Internal Noise Sources Research Articles

More on Classroom Noise

Reduction of classroom noise in order to improve children's learning environment has to be approached from several directions, of which control of internal noise and control of break-in from external sources are the principal ones. Air conditioning noise is a major problem in many North American classrooms, representing the main internal noise source. Responding to this, ASHRAE's Winter Meeting included a highly successful and up-to-the-minute Seminar on Classroom Noise, stressing the need for quieter HVAC systems in order to achieve improved classroom levels.

On the theory of flux gates with output at fundamental frequency

A parametric theory is developed of flux gates with an output at the fundamental frequency. General and particular design expressions were derived. Special emphasis is focussed on the effect of internal parametric amplification of the useful signal that makes possible detection and measurement of weak magnetic fields. The sources of internal flux gate noise were established and ways for their reduction are indicated. The practically achievable noise level was found to be\({{\left( {2 - 5} \right) \cdot 10^{ - 13} T} \mathord{\left/ {\vphantom {{\left( {2 - 5} \right) \cdot 10^{ - 13} T} {\sqrt {Hz} }}} \right. \kern-\nulldelimiterspace} {\sqrt {Hz} }}\).

The Causes and Effects of Distortion and Internal Noise in Hearing Aids

The Causes and Effects of Distortion and Internal Noise in Hearing Aids

A study of methods of characterising the effects of internal noise sources on submarine flank arrays

Passive sonar arrays are used on submarines and surface vessels to detect the presence and position of other ships in the vicinity and locate and identify potential targets. Noise generated by the machinery inside the submarine carrying the array can affect the capacity to detect other vessels. The aim of this work was to develop methods of characterising the effects of this self-noise on the passive flank arrays which are placed close to the outer surface of the hull. This paper presents methods of calculating the pressure field generated close to the hull using measured vibration data and analyses the results obtained by applying beamforming techniques to this calculated noise field. Measurements on a submerged full scale model section of a submarine hull showed that the nearfield pressure field created by internal mechanical sources and the effects on the array output could be predicted from vibration measurements inside the hull.

Letter: Output slave clock short term stability on SDH equipment

AbstractIn the modern synchronous telecommunication networks the quality of distributed clock is of primary importance. Instabilities of slave clocks due to their internal noise sources introduce jitter on transported tributaries. Several parameters can be used to characterize the short term stability of a slave clock. In this paper we use the Time Deviation (TDEV (τ)) to characterize the short term stability of a slave clock based on Phase Locked Loop (PLL), locked to an ideal input reference signal. Different PLL bandwidth values are considered and the possibility to discriminate between the PLL internal noise types is also emphasized, analysing its output TDEV (τ) curves only.

Torque Reversal and Spin‐down of the Accretion‐powered Pulsar 4U 1626−67

Over 5 yr of hard X-ray (20-60 keV) monitoring of the 7.66 s accretion-powered pulsar 4U 1626-67 with the Compton Gamma Ray Observatory/BATSE large-area detectors has revealed that the neutron star is now steadily spinning down, in marked contrast to the steady spin-up observed during 1977-1989. This is the second accreting pulsar (the other is GX 1+4) that has shown extended, steady intervals of both spin-up and spin-down. Remarkably, the magnitudes of the spin-up and spin-down torques differ by only 15%, with the neutron star spin changing on a timescale |ν/dot ν| ≈ 5000 yr in both states. The current spin-down rate is itself decreasing on a timescale |dot ν/bar ν| ≈ 26 yr. The long-term timing history shows small-amplitude variations on a 4000 day timescale, which are probably due to variations in the mass transfer rate. The pulsed 20-60 keV emission from 4U 1626-67 is well-fitted by a power-law spectrum with photon index γ = 4.9 and a typical pulsed intensity of 1.5 × 10^(-10) ergs cm^(-2) s^(-1). The low count rates with BATSE prohibited us from constraining the reported 42 minute binary orbit, but we can rule out long-period orbits in the range 2 days lesssim Porb lesssim 900 days. We compare the long-term torque behavior of 4U 1626-67 to other disk-fed accreting pulsars and discuss the implications of our results for the various theories of magnetic accretion torques. The abrupt change in the sign of the torque is difficult to reconcile with the extremely smooth spin-down now observed. The strength of the torque noise in 4U 1626-67, ~10^(-22) Hz^2 s^(-2) Hz^(-1), is the smallest ever measured for an accreting X-ray pulsar, and it is comparable to the timing noise seen in young radio pulsars. We close by pointing out that the core temperature and external torque (the two parameters potentially relevant to internal sources of timing noise) of an accreting neutron star are also comparable to those of young radio pulsars.

External noise in periodically forced Rayleigh-Bénard convection.

The effect of internal and external noise sources on the onset of Rayleigh-B\'enard convection is studied for the case of time-periodic forcing. Internal noise is modeled through the inclusion of an additive white Gaussian noise field in the Swift-Hohenberg equation. External noise is modeled as multiplicate Gaussian noise. A comparison with available theoretical and experimental data is presented. We conclude that noise in the control parameter of the Swift-Hohenberg equation is not likely to explain the difference between the thermal noise intensity and the one needed to make theory fit the experiments.

COLLOCATED STRUCTURAL CONTROL FOR REDUCTION OF AIRCRAFT CABIN NOISE

Various approaches to cabin noise reduction within turbo-prop aircraft have been suggested. Since the primary source of internal noise is due to propeller-induced fuselage vibration, the noise can be reduced by minimizing the noise transmission through the fuselage using active structural control. The feasibility of a structural control approach using piezoelectric actuators and structural sensors is investigated. Feedback of collocated strain yields an approach which is simple, modular, and extremely robust. Actuator and sensor selection is based on wave-number arguments. Simulations with a simple vibro-acoustic aircraft model indicate that the desired goal of a 10 dBA reduction in peak internal noise is achievable with relatively few actuator. Various actuator/sensor placement and control architecture configurations are investigated with the eventual goal of minimizing the total weight penalty.

AIRCRAFT FUSELAGE NOISE TRANSMISSION MEASUREMENTS USING A RECIPROCITY TECHNIQUE

The cabin noise environment in typical turbo-prop aircraft is high. To evaluate potential noise reduction approaches, a method of predicting the internal noise is desired that avoids costly flight tests. Instead of carrying out a ground simulation of the propeller sound pressure field, a much simpler reciprocal technique can be used if a computed pressure field is available. A capacitive scanner is used to measure the fuselage vibration response on a deHavilland Dash-8 fuselage, due to the internal noise source. The approach is validated by comparing this reciprocal noise transmission measurement with the direct measurement. The fuselage noise transmission information is then combined with computer predictions on the propeller sound pressure field data to predict the internal noise at two points.

Interaction between external and internal noise sources

Harmonic noise (also called quasimonochromatic noise) is used as an external stochastic source term in a non-Markovian oscillatory system with memory-damping. The stationary second moments are obtained by using the adjoint Fokker-Planck operator method. If the internal noise is harmonic then the occurrence of two maxima in the mean energy of the driven system seems to be obvious and depends on the selected relations of the frequencies of the three oscillators. The effects of the frequency-dependent strength of the external harmonic noise on the resulting energy and some generic cases like the interactions between internal harmonic noise and external white noise or exponentially correlated noise have been investigated.

Signal-frequency uncertainty in spectral-shape discrimination: psychometric functions.

Psychometric functions were measured for spectral-shape discrimination (profile analysis) with the signal frequency either fixed, what we call the signal-known condition, or randomly varied, what we call the signal-unknown condition. The functions were obtained using an adaptive, up-down procedure. In the signal-unknown condition, independent tracks for each signal frequency were interleaved within the same block of trials. The mean slope of the psychometric function [k in log d' = k log delta L+C, where delta L = 20 log(1 + delta p/p)] was 1.13 for the signal-known conditions and 1.19 for the signal-unknown condition. The mean signal-to-standard ratio at threshold (Pc = 79.4%) obtained in the signal-unknown condition was 3 to 4 dB higher than that obtained in the signal-known conditions. The psychometric functions of the ideal observer were derived for both the signal-known and signal-unknown conditions. A comparison of the measured and derived psychometric functions suggests that the internal noise component that is statistically independent across frequency channels contributes minimally to the total (internal and external) noise sources that are involved in the decision process.

Carding Fiber Load Measurement

An optical means is presented for measuring areal fiber density on-line on com mercial carding elements. The splitting fractions of the various elements have been evaluated through a process of peeling fiber loads from the elements, and these fractions have subsequently been used in instrument calibration. The optical device is used to monitor fiber loading of the main cylinder while doffer fiber loading and feed roll speed are also being monitored. The instrumentation itself has been studied for sen sitivity, reproducibility, limitations, and possible sources of internal and external noise. The instrumentation is then used on-line to study mechanisms of fiber transfer and storage. Results of dynamic testing under conditions of varying feed rates offer insight into the process of carding. The complex nature of element loading and unloading has been partially elucidated. Hypotheses are proposed and experimental results pro vided as supporting evidence.

Boundary‐element solution of some structural‐acoustic coupling problems using the multiple reciprocity method

AbstractIn the paper we have developed a new formulation for solution of structural‐acoustic coupling problems by boundary elements using the multiple reciprocity method. It is assumed that the structure is composed of plate components and is excited by the external or the internal noise source. The efficiency of the proposed formulation becomes especially remarkable if the boundary‐value problem is to be solved repeatedly for different values of frequency. The accuracy of the numerical computations has been compared with the analytical solution in a test example.

Extracting 1/f noise coefficients for BJT's

We present a method for extracting the BJT SPICE noise model parameters AF and KF based on a general analysis of the small-signal equivalent circuit and the role of the internal BJT noise sources. The analysis is valid even for transistors with poor current gain and large base-collector conductance, for which the output noise characteristics may not be dominated by base flicker and shot noise. The method consists of interpreting the measured 1/f corner frequency versus DC current data in terms of the BJT's internal noisy small signal equivalent circuit. Measured data is presented for an implanted-emitter and two polysilicon-emitter bipolar technologies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of noise in cascaded non‐linear stages

AbstractThis paper presents a method to analyse noise in cascaded non‐linear stages based on Volterra series for multiport systems. the cascaded stages are partitioned such that internal noise sources from different stages are uncorrelated and linear loading between all stages can be assumed. This partitioning scheme implies that the ensemble average of the magnitude‐squared output response from the last stage in the cascade can be determined as a sum of noise contributions from the individual stages. the method allows both unmodulated (independent) and modulated (dependent) noise sources. A special case of the theory and two related examples are presented to illustrate the method.

SIGNAL-TO-NOISE CONSIDERATIONS FOR SKY-SUBTRACTED CCD DATA

ABSTRACT The standard equation for calculating the uncertainty of photometry obtained from CCDs does not correctly consider the random errors, or 'noise', introduced into observations by procedures used in reducing the data. This paper presents a thorough derivation of the theoretical error equation that considers the contributions from all internal noise sources in the signal-to-noise ratio (S/N) of a sky-subtracted image. A simplified version used for estimating the internal errors from empirical data is also derived. The propagation of noise through the data-reduction process is illustrated through a series of equations for the change in S/N that results from a variety of different operations performed on a CCD frame. Comparing these effects with the results expected for an observation made with an ideal detector suggests a number of ways to improve the precision of photometry through the practices employed in obtaining and reducing the observations.

Commutation relations and laser linewidth.

The waves reflected from a resonator must obey the commutation relations of waves in free space. This fact alone, regardless of whether the medium in the resonator is linear or nonlinear, imposes commutation relations on the internal noise sources. The minimum linewidth of an oscillator can be derived from this fact alone. This paper analyzes the special cases of a nondegenerate parametric oscillator and diode laser oscillator and shows how the physical noise sources satisfy the commutation relations and how the linewidth is determined from them.

Control of the Noise Environment for Passengers in Railway Vehicles

Modern rolling stock is significantly quieter, both internally and externally, than previous designs of carriage on British Railways (BR). However, there remains scope for improvement on future builds of vehicle in order to provide a better acoustic environment for passengers. Attention needs to be given, in particular, to the shape of the sound spectrum. This paper discusses the acoustic characteristics which influence passenger comfort and the extent to which a satisfactory acoustic environment is achievable. Sources of internal noise are identified, and the mechanisms by which the sound energy enters the passenger areas are discussed. Measures which can be taken to influence these mechanisms, preferably at the design stage, are indicated.

Black box analysis of the skin senses as a multiple communication channel.

The skin senses have been investigated as a multichannel parallel information system. In psychophysical experiments using two cold two warmth and two vibration stimuli the capability of these senses have been determined for transmitting independent parallel information. A black box model is used to describe interactions within the system between pairs of signals causing two different response components in a multiple response after multimodal activation. The theory distinguishes two kinds of interaction, those between internal noise sources and between internal signal sources. It appears that the skin senses show only a very small interaction of their respective noise sources whereas their signal sources interact extensively. The repercussions for the skin as a parallel information transmitter is discussed shortly in the context of a development of an optimum "skin vision" system, which should project an environment onto the skin, employing both the sense of touch and the sense of temperature.

Boundary element method applied to certain structural-acoustic coupling problems

This paper is concerned with an integral equation approach to certain structural-acoustic coupling problems. It is assumed that the structure is composed of plate components and is excited by the external or the internal noise source. The vibration of the cavity or the ambient atmosphere is governed by the Helmholtz equation, and that of the structure by the reduced out-of-plane bending vibration equation. These differential equations are transformed into the set of boundary integral equations, and they can be solved means of the usual boundary element method and the boundary-domain element method. Taking account of the coupling conditions on the surface of the structure, we can finally obtain the coupled set of linear simultaneous equations with respect to nodal values. The solution of these equations enables us to investigate the properties of the structural-acoustic coupling system under consideration. The potential usefulness of the proposed method is demonstrated through some sample computations.