Uncorrelated Noise Sources Research Articles

Application of Frequency Importance Functions to Directivity for Prediction of Benefit in Uniform Fields

The purpose of the current investigation was to systematically examine two of the assumptions central to the application of Articulation Index weighted Directivity Index (AI-DI) to the prediction of directional benefit across three groups of listeners differentiated by degree and configuration of hearing loss. Specifically, the assumption that (1) changes in speech recognition performance are predictable from frequency specific changes in calculated audibility after applying directivity index (DI) values and (2) applying appropriate frequency importance functions would increase the accuracy of AI-DI predictions of directional benefit were evaluated. The output of a single hearing aid for a speech in noise input was recorded to produce high and low directivity (directional and omnidirectional microphone modes) segments. These segments were then high-pass and low-pass filtered into low- and high-frequency regions and acoustically mixed to generate the eight frequency-specific directivity combinations. All recordings were made through an acoustic manikin placed in a single room, surrounded by five uncorrelated noise sources. The aided sentence recognition, in noise, for three groups of 12 adult participants with symmetrical sensorineural hearing impairment, was then measured across the eight listening conditions. The three groups were differentiated by degree and type of hearing loss including "sloping," "flat," and "severe" configurations. The frequency-specific DI values for each of the eight listening conditions were applied to the calculation of frequency specific noise levels. These corrected noise levels were then used to calculate an Articulation Index using the Speech Intelligibility Index (SII, ). These SII values were then compared with measured speech recognition under the same eight listening conditions. Directional benefit values were then calculated by subtracting the performance of individual participants on the Connected Speech Test (CST) in omnidirectional mode from performance in all other filter conditions. The changes in average DI and AI-DI (using three different frequency importance functions) that existed between omnidirectional and the other seven filter conditions were then calculated for comparison to directional benefit values. The speech recognition data revealed a complex interaction between filter condition and group. Despite this interaction, highly significant positive correlations were found between participants' speech recognition scores and the corresponding SII calculation for all three hearing loss groups. Individual subjects' measured directional benefit was highly correlated with changes in DI. Similar correlations were found for average DI and all three AI-DI weighting methods. As expected, performance and calculated SII values were in good agreement across conditions supporting the hypothesis that DI provides a reasonable frequency-specific estimate of signal-to-noise ratio changes in the test environment. The results further support the use of AI-DI or average DI for prediction of directional benefit. The choice of importance weighting across frequency (flat or frequency importance function based), however, did not improve the accuracy of these predictions; therefore, a simple average DI is advocated. Further, the prediction of absolute directional benefit across hearing loss groups from traditional AI-DI calculations may lead to error if the negative effects of hearing loss on speech understanding, and how these effects vary with degree of hearing loss, are not considered as a contributing factor.

A simulated diffuse field for in situ microphone measurements

The asymmetric polar response of an in situ microphone system requires calculation of the Directivity Index (DI) from the random incidence response, as a calculation of the DI from polar response data in a single plane is inadequate. A technique is presented enabling diffuse field measurements to be performed, without a reverb chamber, in a moderately sized, untreated room over a wide bandwidth. The limitations and practicality of alternative methods are discussed. The use of four appropriately positioned, uncorrelated noise sources is shown to be simple, fast, and practical in comparison to roving source free‐field tests. This also greatly simplifies the DI calculation. The importance of carefully positioning and calibrating the uncorrelated noise sources is shown. The diffusivity of the sound field is further improved by rotation of the test object during the measurement. Methods of verifying the diffusivity of the simulated sound field are discussed. Results are shown for several nonsimple microphone devices including a measurement manikin, a behind‐the‐ear hearing aid mounted on a manikin, and an in‐the‐ear hearing aid mounted on a manikin. The method is shown to be applicable to any asymmetrical polar response microphone system including, but not limited to headsets, lavalier microphones, and loudspeaking telephones.

Range estimation of broadband noise sources in an ocean waveguide using the array invariant

A method is developed for range estimation of broadband noise sources in a horizontally stratified ocean waveguide without a priori knowledge of the environment. It has previously been shown that the range of a transient source can be estimated using the ‘‘array invariant’’ method by analyzing instantaneous beam-time intensity data [Lee and Makris, J. Acoust. Soc. Am. 116, 2646 (2004)]. This method is now extended to range estimation of continuous, broadband noise sources in an ocean waveguide. It is shown that the cross-correlation of instantaneous beam-time intensity asymptotically reaches the array invariant. The range of the source can then be determined without knowledge of the environmental parameters. This method is applied to localize multiple uncorrelated noise sources in a horizontally stratified ocean waveguide without ambiguity.

Voice tracking and spoken word recognition in the presence of other voices

We study the human hearing process by modeling the hair cell as a thresholded Hopf bifurcator and compare our calculations with experimental results involving human subjects in two different multi-source listening tasks of voice tracking and spoken-word recognition. In the model, we observed noise suppression by destructive interference between noise sources which weakens the effective noise strength acting on the hair cell. Different success rate characteristics were observed for the two tasks. Hair cell performance at low threshold levels agree well with results from voice-tracking experiments while those of word-recognition experiments are consistent with a linear model of the hearing process. The ability of humans to track a target voice is robust against cross-talk interference unlike word-recognition performance which deteriorates quickly with the number of uncorrelated noise sources in the environment which is a response behavior that is associated with linear systems.

Better Speech Perception in Noise With an Assistive Multimicrophone Array for Hearing Aids

To evaluate the improvement in speech intelligibility in noise obtained with an assistive real-time fixed endfire array of bidirectional microphones in comparison with an omnidirectional hearing aid microphone in a realistic environment. The microphone array was evaluated physically in anechoic and reverberant conditions. Perceptual tests of speech intelligibility in noise were carried out in a reverberant room, with two types of noise and six different noise scenarios with single and multiple noise sources. Ten normal-hearing subjects and 10 hearing aid users participated. The speech reception threshold for sentences was measured in each test setting for the omnidirectional microphone of the hearing aid and for the hearing aid in combination with the array with one and three active microphones. In addition, the extra improvement of five active array microphones, relative to three, was determined in another group of 10 normal-hearing listeners. Improvements in speech intelligibility in noise obtained with the array relative to an omnidirectional microphone depend on noise scenario and subject group. Improvements up to 12 dB for normal-hearing and 9 dB for hearing-impaired listeners were obtained with three active array microphones relative to an omnidirectional microphone for one noise source at 90 degrees . For three uncorrelated noise sources at 90 degrees, 180 degrees, and 270 degrees, improvements of approximately 9 dB and 6 dB were obtained for normal-hearing and hearing-impaired listeners, respectively. Even with a single noise source at 45 degrees, benefits of 4 dB were achieved in both subject groups. Five active microphones in the array can provide an additional improvement at 45 degrees of approximately 1 dB, relative to the three-microphone configuration for normal-hearing listeners. These improvements in signal-to-noise ratio can be of great benefit for hearing aid users, who have difficulties with speech understanding in noisy environments.

Distance and Reverberation Effects on Directional Benefit

Understanding the potential benefits and limitations of directional hearing aids across a wide range of listening environments is important when counseling persons with hearing loss regarding realistic expectations for these devices. The purpose of this study was to examine the impact of speaker-to-listener distance on directional benefit in two reverberant environments, in which the dominate noise sources were placed close to the hearing aid wearer. In addition, speech transmission index (STI) measures made in the test environments were compared to measured sentence recognition to determine if performance was predictable across changes in distance, reverberation and microphone mode. The aided sentence recognition, in noise, for fourteen adult participants with symmetrical sensorineural hearing impairment was measured in six environmental conditions in both directional and omnidirectional modes. A single room, containing four uncorrelated noise sources served as the test environment. The room was modified to exhibit either low (RT60 = 0.3 sec) or moderate (RT60 = 0.9 sec) levels of reverberation. Sentence recognition was measured in both reverberant environments at three different speech loudspeaker-to-listener distances (1.2 m, 2.4 m, and 4.8 m). STI measures also were made in each of the 12 listening conditions (2 microphone modes x 3 distances x 2 reverberation environments). A decrease in directional benefit was measured with increasing distance in the moderate reverberation condition. Although reduced, directional benefit was still present in the moderately reverberant environment at the farthest speech speaker-to-listener distance tested in this experiment. A similar decrease with increasing speaker-to-listener distance was not measured in the low reverberation condition. The pattern of average sentence recognition results across varying distances and two different reverberation times agreed with the pattern of STI values measured under the same conditions. Although these data support increased directional benefit in noise for reduced speaker-to-listener distance, some benefit was still obtained by listeners when listening beyond "effective" critical distance under conditions of low (300 msec) to moderate (900 msec) reverberation. It is assumed that the directional benefit was due to the reduction of the direct sound energy from the noise sources near the listener. The use of aided STI values for the prediction of average word recognition across listening conditions that differ in reverberation, microphone directivity, and speaker-to-listener distance also was supported.

OPTIMAL TRANSITION RATE AND STOCHASTIC RESONANCE IN A BISTABLE SYSTEM DRIVEN BY POWER-LAW NOISE

In this work, we study the stochastic resonance phenomenon in a bistable nonlinear dynamical system in the presence of an uncorrelated noise source whose distribution decays asymptotically as P(ξ) ∝ 1/ξ2α. We investigate the influence of the decay exponent α on the transition rate and on the optimal noise intensity giving the maximum signal-to-noise ratio when a weak periodic signal is superposed to the external noise. We find that the transition rate achieves a maximum for a finite decay exponent α. However, the optimal noise intensity for stochastic resonance depicts a monotonic power-law correction relative to the usual behavior of nonlinear dynamical systems driven by Gaussian noises.

Two-dimensional noise source imaging with a T-shaped microphone cross array

Spatial cross-correlation beamsteering with a T-shaped sparse array can provide an efficient procedure to identify the directions and to obtain the emission levels of multiple uncorrelated noise sources. The procedure can be applied to stationary or moving sources. It provides two-dimensional image resolution equivalent to that of a full array, but requires significantly fewer microphones. This results in substantially reduced requirements for data acquisition and signal processing, with a material cost reduction. Simulation, laboratory measurement, and field measurement of a train moving at high speed illustrate application of the procedure.

Heating of trapped ions from the quantum ground state

We have investigated motional heating of laser-cooled 9Be+ ions held in radio-frequency (Paul) traps. We have measured heating rates in a variety of traps with different geometries, electrode materials, and characteristic sizes. The results show that heating is due to electric-field noise from the trap electrodes which exerts a stochastic fluctuating force on the ion. The scaling of the heating rate with trap size is much stronger than that expected from a spatially uniform noise source on the electrodes (such as Johnson noise from external circuits), indicating that a microscopic uncorrelated noise source on the electrodes (such as fluctuating patch-potential fields) is a more likely candidate for the source of heating.

Field Trial Evaluations of a Switched Directional/Omnidirectional In-the-Ear Hearing Instrument

The use of directional microphones is one of the few methods available for hearing aids to increase the signal-to-noise ratio. The smaller microphones available with today's technology have increased the viability of their application for in-the-ear (ITE) hearing aids. This study evaluated an ITE hearing aid containing two nondirectional microphones that provides wearer-selectable omnidirectional/directional operating modes. Ten sensorineural hearing-impaired patients were fitted binaurally. During the first trial period, the low-frequency gain decrease produced by the directional mode was not compensated for. The frequency responses were matched during the second trial period. For both trial periods, Hearing in Noise Test results using two uncorrelated noise sources indicated significant speech recognition improvements for the directional mode relative to the omnidirectional mode. Responses on Abbreviated Profile of Hearing Aid Benefit questionnaires, paired-comparison judgments, and interview data revealed that most subjects preferred the directional mode in noisier environments, but many also preferred the omnidirectional mode in quiet listening.

Adaptive noise cancellation with directional microphones

The use of adaptive noise cancellation in communication devices has now been under investigation for more than two decades. Early studies predicted large SNR gains that were rarely obtained when actual devices were tested in typical acoustic noise environments. One of the problems encountered was the lack of coherence between the ‘‘noise’’ sensor and the actual corrupting noise signal. The measured low coherence values were most probably due to time-varying multipath, multiple uncorrelated noise sources, and nonlinearities in the signal and noise transmission paths. This talk will address the expected noise cancellation that can be obtained by using directional microphones to spatially filter the acoustic field into signal-plus-noise and noise channels. Analytical results for spherically and cylindrically isotropic noise fields will be shown and discussed.

Performance analysis of a crosstalk resistant adaptive noise canceller

The Adaptive Noise Canceller (ANC) is a commonly used adaptive filter for estimating signal in additive noise. An important factor that effects the performance of the ANC is the signal crosstalk between the primary and reference channels of the ANC. This results in reduced output Signal-to-Noise Ratios (SNR's) and undesirable signal distortion. A Crosstalk Resistant Adaptive Noise Canceller (CRANC), which is a cascade of two ANC's, can be designed to output a distortion free signal. This paper undertakes the performance analysis of the CRANC filter under the influence of various parameters. It is shown that the CRANC's performance is sensitive to the presence of uncorrelated noise sources. Equations are derived to show the effect of uncorrelated noise sources on the CRANC filter structure. The CRANC's performance in estimating the stimulus evoked nerve signal embedded in larger muscle interference is then evaluated through simulations. The results show that with no or little uncorrelated noise present, the CRANC gives a superior performance compared to the ANC. With higher levels of uncorrelated noise power, however, the CRANC's performance is shown to be comparable to that of the ANC. An alternate multichannel CRANC structure is then utilized to mitigate the effects of the uncorrelated noise sources, and its performance in estimating the nerve signal from the muscle interference is evaluated through simulations.

On-board phase and modulus calibration of large aperture synthesis radiometers: study applied to MIRAS

On-board calibration of bidimensional aperture synthesis radiometers with a large number of antennas by the standard correlated noise injection method is technologically very critical because of the stringent requirements on mass, volume, and phase equalization of the noise distribution network. A novel approach, which makes use of a set of uncorrelated noise sources uniformly distributed in the array, is proposed. Each noise source drives correlated noise only to a small set of adjacent antennas. These sets of antennas are overlapped in order to maintain phase and modulus track along the array. This approach reduces drastically mass and volume of the noise distribution network. Moreover, its phase matching requirement is strongly relaxed because it is only necessary within small sets of adjacent antennas. Power stability of the uncorrelated noise sources is also not a stringent requirement. This procedure allows independent phase and modulus calibration by making use of a reduced number of redundant correlations.

Constant output noise temperature of the superconducting quasiparticle mixer

The output noise temperature, rather than the standard input noise temperature, is shown to be the fundamental measure of the shot noise of the superconducting (SIS) quasiparticle mixer. This conclusion is based on extensive numerical computations of the properties of optimized SIS (superconductor-insulator-superconductor) receivers and also on analytic calculations with separate consideration of correlated and uncorrelated noise sources. The quantum theory of mixing is used to derive a simple analytic expression for the mixer output noise which is independent of frequency, and qualitatively explains the dependence of the output temperature on each of the receiver's operating parameters. The derivation leads to a novel understanding of the SIS mixer which, distinguishes between its correlated and uncorrelated noise sources. The authors predict T(receiver)=2*T(mixer) for many SIS receivers. Numerous experiments substantiate the analysis. >

Noise analysis of soliton communication systems-a new approach

A noise analysis that is applicable to any space-time soliton propagation problem with perturbation by spatially uncorrelated white or nonwhite noise sources is developed. It leads to particularly simple expressions for the variation of the inner products of the pulse with the weighting functions. The method is used to evaluate the effect of filters on the Gordon-Haus effect of amplified soliton propagation. >

Corona discharge streamer model

Starting from the motion of a charged particle in the electric field set up by the conductor system of a power transmission line, the paper develops a model for the streamers released when the voltage stress at the surfaces of conductors leads to corona discharges from them. The frequency content of streamer pulses defines the frequency spectrum of radio noise-fields radiated from transmission lines in corona. This is a definition which is achieved for the first time in analytical form in the present paper. From the basis which the waveshape of a single streamer pulse in the time domain provides, multiple point discharges are interpreted as random uncorrelated noise sources in power spectral density form. Comparisons are made between this frequency domain function and previously published corona excitation functions derived from measurement. Further validation studies are reported in which noise-interfering fields found from the streamer model which the paper develops are correlated with the results of recent measurements on a 500 kV double-circuit line energised for test purposes to 522 kV.

A VLSI-efficient technique for generating multiple uncorrelated noise sources and its application to stochastic neural networks

A method for generating multiple arbitrarily shifted pseudorandom bit streams from a single linear feedback shift register (LFSR) is presented. Each bit stream is obtained by tapping the outputs of selected LFSR cells and feeding these tapped cell outputs through a set of exclusive-OR gates. This enables many neurons to share a single LFSR, resulting in an acceptably small overhead for VLSI implementation. >

Matched‐field processing and the effect of realistic noise correlation models

In simulating acoustic source localization by matched‐field processing, it is desirable to use a realistic ambient noise model. Two general classes of noise model were considered: a planar isotropic noise source distribution and a line source distribution. The former was implemented based on the work of Kuperman and Ingenito [J. Acoust. Soc. Am. 67, 1988–1996 (1980)]. Their expressions for the correlation integrals were adapted to compute the correlation function for both simple and realistic propagation models, through the use of analytical and numerical Green's functions, respectively. The line source model involved the assumption of uncorrelated noise sources along an infinite line and was designed to model a shear zone in a shallow ice‐covered ocean. Using the corresponding Green's functions for a waveguide, the line model correlation function could be reduced to a summation over all mode pairs, each term of which involved an integration over wavenumber. In this paper, the line source correlation model is presented and correlation functions given by both noise models are illustrated. The effect of using these models on the localization of a discrete source by a grid search‐optimization procedure is then examined.

Response of a Twisted Pair Excited by the Nonuniform Electromagnetic Fields of Nearby Loops

This paper derives equations for the noise induced in a twisted pair when the pair is embedded in a nonuniform electromagnetic field of a small current loop. The derived equations are applied to calculate the noise induced in a typical 135-? twisted pair. In the frequency range of 10 kHz-100 MHz, the noise induced in this twisted pair is shown to be less by about 70-135 dB when compared to a similar parallel-conductor line without twist that is embedded in the same field. Included is a simplified method of applying the derived equations to the problem of finding the response of the twisted pair to distinct uncorrelated noise sources, assuming that the sources can be approximated by small current loops.

A functional GaAs FET noise model

It is the purpose of this paper to develop a theory upon which the design of low noise FET amplifiers can be based. This is not a fundamenta model of the noise mechanisms in GaAs FET's, but rather, an endeavor to relate physically measurable device capacitances and resistances to the device noise figure and optimum noise source impedance. I will be shown that the noise performance of an FET can be adequately described by two uncorrelated noise sources. One, at the input of the FET, is the thermal noise generated in the various resis, tances in the gate-source loop. This noise source is frequency dependent and it can be calculated from the equivalent circuit of the FET. The second noise source, in the Output of the FET, is frequency independent, and not recognizably related to any measured parameters. This output nise is a function of drain current and voltage. The decomposition of the FET noise into two uncorrelated sources simplifies the design of broad-band low noise amplifiers. Once the equivalent circuit of a device and its noise figure at one frequency are known, the optimum noise source impedance and noise figure over a broad range of frequencies may be calculated. For the device designer this model also may be helpful in balancing input-output noise tradeoffs.