Increased Background Noise Research Articles

Background Noise Effects on Combustor Stability

This paper considers the effects of background turbulent fluctuations upon a combustor's stability boundaries. Inherent turbulent fluctuations act as both additive and parametric excitation sources to acoustic waves in combustors. Although additive noise sources exert primarily quantitative effects upon combustor oscillations, parametric noise sources can exert qualitative impacts upon its dynamics; particularly of interest here is their ability to destabilize a system that is stable in the absence of these noise sources. The significance of these parametric noise sources increases with increased background noise levels and, thus, can play more of a role in realistic, high-Reynolds-number systems than experiments on simplified, lab-scale combustors might suggest. The objective of this paper is to determine whether and/or when these effects might be significant. The analysis considers the effects of fluctuations in damping rate, frequency, and combustion response. It is found that the effects of noisy damping and frequency upon the combustor's stability limits is relatively small, at least for the fluctuation intensities estimated here. The effects of a noisy combustion response, particularly of a fluctuating time delay between flow and heat-release perturbations, can be quite significant, however, in some cases for turbulence intensities as low as u' r m s /u ∼ 5-10%. These results suggest that deterministic stability models calibrated on low turbulence intensity, lab-scale combustors might not adequately describe the stability limits of realistic, highly turbulent combustors.

Changes in the Prevalence of Vocal Symptoms Among Teachers During a Twelve-Year Period

Vocal symptoms and voice disorders among teachers were studied in 1988 using a questionnaire designed to obtain information on six vocal symptoms that had appeared during the past 2 years. Twelve percent of the 478 respondents reported vocal symptoms occurring weekly or more often. The study was repeated using the same questionnaire in 2001. The results of this second study (n=241) indicate that vocal symptoms had increased considerably. Twenty-nine percent of the teachers reported symptoms occurring weekly or more often, and 20% reported two symptoms or more occurring at least once a week, which is significantly more than in 1988. Accordingly, voice disorders are probably a growing problem among teachers. Several factors may explain these increases. In 2001, the teachers complained more often about increases in the size of their classes. Factors that disturbed normal work routines, such as noisy or misbehaving pupils, had also increased significantly. A growing number of misbehaving pupils probably cause increased background noise and stress and, thus, increase the vocal symptoms in teachers.

Light-in-flight holographic particle image velocimetry for wind-tunnel applications

Holographic particle imaging techniques for air-flow investigations are mainly limited tosmall-scale laboratory experiments. The two main reasons are the limited light poweravailable in conjunction with small tracer-particle sizes, which must be in the order of1 µm to properly probe air flows, and the increased background noise from out-of-focusparticles in deep volumes preventing investigations with higher particle densities. Toensure a good accuracy of the velocity measurements by faithful reconstructiongeometry, the evaluation of particle images is often conducted in the originalrecording set-up. The time-consuming scanning process, however, blocks the flowfacility during evaluation—a disadvantage for measurements in costly industrialwind tunnels. For an alternative, we have introduced off-site reconstruction andevaluation. In recent papers we have furthermore demonstrated principles tocope with little object light and to suppress background noise. Here we present asuccessful implementation of theses principles for wind-tunnel measurements.

Variation of CS salience reveals group II mGluR-dependent and -independent forms of conditioning in the rat

There is good evidence that metabotropic glutamate receptors (mGluRs) are involved in some types of learning, and we have previously suggested that this involvement may reflect the modulation by mGluRs of the signal-to-noise ratio in neural networks. This hypothesis supposes that unspecific activation of mGluRs increases background noise level, so reducing the effectiveness of behaviourally relevant stimuli as signals in the network. We report here that intraperitoneal (i.p.) injection of 4-aminopyrrolidine-2,4-dicarboxylic acid (APDC), a specific agonist of group II mGluRs, disrupts conditioning to context (but not to cue) using conventional procedures. The hypothesis predicts, however, that the effect of the drug should be counteracted by the use of more salient stimuli, which would provide stronger signals to the network. In accordance with this prediction, we find that increases in the salience of either the CS (context) or the UCS (shock) abolish the drug-induced disruption of conditioning. These results suggest that group II mGluRs modulate neural networks involved in association formation.

Sheath liquid effects in capillary high-performance liquid chromatography–electrospray mass spectrometry of oligonucleotides

Fused-silica capillary columns of 200 μm inner diameter were packed with micropellicular, octadecylated, 2.3 μm poly(styrene–divinylbenzene) particles and applied to the separation of oligonucleotides by ion-pair reversed-phase high-performance liquid chromatography. Oligonucleotides were eluted at 50°C with gradients of 3–13% acetonitrile in 50 m M triethylammonium bicarbonate. Addition of sheath liquid to the column effluent allowed the detection of oligonucleotides by electrospray ionization mass spectrometry using full-scan data acquisition with a detectability comparable to that obtained with UV detection. The signal-to-noise ratios with different sheath liquids increased in the order isopropanol<methanol<acetonitrile. The incorporation of volatile acids or bases such as triethylamine or hexafluoroisopropanol into the sheath liquid was found to influence the charge state distribution of oligonucleotides longer than 20 nucleotide units whereas no significant effect was observed with shorter oligonucleotides. Organic acids and bases in the sheath liquid generally deteriorated the signal-to-noise ratios in the chromatograms and mass spectra mainly because of increased background noise. Only a few charge states were observed in the mass spectra of oligonucleotides because of charge state reduction due to the presence of carbonic acid in the eluent. With triethylammonium hydrogencarbonate as chromatographic eluent and acetonitrile as sheath liquid, very few cation adducts of oligonucleotides were observed in the mass spectra. However, the presence of small amounts of monopotassium adducts enabled the calculation of the charge state of multiply charged ions. With acetonitrile as sheath liquid, 710 amol of a 16-mer oligonucleotide were detected using selected ion monitoring data acquisition with a signal-to-noise ratio of 3:1. Finally, capillary ion-pair reversed-phase high-performance liquid chromatography–electrospray ionization mass spectrometry was applied to the separation and characterization of heterooligonucleotide primers.

Sample handling for proteome analysis.

The main factor limiting the sensitivity range for the identification of proteins isolated by two-dimensional (2-D) electrophoresis is sample handling: protein detection limits on the gel, losses during extraction and digestion, as well as interference of gel contaminants and detergents with the mass spectrometry (MS) detection increasing background noise. At the one hundred picomole level, losses are fairly negligible but when the amounts drop below 1 picomole (and subfemtomole peptide detection limits have been reported recently by MS), the losses become a critical point. In order to extend proteome analysis to include very low copy number proteins, methods must be developed to minimize losses and handling steps, maximize digestion and extraction yields, as well as to lower chemical noise. We present several methods that we have developed in our laboratory to: (i) increase the amount of material available in a sodium dodecyl sulfate (SDS)-free form which does not require staining, (ii) increase protein extraction and digestion yields and lower the contamination by autoproteolytic products, and (iii) allow direct modification of the peptide mixture to generate sequence tags.

Rapid peptide mapping by reversed-phase liquid chromatography on nonporous silica with on-line electrospray time-of-flight mass spectrometry.

Reversed-phase liquid chromatography (LC) using a nonporous silica support has been combined with electrospray (ES) time-of-flight (TOF) mass spectrometry (MS) for the fast separation and mass detection of peptides. Using this LC method, the resolution of a peptide mixture can be completed is less than 35 s. The resulting chromatographic peak widths are less than 1 s wide. Because of the unique nature of a TOF mass analyzer, complete mass spectra can be acquired at a rate which is sufficient to sample these narrow peaks. When compared with conventional LC, the same separation takes nearly 20 min to complete, and the signal-to-noise ratio observed in the total ion chromatogram is dramatically lower due to the influence of increased background noise in the mass spectra. The limit of detection for a low molecular weight peptide, Val-Pro-Leu, was found to be 6 pmol with the total ion chromatogram and 500 fmol with the reconstructed ion chromatogram. A peptide map of horse heart myoglobin, completed in 3.5 min, is shown as an example of the results which can be obtained from combining this fast LC method with fast ES/TOF/MS detection capability.

On spatial sampling using the scanning intensity technique

Based on a very accurate temporal and spatial sampling of the radiated intensity and pressure field over a thin vibrating aluminium plate, different scanning procedures and patterns have been simulated by averaging subsets of the basic data. In general, the different simulated scanning patterns gave almost identical results in very good agreement with the ‘ideal’ averaging i.e. averaging all data. Changing the line density, changing the starting point, excluding data at the edges and not scanning after straight lines gave far less deviations than the deviations in the ‘ideal’ averaged intensities with increasing background noise levels.

Numerical investigation of error mechanisms in near‐field acoustic holography (NAH)

Near‐field acoustic holography (NAH) has been validated in precisely controlled laboratory conditions for small scale models. The practical validity of the approach in nonlaboratory settings for larger scale models, where increased background noise, inexact sensor positioning, and other environmental contaminants are likely, needs to be established. In this paper, the effect of these error mechanisms on the accuracy of cylindrical near‐field acoustic holography are investigated numerically. This is accomplished using simulated hologram plane pressure data obtained from surface pressure and velocity results. The surface pressure and velocity are obtained using FEM/BEM calculations (SARA) for a ribbed cylindrical model. The simulated hologram plane data are obtained from the SARA results using a numerical implementation (Gaussian quadrature) of the Helmholtz integral. The errors in the holographic reconstructions associated with background noise, sensor placement, and other error mechanisms are revealed by comparing the holographically reconstructed results with the original SARA results. Since the ribbed cylindrical model under investigation is quite detailed, this approach is a good test of the ability of NAH to appropriately reproduce the complex surface behavior. [Work supported by ONR.]

Activation of calcium current in voltage‐clamped rat glomerulosa cells by potassium ions.

1. We examined Ca2+ influx mechanisms using the whole-cell patch-clamp technique in primary cultures of rat glomerulosa cells. 2. Depolarization of the plasma membrane, as studied by a stepwise or ramp depolarization technique, activated low-threshold, transient (T-type) and high-threshold, long-lasting (L-type) voltage-dependent calcium channels (VDCCs). 3. Extracellular K+ activated an inward current (Ig1), even in voltage-clamped cells. This phenomenon was observed within the physiological concentration range, beginning at 4.6 mM K+o (as opposed to the control level of 3.6 mM K+o). Increased cell conductance and increased background noise indicated that Ig1 is evoked by enhanced channel activity. Potassium induced no outward current in the voltage range examined (-120 to +60 mV). 4. When non-permeable anions were present only in the pipette and Na+ and Mg2+ were omitted from the bath, K+ still activated the current. Ig1 was blocked by 100 microM cadmium but was insensitive to 2 microM nifedipine or to 300 microM Ni2+. 5. In fluorimetric studies elevation of the cytoplasmic Ca2+ concentration in response to K+ (5.6-13.6 mM) was reduced only partially when VDCCs were blocked with Ni2+ (200 microM) and nifedipine (2 microM). 6. Elevation of the K+ concentration shifted the threshold potential of the T-type calcium channel in the negative direction. 7. In summary, K+ as a ligand activates Ca(2+)-permeable channels in rat glomerulosa cells. This current may contribute to the development of Ca2+ signals in response to stimulation with K+ in the physiological range. The reduction of the activation threshold of the T-type current by K+ may also be of physiological significance.

A note on the seafloor coupling characteristics of the new ONR ocean-bottom seismometer

A series of transient tests were conducted to determine the seafloor coupling characteristics of a new ocean-bottom seismometer (OBS) developed for the United States Office of Naval Research (ONR). The OBS comprises a large recording package and a separate sensor package that is deployed from the recording package. In addition to the coupling characteristics of both the sensor and the recording packages, the seismic energy radiated from the main recording package as a result of motion of the recording package was measured. The observed vertical coupling resonances of both the recording package and the sensor package are in good agreement with those predicted by a simple model of soil-structure interaction. The most important result of this study is that significant energy is radiated from the recording package in response to horizontal motions of the recording package. When the sensor package is 1 m from the recording package, the amplitude of the recorded signal is similar to that recorded in the recording package. In the field, this effect will result in distortion of seismic signals and increased background noise recorded by the sensor package if the recording package is disturbed by seafloor currents or biological activity. The amplitude of this signal attenuates by approximately a factor of two as sensor/recorder separation is increased from 1 to 6 m, suggesting that an improved response can be achieved by increasing the separation between the recording package and the sensors. This effect is much less severe for vertical disturbances of the recording package.

Sea surface signatures related to subaqueous dunes detected by acoustic and radar sensors

Side-scan sonar records and radar images of the Lister Tief in the German Bight of the North Sea have been analysed. The radar data show signatures on the sea surface which are related to irregularities in the submarine seabed. Some side-scan and radar data from the test area were taken at different dates, but at the same tidal phase and under comparable weather conditions. Existing one-dimensional models of the radar imaging mechanism predict extremes in radar backscatter above maximum slope regions of subaqueous dunes. However, the acoustic data obtained during the ebb tidal phase do not always show an enhanced background noise and backscattering strength modulation directly above maximum slopes of the dunes. A large variation of the position of background noise has been observed. The experimental acoustic data contradict the results of existing radar imaging models. The sonographs showed that regions with increased background noise at close range (<5 m) are often associated with signatures of enhanced backscatter at ranges farther away (<40 m) or at lower grazing angles (<30°). We conclude that the modulation of scattering strength can be attributed to regions of air bubbles generated by turbulence and breaking water waves. Simulations of the radar cross-section modulation above the large slopes of dunes are too large to remain within the bounds of the weak hydrodynamic interaction theory in the relaxation time approximation. Therefore, this theory is not applicable in the sea area of the Lister Tief. Furthermore, the hydrodynamic mechanism of standing waves or stationary surface deformations associated with dunes is discussed.

Noise levels that are tolerable when listening to speech

It is known that speech understanding declines with increased background noise. It is unknown, however, what noise levels are tolerated by individuals when they are listening to speech. A story recorded by a female was played through an earphone at a comfortable level. A noise was added and the subject was asked to set the noise level at the highest level that would not exceed tolerable listening conditions. The interferences were (1) a babble of 12 voices, (2) speech spectrum‐shaped noise, (3) traffic noise, (4) music in a Muzak style, and (5) noise from a pneumatic drill. The subjects were (1) young adults with normal hearing, (2) elderly (at least 65 yr of age) with essentially normal hearing, (3) elderly who were successful hearing aid users, and (4) elderly who rejected their hearing aids. The following results will be discussed: (1) differences between young and elderly subjects, (2) differences among noises, and (3) differences between successful and unsuccessful hearing aid users. [Research supported by AARP Andrus Foundation.]

Continuum source excited atomic fluorescence in the metastable nitrogen plasma

Source-modulated continuum source excited atomic fluorescence (AFC) detection limits were measured for 13 elements in a metastable nitrogen plasma atom cell which used a carbon furnace for sample introduction. The AFC detection limits were compared to wavelength-modulated atomic emission (AE) detection limits in the same atom cell under the same plasma conditions. AFC detection limits were 5 to 2500 times poorer than the AE detection limits. Increased background noise due to analyte AE was shown to be the primary reason for the poor AFC results.

Adsorptive stripping voltammetry - A new electroanalytical avenue for trace analysis

nickel can also be determined using a copper anode. Conditions are similar to those for nickel except that the applied potentials must be 200 mV more positive than for nickel. The anodic reaction rate constants with copper are typically somewhat greater than with nickel, however the applied potential results in increased background noise so that copper offers no signal-to-noise advantage over nickel. When the background electrolyte contains nickel as suspended nickel hydroxide as described above, copper, nickel, cobalt, silver, platinum, and gold electrodes yield similar analytical currents. These results indicate that suspended nickel hydroxide adsorbs to the metal electrode surfaces and essentially converts them to nickel oxide electrodes. Silver iodide in its room-temperature crystal form is sufficiently conducting so that it can carry the currents necessary for analytical amperometry. The electrode material is contacted using silver epoxy. The cathodic electrode mechanism involved generation of iodide at the silver:silver iodide interface yielding a current proportional to the rate of oxidation of iodide ions at the silver iodide:solution interface. Hypochlorous acid at sub-parts-per-million level thus yields cathodic currents for pH 6 solutions whereas pH 3 must be used for cathodic amperometry of monochloramine [4]. Accordingly a direct, linear response analytical technique for concentrations from more than 5 mg chlorine per liter down to about 10 gg chlorine per liter was developed for either monochloramine or the sum of hypochlorous acid plus monochloramine. The concentrations determined were sufficiently low to allow determination of the rate constant for monochloramine formation under realistic water treatment concentration, pH, and ionic strength conditions. The rate constant obtained was 3.2X 106 L/mol/s, in agreement with earlier reported values extrapolated from higher concentrations and less moderate pH solutions.

Judgments of aircraft noise in a traffic noise background

An investigation was conducted to determine subjective response to aircraft noise in different road traffic backgrounds. In addition, two laboratory techniques for presenting the aircraft noise with the background noise were evaluated. For one technique, the background noise was continuous over an entire test session; for the other, the background noise level was changed with each aircraft noise during a session. Subjective response to aircraft noise was found to decrease with increasing background noise level, for a range of typical indoor noise levels. Subjective response was found to be highly correlated with the Noise Pollution Level (NPL) measurement scale.

Effects of a traffic noise background on judgments of aircraft noise

Subjects were asked to rate their annoyance to recorded aircraft noises when combined with several levels of road traffic noise in two sets of numerical category scaling tests. The aircraft and traffic noise levels were typical of a closed-window indoor environment. For one set of tests, the mean background noise level was continuous, whereas for the other set of tests the mean background noise level was changed between each aircraft noise. Under the continuous background noise conditions, the rated annoyance was found to decrease consistently with increasing background noise level. For example, when the aircraft and traffic noise levels were of equal Intensity, the reduction in annoyance was the equivalent of a 5-dB reduction in aircraft noise. For the discontinuous background tests, no consistent change was noted. Regression analyses of the subjective annoyance ratings showed high correlation with the incremental noise pollution level, LNP, developed by D. W. Robinson [NPL Acre Report AC49 (1971)].

Optical Filter Analysis as a Function of Nonoptimum Noise Conditions

An experimental study was conducted to determine the effects of nonoptimum noise conditions upon the SNR of optical spatial filters. The experiments were performed utilizing matched and inverse filters that are generally recognized as the best types of filters for pattern recognition. Their signal-to-noise ratios were compared as a function of varying background noise. The results indicate that (1) the SNR is a decreasing function of increasing background noise size, and (2) the performance difference between the inverse and matched filters increases as the noise size decreases.