Spectral Noise Level Research Articles

Deep Sound: A Free-Falling Sensor Platform for Depth-Profiling Ambient Noise in the Deep Ocean

AbstractAmbient noise in the deep ocean is traditionally monitored using bottom-mounted or surface-suspended hydrophone arrays. An alternative approach has recently been developed in which an autonomous, untethered instrument platform free falls under gravity from the surface to a preassigned depth, where a drop weight is released, allowing the system to return to the surface under buoyancy. Referred to as Deep Sound, the instrument records acoustic, environmental, and system data continuously during the descent and ascent. The central component of Deep Sound is a Vitrovex glass sphere, formed of two hemispheres, which houses data acquisition and storage electronics, along with a microprocessor for system control. A suite of sensors on Deep Sound continuously monitor the ambient noise, temperature, salinity, pressure, and system orientation throughout the round trip from the surface to the bottom. In particular, several hydrophones return ambient noise time series, each with a bandwidth of 30 kHz, from which the noise spectral level, along with the vertical and horizontal coherence, are computed as functions of depth. After system recovery, the raw data are downloaded and the internal lithium ion batteries are recharged via throughputs in the sphere, which eliminates the need to separate the hemispheres between deployments. In May 2009, Deep Sound descended to a depth of 6 km in the Philippine Sea and successfully returned to the surface, bringing with it a unique data set on the broadband ambient noise within and below the deep sound channel. The next deep deployment is planned for November 2009, when Deep Sound will descend almost 11 km, to the bottom of the Challenger Deep at the southern end of the Mariana Trench. If successful, it will return with continuous acoustic and environmental recordings taken from the sea surface to the bottom of the deepest ocean on Earth.

A validation test for the Acoustic Footprint Mission Planning System

QinetiQ North America, Technology Solutions Group (QNA) and Wyle Laboratories have integrated two mature, proprietary technologies into a prototype Acoustic Footprint Mission Planning System. Wyle's Noise Model Simulation (NMSim) uses validated linear noise propagation routines in combination with ray-tracing algorithms based on weather data provided by QNA's Precision Airdrop System (WindPADS) to predict the time history of spectral noise levels from a flying aircraft at any number of user-defined receptor locations. Through interpolation over a ground-based receptor grid, the impact of noise propagated through a stratified atmosphere may be quantified in terms of audibility at any point inside the acoustic footprint. Weather data for the NMSim ray-tracing routines is extracted from the WindPADS three-dimensional forecast of wind, pressure, temperature and humidity in the atmosphere as a function of time. The three-dimensional field is the result of physical and dynamic assimilation of forecast and measured atmospheric data, with wind-forcing by the underlying terrain. This paper describes the results of a validation test performed at Yuma Proving Ground, AZ. It is shown that the noise propagation model predictions agree satisfactorily with the measurements of a C-130 cargo plane. [Work supported by Natick Soldier Center.]

Compensation of gradient-induced magnetic field perturbations

Pulsed magnetic field gradients are essential for MR imaging and localized spectroscopy applications. However, besides the desired linear field gradients, pulsed currents in a strong external magnetic field also generate unwanted effects like eddy currents, gradient coil vibrations and acoustic noise. While the temporal magnetic field perturbations associated with eddy currents lead to spectral line shape distortions and signal loss, the vibration-related modulations lead to anti-symmetrical sidebands of any large signal (i.e. water), thereby obliterating the signals from low-concentration metabolites. Here the measurement, characterization and compensation of vibration-related magnetic field perturbations is presented. Following a quantitative evaluation of the various temporal components of the main magnetic field, a digital B 0 magnetic field waveform is generated which reduces all temporal variations of the main magnetic field to within the spectral noise level.

Determination of number of significant components and key variables using genetic algorithms in liquid chromatography-nuclear magnetic resonance spectroscopy and liquid chromatography-diode array detection

A new method is proposed for determination of the number of significant components in hyphenated chromatographic data. The method is based on the application of genetic algorithms (GA). The method is applied to five datasets from on-flow liquid chromatography with nuclear magnetic resonance spectroscopy (LC-NMR) and on three datasets from liquid chromatography with diode array detector (LC-DAD). The effect of different factors such as the application of technique on time or spectral direction, noise level, baseline, chromatographic resolution and relative peak height has been studied. The method is compared with other chemometric approaches for rank analysis and variable selection. The results produced by GA are promising on both types of data.

Near-infrared spectroscopic measurement of urea in dialysate samples collected during hemodialysis treatments.

Single-beam spectra were collected over the combination region of the near-infrared spectrum for 80 samples collected from 15 people over a two-week period. Partial least-squares (PLS) regression was used to generate an optimized calibration model for urea. PLS calibration models accurately measure urea in the spent dialysate matrix. Prediction errors are on the order of 0.15 mM, which is sufficient for the clinical assessment of the dialysis process. In addition, the feasibility of a global calibration model is demonstrated by generating a calibration model from samples and spectra obtained from 12 people to predict the level of urea in samples collected from 3 different people. In this case, the standard error of prediction is 0.09 mM. Spectra were modified in order to systematically examine the impact of resolution and noise. Little impact is observed by altering the spectral resolution from 4 to 32 cm-1. Spectral noise, however, plays an important role in the accuracy of these calibration models. Increasing the magnitude of the spectral noise increases the prediction errors and increases the width of the spectral range necessary for extracting the analytical information. The utility of the method is demonstrated by analyzing dialysate samples collected during actual dialysis treatments. In addition, the necessary resolution and spectral quality necessary for reliable on-line urea monitoring is identified. These findings indicate that a dedicated, on-line urea spectrometer must posses a resolution of 16 cm-1 coupled with a sample thickness of 1.5 mm and spectral noise levels on the order of 25 micro-absorbance units when measured as the root-mean-square (RMS) noise of 100% lines.

Normatização de medidas acústicas da voz normal

Introdução: A análise acústica da voz com o avanço da tecnologia digital surge como um exame complementar promissor para aumentar a precisão diagnóstica em laringologia. Objetivo: Normatização das medidas acústicas de freqüência fundamental, perturbação e ruído na voz normal de brasileiros dos sexos masculino e feminino. Forma de estudo: Clínico prospectivo. Material e Método: Pesquisa no período de março a agosto de 1997, no Ambulatório de Otorrinolaringologia do Hospital das Clínicas da Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo, com 80 voluntários da cidade de Ribeirão Preto e região, sendo 40 do sexo masculino e 40 do feminino, selecionados por meio de triagem, foram realizadas gravações digitais dos fonemas /a/, /e/ e /i/, originando uma amostra de 240 sinais acústicos submetidos ao Programa de Análise Acústica da Voz, desenvolvido na Escola de Engenharia de São Carlos da Universidade de São Paulo com obtenção das medidas acústicas. Resultados: Foram obtidos valores médios em relação a sexo e fonemas das medidas de: Freqüência fundamental, Jitter (Directional Perturbation Factor, Perturbation Variation Index, Jitter Ratio, Jitter Factor, Period Perturbation Quotient), Shimmer (Directional Perturbation Factor, Amplitude Variation Index, Amplitude Perturbation Quotient), Spectral Noise Level, Harmonic-to-noise ratio, Harmonic-to-noise ratio cepstrum, Normalized Noise Energy e Breathiness ratio. Conclusão: A normatização das medidas acústicas é necessária para conhecer melhor a voz normal. Os valores obtidos são compatíveis em sua grande maioria com à literatura existente.

Long time‐base measurements of surf noise

Preliminary results from an experiment conducted over a year to measure surf noise will be presented. In July of 1997 a surf‐monitoring sensor array comprised of four broadband hydrophones and a pressure sensor were deployed in 7 m of water, 200 m southwest of Scripps Pier and roughly 200 m from the surf line. Underwater ambient noise over the frequency range of 50 Hz to 22.5 kHz and wave height data from the pressure sensor has been acquired for 9 min every hour almost continuously since deployment. The resulting data set is in excess of 20 000 9‐min samples, providing data through a wide range of incident wave fields and weather conditions, including several winter storms. Surf noise predominates on the frequency band of a few hundred Hertz up to 1 kHz. Variations in the surf noise levels can be seen on time scales of seconds, due to individual breaking waves; hours, due to tidal variations; and days, due to variations in the incident wave field amplitude. A preliminary analysis of the correlation between surf noise spectral levels and environmental factors, such as wave height and tidal variations, will be presented. [Work supported by ONR.]

Near-IR Reflectance Spectroscopy for the Determination of Motor Oil Contamination in Sandy Loam

We have investigated the application of near-IR reflectance spectroscopy to the determination of motor oil contamination in sandy loam. Although the present work is concerned with a specific case of contamination, we discuss the possibility of applying the method to other organic contaminants and other types of soil. The spectral region considered was 1600–1900 nm, which contains the first overtone of the CH stretch. Using a commercial Fourier transform spectrometer together with cross-validated partial least-squares data analysis, the one-sigma precision for the determination of motor oil in sandy loam was 0.17 wt % (0.13 to 0.26 wt % at the 95% confidence level). The largest contribution to the precision of the determination was sampling error, or inhomogeneity in each sample. Given the precision limit imposed by the sampling error, we found that the performance of the spectrometer could be lowered without affecting the overall precision. In a modeling exercise, adequate performance was obtained with a spectrometer having only seven spectral channels with a spectral resolution of 10 nm and a spectral noise level of 10−3 absorbance units. A design for an inexpensive miniature instrument is presented.

Magnetometer spacing criterion for biomagnetic source current imaging.

A criterion for determining the maximum spacing between magnetometers for measuring the magnetic field is derived. A two-dimensional (2-D) filter model is employed to determine the maximum spatial frequency component present in the magnetic field that is above the spectral noise level. This maximum frequency component is then sampled at a rate greater than twice per period as indicated by the Nyquist criterion, yielding the required magnetometer spacing. It is shown that the rule-of-thumb employed in current clinical biomagnetic array systems, that the spacing between the coils should be approximately equal to the depth of the source, is adequate when the signal-to-noise power ratio is less than 28.4 (14.5 dB). The analysis also quantitatively demonstrates that reducing the separation between the measurement and source planes has a greater effect on the resolution than decreasing the noise level by the same factor. This result is important for employing high Tc superconductor magnetometers that allow thinner thermal insulating layers at the cost of higher thermal noise.

Performance of Mean-Frequency Estimators for Doppler Radar and Lidar

The performance of mean-frequency estimators for Doppler radar and lidar measurements of winds is presented in terms of two basic parameters: Φ, the ratio of the average signal energy per estimate to the spectral noise level; and Ω, which is proportional to the number of independent samples per estimate. For fixed Φ and Ω, the Cramer-Rao bound (CRB) (theoretical best performance) for unbiased estimators of mean frequency (normalized by the spectral width of the signal), signal power, and spectral width are essentially independent of the number of data samples M. For Φ, the estimators of mean frequency are unbiased and the performance is independent of M. The spectral domain estimators and covariance based estimators are bounded by the approximate periodogram CRB. The standard deviation of the maximum-likelihood estimator approaches the exact CRB, which can be more than a factor of 2 better than the performance of the spectral domain estimators or covariance-based estimators for typical Ω. For small Φ, the estimators are biased due to the effect of the uncorrelated noise (white noise), which results in uniformly distributed “bad” estimates. The fraction of bad estimates is a function of Φ and M with weak dependence on the parameter Ω. Simple empirical models describe the standard deviation of the good estimates and the fraction of bad estimates. For Doppler lidar and for large Φ, better performance is obtained by using many low-energy pulses instead of one pulse with the same total energy. For small Φ, the converse is true.

Pitch Effects on Vowel Roughness and Spectral Noise for Subjects in Four Musical Voice Classifications

This study was designed to investigate the effects of vocal fo on vowel spectral noise level (SNL) and perceived vowel roughness for subjects in high- and low-pitch voice categories. The subjects were 40 adult singers (10 each sopranos, altos, tenors, and basses). Each produced the vowel /a/ in isolation at a comfortable speaking pitch, and at each of seven assigned pitches spaced at whole-tone intervals over a musical octave within his or her singing pitch range. The eight /a/ productions were repeated by each subject on a second test day. The SNL differences between repeated test samples (different days) were not statistically significant for any subject group. For the vowel samples produced at a comfortable pitch, a relatively large SNL was associated with samples phonated by the subjects of each sex who manifested the relatively low singing pitch range. Regarding the vowel samples produced at the assigned-pitch levels, it was found that both vowel SNL and perceived vowel roughness decreased as test-pitch level was raised over a range of one octave. The relationship between vocal pitch and either vowel roughness or SNL approached linearity for each of the four subject groups.

Relationship of Spectral Noise Levels to Psychophysical Scaling of Vowel Roughness

Direct magnitude estimation (DME) and equal-appearing interval (EAI) ratings of perceived roughness and spectral noise level (SNL) measurements of isolated sustained vowel samples (/a/ and /i/) were obtained. The linear and curvilinear relationships between the resulting roughness ratings and SNL were then examined. In the present study, the comparison between the two models resulted in conflicting evidence regarding the model that best described the relationship between roughness ratings and SNL measurements. In all comparisons, however, a significant linear component was indicated. Additionally, when a significant curvilinear component was indicated, the difference between the lines and curves that were fit to the data was minimal. The present findings suggest, therefore, that a high degree of linearity exists between both DME and EAI roughness ratings and SNL.

Harsh voice: Vocal effort perceptual ratings and spectral noise levels of hearing-impaired children

This study investigated the relationship between spectral noise levels and ratings of perceived vocal effort in the voices of severely to profoundly hearing-impaired children as compared to normal-hearing children, aged 6–11. Spectral noise levels served as an index of the inharmonic (noise) components present from 100 to 2500 Hz for sustained vowels. A nine-point perceptual rating scale was devised as the subjective measure of vocal effort. The severely to profoundly hearing-impaired children could be differentiated on the basis of significantly higher ratings for perceived vocal effort in addition to higher physical measurements of spectral noise levels. Tentative therapeutic implications are discussed.

Spectral noise level measurements used to track voice improvement in one patient

In this report we present one example of the utilization of an acoustic measurement to aid in describing voice roughness objectively in a clinical context. Vowel spectral noise level (SNL) and fundamental voice frequency ( F 0) measurements were acquired for five sustained vowels produced by one patient having bilateral vocal fold nodules. The measurements were obtained at specific time intervals while the patient underwent voice therapy. Clinically observed changes over the course of therapy included an improvement in perceived voice quality, a general reduction in vowel SNL, and an increase in vowel F 0. These observations were accompanied by visually detected laryngeal tissue changes. The results suggest that acoustic SNL measurements can be employed clinically to verify and support perceptual judgements of voice quality.

Offshore measurements of the underwater acoustic noise spectrum during rainfall

The underwater acoustic noise spectrum during rainfall has been measured in a coastal ocean environment for a wide range of sea states and rainfall intensities. The relationship between spectral noise level and rain rate was examined and a power law function fitted to data at 5, 8, 15, 20, and 30 kHz. Finally, a family of curves representing the acoustic noise spectrum for sea states 12, 1, 2, 3, 4, and 5 and rain conditions of 1.0, 3.0, 5.0, and 7.0 mm/h was derived. Results indicate a domination of the acoustic noise spectra by rain‐generating mechanisms at frequencies in the band 7–25 kHz and sea states less than 4. Furthermore, the smearing of the rain‐generated spectral peak near 15 kHz strongly supports the postulation of a strong dependence of rain‐generated acoustic noise on sea surface conditions. [Work supported by DSS Unsolicited Proposals Program, IOS, and DND.]

Near-surface ocean ambient noise measurements at very low frequencies

Ambient noise in the frequency range 1–20 Hz was measured for 1-day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface-suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow-band spectral analysis was performed at 1-h intervals to form time series of noise spectral level for each site. Statistical analyses of the power-spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind-speed dependence of mean spectral level. Comparisons with data from bottom-mounted hydrophones and studies of inter- and intra-array coherence indicate that the data were, with few exceptions, uncontaminated by self-noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5- to 3-Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was much weaker, suggesting that the noise sources at this frequency are more distant. Estimates of noise directionality and tests for normality were also made on the data between 1 and 5 Hz.

Theoretical design of acoustic treatment for noise control in a turboprop aircraft

An analytical procedure has been developed for design of acoustic treatment for cabin noise control of light aircraft. Using this approach acoustic add-on treatments capable of reducing the average noise levels in the cabin by about 17 dB from the untreated condition are developed. The added weight of the noise control package is about 2% of the total gross takeoff weight of the aircraft. The analytical model uses modal solutions wherein the structural modes of the sidewall and the acoustic modes of the receiving space are accounted for. The input noise spectral levels are selected utilizing experimental flight data. The add-on treatments include aluminum honeycomb panels, constrained layer damping tape, porous acoustic materials, noise barriers, limp trim panels, and tuned dampers. To reduce the noise transmitted through the double-wall aircraft windows to acceptable levels, changes in the design of the aircraft window are recommended.

The tidal signal in inverted echo-sounder records

Four IES records of several months duration from the western equatorial Atlantic are analysed with principal interest in their tidal content. Spectral noise level in the tidal bands is some two orders of magnitude higher than in comparable sea-level records, but the main constituents of both diurnal and semi-diurnal tides stand out with usable coherence with the tidal potential. A mid-ocean record, FLAVIA, gives amplitudes and phases that correspond closely with the surface tide, but three other records in a region of disturbed bathymetry near the continental shelf give amplitudes and phases which differ from the expected surface effect, indicating relatively strong coherent internal tides in the region as well as an evident incoherent tidal signal. Two of the latter records also show second-harmonic distortion, which is characteristic of internal tides, in the present case corresponding to a steeppened forward face of the internal wave. Theory, following the analysis of Long (1972 Tellus, 24, 88–89), suggests that this form of wave steepening is due to the steady shear in the surface layer. The physical theory of acoustic time-delay in vertical transmission through long internal waves with and without surface elevation is analysed quantitatively. In the region studied a pure internal tide of amplitude 10 m at 200-m depth would produce the observed changes in tidal signal. Wave amplitudes resulting from tidal flow over a 2.4-km high ridge, computed from the linear theory of Zeilon (1912 Kungliga Svenska Vetenskapsakademiens Handligar, 47, 1–45), are only about 1.2 m maximum, but the theory does suggest a likely mechanism for producing coherent internal tidal motion, possibly by invoking the shelf edge.

Improving the Quality of a Noisy Speech Signal

In this paper we discuss the problem of reducing the noise level of a noisy speech signal. Several variants of the well-known class of “spectral subtraction” techniques are described. The basic implementation consists of a channel vocoder in which both the noise spectral level and the overall (signal + noise) spectral level are estimated in each channel, and the gain of each channel is adjusted on the basis of the relative noise level in that channel. Two improvements over previously known techniques have been studied. One is a noise level estimator based on a slowly varying, adaptive noise-level histogram. The other is a nonlinear smoother based on inter-channel continuity constraints for eliminating the so-called “musical tones” (i.e., narrow band noise bursts of varying pitch). Informal listening indicates that for modest signal to noise ratios (greater than about 8 dB) substantial noise reduction is achieved with little degradation of the speech qualify.

Identification of normal and abnormally rough vowels by spectral noise level measurements

Our goal in this study was to learn whether vowel spectral noise level (SNL) measurements might differentiate between normal and abnormally rough test vowel samples. The vowel samples studied were obtained originally for two previous investigations. In one study, 20 normal-speaking subjects phonated in isolation each of five test vowels (/u/, /i/, /λ/, /a/, /æ/) both normally and with simulated abnormal vocal roughness. In the other, 20 subjects who individually presented a laryngeal pathology phonated each of the same test vowels in isolation. The 300 test samples available from the two studies were obtained at one intensity and mouth-to-microphone distance (75 dB SPL at 15 cm). From the original studies, narrow-band (3-Hz) acoustic spectra of the vowel samples and SNL measurements were available. For this study, a mean of 25 noise level measurements per vowel sample served as an SNL index for each test production. Additionally, for this study, the test samples were individually classified dichotomously as normal or as abnormally rough in quality by a panel of nine listeners. Data analysis resulted in the identification of an SNL mean value (termed the SNL “criterion”) that appeared to differentiate effectively between the perceptually normal and abnormally rough samples of each test vowel. In general, the findings appeared to support the hypothesis that vowel SNL measurements could be used to identify normal and abnormally rough vowel samples.