough to ignore. CD uniformity is <0.1 nm, even for the current gap of 2 mm between the mask and pellicle. However, wrinkling of the EUV pellicle, caused by heat, can cause serious image distortion because a wrinkled EUV pellicle experiences both transmission loss variation as well as CD nonuniformity. The local angle of a wrinkle (as opposed to the period or amplitude of a wrinkle) is the main factor that influences CD uniformity, and a local angle of <∼16 deg is needed to achieve 0.1-nm CD uniformity for a 16-nm L/S pattern.

A fiber-optic interferometer based on non-adiabatic fiber taper and long-period fiber grating for simultaneous measurement of magnetic field and temperature

A dual-parameter sensor based on a fiber-optic interferometer consisting of a non-adiabatic fiber taper and a long-period fiber grating (LPFG) integrated with magnetic nanoparticle fluids has been proposed and experimentally demonstrated. Due to the Mach–Zehnder interference induced by the concatenation of the fiber taper and long-period grating, an interferometric spectrum could be acquired within the transmission resonance spectral envelope of the LPFG. Thanks to different magnetic field and temperature sensitivities of difference interference dips, simultaneous measurement of the magnetic field intensity and environmental temperature could be achieved. Moreover, due to the variation in coupling coefficients of the fiber taper and the LPFG in response to the change of the applied magnetic field intensity, some of the interference dips would exhibit opposite magnetic-field-intensity-dependent transmission loss variation behavior. Magnetic field intensity and temperature sensitivities of 0.017 31 dB Oe−1 and 0.0315 dB K−1, and −0.024 55 dB Oe−1 and −0.056 28 dB K−1 were experimentally acquired for the experimentally monitored interference dips.

Temporal variation of transmission loss by internal tide in the southern sea of Jeju island in summer

Temporal variations of acoustic transmission loss (TL) affected by internal tide are simulated using oceanic data measured at two sites in the southern sea of Jeju island. Temperature and salinity were measured every hour for 25 hours during July 27th and 28th 2009. The periodic fluctuation of temperature due to the internal tide was observed and its vertical displacement was more than 10m. In order to investigate the variation of TL, acoustic propagation between two measurement sites (3.8 km distance) was computed at a source depth of 10 m. Acoustic propagation model RAM was used for the simulation. Standard deviations of TL variation were 4.2 dB and 3.7 dB for center frequencies of 100 Hz and 1 kHz with 1/3 octave band, respectively. The lower frequency was more correlated with the tidal period than the higher one. Detection range was also varied by the internal tidal, up to 1 km for the 60 dB detection level. These results imply that tidal variation of TL should be considered for acoustic researches at...

Optical fiber magnetic field sensor based on cascaded down-taper and spherical structure

An optical fiber magnetic field sensor using intermodal interferometer coated by magnetic fluid (MF) is proposed. The interferometer consists of down-taper and spherical structure formed on the standard single-mode fiber (SMF) by a fusion splicer. Since the refractive index (RI) of the MF is sensitive to external magnetic field, the interferometer coated by MF can be used for magnetic field sensing. Two interference valleys of the interferometer integrated with ferrofluid under different magnetic field intensities have been experimentally analyzed. The experimental results show that there is a linear relationship between the valley wavelength shift and magnetic field intensity for a range of 0 to 20 mT, and the maximum sensitivity reaches up to −0.195 nm/mT. In the range of 0 to 12 mT, the variation of transmission loss at valley wavelength with a magnetic field has a maximum sensitivity of 0.106 dB/mT.

동중국해 북부해역에서 음파전달 특성의 시공간적 변동성

Acoustic propagation in shallow water with changing environments is a major concern of navy. Temporal and spatial variability of acoustic propagation in the northern East China Sea (ECS) is studied, using the 11 years hydrographic data and the Bellhop acoustic model. Acoustic propagation in the northern ECS is highly variable due to extensive interaction of various ocean currents and boundaries. Seasonal variations of transmission loss (TL) with various source depths are highly affected by sharp gradient of sound speed and bottoms interaction. Especially, various bottom sediment types lead to severely degrading a waterborne propagation with bottom loss. In particular, the highly increased TL near the ocean front depends on the source position, and the direction of sound propagation.

Simulation of Temporal Variation of Acoustic Transmission Loss by Internal Tide in the Southern Sea of Jeju Island in Summer

In this paper, temporal variations of acoustic transmission loss (TL) affected by internal tide are studied by computer simulation using oceanic data measured in the southern sea of Jeju Island in summer. Temperature was measured with depth (bottom depth are nearly 80 m) in two sites near Seogwipo coast every one hour for 25 hours during July 27 and 28, 2009. The periodic fluctuation of temperature due to the internal tide was observed and its vertical displacement was more than 10 m. In order to investigate temporal variation of TL by internal tide, acoustic propagation between two measurement sites (3.8 km distance) was simulated with a source depth of 10 m. TL variation for 1/3 octave band of 100 Hz center frequency highly coincided with tidal period but more complex variation with indistinct tidal period was observed for 1 kHz. Maximun standard deviation of TL variation was 4.2 dB for 100 Hz at 2.8 km distance from a source and it was 3.7 dB for 1 kHz. The tidal variation was also shown in detection range and its maximum variance was less than 1 km. These results imply that temporal variation of TL should be considered for acoustic researches at the southern sea of Jeju Island.

Magnetic Field Sensing Based on Magnetic-Fluid-Clad Fiber-Optic Structure With Up-Tapered Joints

A kind of magnetic field sensor based on mode interference effect is proposed. The sensing arm consists of two special up-tapered joints formed on the traditional single-mode fiber by fusion tapering technique. Magnetic fluid is used as the cladding of the structure. The interference valley wavelength or the transmission loss of the sensing structure is sensitive to the external magnetic field, which is utilized for magnetic field sensing. The sensitivity of shift of interference valley wavelength with magnetic field can be up to 325.3 pm/mT in the range of 0-16 mT. The variation of transmission loss at valley wavelength with magnetic field has a sensitivity of -0.2121 dB/mT in the range of 2-30 mT.

Spatial uncertainty in higher fidelity acoustic models, and impact on model-data comparisons

Acoustic propagation models, such as the parabolic equation (PE), are typically parameterized by a single source depth and one or more receiver depths and ranges. At the higher frequencies, propagation angles, and ranges enabled by advances in computer hardware and refinement of algorithms, the acoustic field predicted by the model can contain features at considerably finer scale than the resolution of the requested output points. The typical practice of simply sampling the field at the output points can introduce artificial large scale structure in the output, due to aliasing. However, averaging out the fine-scale spatial variability can also result in a misleadingly precise value for transmission loss (TL) at a given point. We show that reconciliation of acoustic model predictions with experimental data can be greatly improved by presenting the TL variability due to the expected uncertainty of source and receiver position within an otherwise deterministic model. Even with good estimates of source and receiver positions, this variability can be greater than that due to uncertainty in the model's environmental inputs. Model-data comparisons are demonstrated with examples from a recent ONR effort in the eastern Gulf of Mexico which included controlled TL measurements at 0.4–2 kHz in shallow water.

Measurements and modeling of transmission loss variability in Barrow Strait

During the summer of 2012, a field trial was held in Barrow Strait, south of Devon Island in the Canadian Arctic. The trial included a set of acoustic transmission loss experiments recorded on Starfish Sensor Cubes, which include a 1-m cube of seven hydrophones operating in the frequency range of 5 - 750 Hz. The transmission loss runs consisted of 10-minute and 20-minute duration transmissions of 400 Hz and 500 Hz tones made at a discrete set of distances up to 60 km from the recorders. Supporting environmental measurements included sets of CTD (conductivity-temperature-depth) profiles and bathymetric measurements. The effects of the measured environmental properties and variability are investigated via propagation modeling, and compared to the experimental data acquired during these experiments.

유동속도가 단순확장관 음향투과손실에 미치는 영향 해석

Acoustic power transmission loss(TL) is an important performance of the muffler system. TL will be affected by the velocity of the fluid in duct since acoustic pressure varies according to the fluid velocity. In this paper, two kinds of fluid model, potential flow and turbulent flow, for the fluid flowing in simple expansion chamber are considered. The effects of their two fluid models in acoustic TL are investigated for the straight and L-shaped simple expansion chamber. In higher frequency range, the characteristics of TL of the two fluid models show different results. The variation of TL according to the fluid velocity is shown more distinctly when turbulence model is used. Turbulent flow model should be used to obtain better estimation of acoustic TL in higher frequency range.