Showing papers on "Noise reduction published in 1981"

Active adaptive sound control in a duct: A computer simulation

Abstract: Most active sound cancellation systems reported in the literature use open‐loop control, depend on near‐zero phase delay in control system elements, and require constant acoustic signal transit time from a signal pickup (microphone) to a control sound source (loudspeaker). The applicability of such systems can be significantly enhanced by using closed‐loop control. This study concerns a digital computer simulation of adaptive closed‐loop control for a specific application, sound cancellation in a duct. The key element is an extension of Sondhi’s adaptive echo canceler and Widrow’s adaptive noise canceler from signal processing to control. The adaptive algorithm is thus based on the LMS gradient search method. The simulation shows that one or more pure tones can be canceled down to the computer bit noise level (−120 dB). In the presence of additive white noise, pure tones can be canceled to at least 10 dB below the noise spectrum level for SNR’s down to at least 0 dB. The underlying theory implies that the algorithm allows tracking tones with amplitudes and frequencies that change more slowly with time than the adaptive filter adaptation rate. The theory implies also that the method can cancel narrow‐band sound in the presence of spectrally overlapping broadband sound. The method can be applied more widely, particularly to control systems that involve transport delay.

Image Reconstruction and Noise Evaluation in Photon Time-of-Flight Assisted Positron Emission Tomography

Abstract: In positron CT, the path difference of annhilation pair gamma rays can be measured by time-of-flight (TOF) difference of pair gamma rays. This TOF information gives us rough position information along a projection line and will reduce noise propagation in the reconstruction process. A reconstruction algorithm for TOF-positron computed tomography (PCT) based on the back-projection with 1-dimensional weight and 2-dimensional filtering is presented. Also a formula to evaluate the variance of the reconstructed image and the optimal back-projection function are presented. The advantage of TOF-PCT over conventional PCT was investigated in view of noise figure. An example of such noise figure evaluations for CsF and liquid Xenon scintillators is given.

Low-noise fiber-optic rotation sensing

Abstract: A major source of noise in fiber-optic Sagnac interferometers was identified to be the fluctuation of the phase of Rayleigh backscattering resulting from minute temperature variations and vibration. This noise can be reduced either by introducing a phase modulation into the fiber ring or by the use of a low-coherence source. A noise-equivalent rotation rate of 8 x 10(-4) deg/sec was achieved.

Techniques For Speckle Noise Removal

Abstract: In this paper, several techniques to reduce speckle noise (more generally signal independent multiplicative noise) in images are studied. The techniques include gray scale modification, frame averaging, low-pass filtering in the intensity and density domains, and application of the short space spectral subtraction image restoration technique in the density domain. Some discussions on the theoretical basis of the techniques studied are given and their performances are illustrated by way of examples.

Speckle Noise Reduction in Fiber Optic Analog Video Transmission Using Semiconductor Laser Diodes

Abstract: This paper investigates the effects of speckle noise on analog video transmission systems using semiconductor laser diodes. The system linearity degradation due to speckle noise is examined using different fiber types. Then this paper proposes a new modulation technique employing a superimposed pulse scheme to reduce speckle noise. It is experimentally confirmed that the proposed modulation method is effective in reducing speckle noise. Based on the above investigations, analog video transmission experiments are performed using single-mode fibers, step-index multimode fibers, and graded-index multimode fibers in the 0.8 and 1.3 μm wavelength regions. The results of the transmission tests have confirmed the feasibility of analog video transmission using semiconductor laser diodes.

Vibration/noise reduction device for electrical apparatus

Abstract: A vibration/noise reducing device for applying vibration/noise-reducing vibrations/sound waves of substantially opposite phase to vibrations/noises generated from an electrical apparatus is disclosed, in which the vibrations/noises from the electrical apparatus is sensed by a sensor (12, 70), the sensed analog time-domain signal (14) is A/D converted, the resulting digital time-domain signal (18) is then Fourier-transformed, the resulting Fourier-transformed digital frequency-domain signal (22) is modified in its amplitude/phase to produce a second digital frequency-domain signal (26) for generating a vibration/noise reducing control signal, the second digital frequency-domain signal (26) is inverse-Fourier-transformed, the resulting inverse-Fourier transformed second digital time-domain signal (30) is D/A converted, and the resulting analog time-domain signal (34) is used as the control signal to generate the vibration/noise-reducing vibrations/sound waves to be applied to the virbrations/noises.

Fast Digital Filters with Low Round-Off Noise

Abstract: A novel approach to the realization of fast and efficient IIR digital filters is presented. The realization is based on recently introduced state-space structures and retains and enhances their low noise and sensitivity properties while reducing the number-of-required multiplications. This permits the implementation of higher order optimal forms requiring only 1.34 to 1.65 times the number of multiplications used in the direct form (well known for 'its poor coefficient sensitivity and high output noise levels). The high inherent parallelism of the state-space structure is also further enhanced by the proposed fast digital filter (FDF). The resulting block-processing algorithm is suitable for high speed implementation of digital filters on parallel processing systems. These systems employ fast and efficient techniques, such as distributed arithmetic or multimicroprocessor techniques, to perform the required computation of the long-independent inner products. In addition to low quantization noise, the FDF structure has other desirable properties including low sensitivity and improved limit cycle behavior. For output decimating IIR filters additional savings in the number of multiplications is achieved with the proposed structure.

An error cancellation digital-filter structure and its distributed-arithmetic implementation

Abstract: This note presents a novel digital-filter structure which produces less roundoff noise than that which is produced by the conventional canonical form. This filter structure minimizes the roundoff-noise gain by introducing additional (or altering existing) zeros for the roundoff noise but not for the signal. Since the filter structure is based upon a general noise-canceling technique associated with the quantizer, it can be applied to a variety of forms of digital filters. A distributed-arithmetic ROM implementation of the new structure is presented as an application example to demonstrate its extreme simplicity.

Noise transmission and control for a light, twin-engine aircraft

Abstract: One of the dominant source-path combinations for cabin noise in light twin-engine aircraft is propeller noise being transmitted through the fuselage sidewall. This source-path was investigated and candidate sidewall add- on treatments were installed and tested using both an external sound source and the propeller in ground static engine runs. Results indicate that adding either mass or stiffness to the fuselage skin would improve sidewall attenuation and that the honeycomb stiffness treatment provided more improvement at most frequencies than an equal amount of added mass. It is proposed that double-wall construction in conjunction with skin stiffening should provide a good weight-efficient combination for the aircraft studied. NE of the principal source-path combinations of cabin noise in light, twin-engine aircraft is propeller noise transmitted through the fuselage sidewall. Improved methods of controlling this cabin noise are needed to provide a comfortable passenger environment, while at the same time controlling aircraft weight and fuel consumption. Lighter weight noise control methods are needed to replace traditional approaches which have relied largely on relatively heavy damping and mass treatments. A number of approaches have been investigated for reducing cabin noise for this type of aircraft. Flight tests indicated interior noise can be reduced about 3.5 dB(A) by a reduction of engine rpm in an aircraft with variable pitch propellers.1 Design of propeller configurations is being in- vestigated as a means of reducing the noise generated at the source.2 Theoretical prediction methods for sidewall noise transmission have been developed to aid the search for noise- resistant sidewall structures. Theoretical analysis of interior noise transmission has included mechanical analogy models, rigid-stiffener/flexible-panel models, and more complex flexible-stiffener/flexible-panel models. 3"5 The analyses have been compared with laboratory test data for verification and have been used to examine a number of candidate noise control treatments including variations of skin thickness, stiffener stiffness, and structural damping, and addition of damping, mass, and honeycomb panel stiffening. Previous work has not included evaluation of candidate noise control treatments in an experimental situation using an actual aircraft. Such studies are needed to evaluate and compare candidate treatments, and to guide further development of noise control treatments. The purpose of this paper is to describe an experimental program of evaluation of three noise control treatments. The work is focused on added stiffness in the form of honeycomb panels. Also, two mass treatments are included for comparison. The tests were carried out using a light twin-engine aircraft (Fig. 1). Can- didate treatments were developed using the aircraft with a horn noise source in the laboratory. The performance of the stiffness treatment was verified using ground static runs of the aircraft engines. The laboratory portion of this investigation is described in Ref. 6.

An Improved Prediction Method for Noise Generated by Conventional Profile Coaxial Jets

Abstract: A semiempirical model for predicting the noise generated by conventional velocity profile jets exhausting from coaxial nozzles is presented and compared with small scale static and simulated flight data. Improvements to the basic circular jet noise prediction are developed which improve the accuracy, especially at high jet velocity and near the jet axis.

Noise reduction in digital fluoroscopy systems

Abstract: Apparatus for reducing the effect of x-ray statistical noise and electronic noise in a fluorographic system that displays an x-ray image on a television screen. Analog video signals based on the x-ray image are amplified logarithmically and digitized to yield live pixel signals. Processed pixel signals are averaged in a full image store or memory. Motion is detected by subtracting the stored pixels from the live pixels on a pixel-by-pixel basis in an ALU. The difference resulting from subtraction is used as part of an address to a look-up table (LUT) which contains values equivalent to the difference signals times a noise reduction multiplicative factor, K. The other part of the addresses is the live pixel value. There are several replications of the look-up tables each relating to a particular brightness level range. The one selected is determined by the live signal part of the address which relates to brightness. The K times the difference signals in the ranges are chosen so the amount of noise reduction varies with brightness level as desired for logarithmic signals. The pixels processed as explained above are added in-phase with the stored and averaged pixels and returned to the corresponding full image memory locations.

Iterative image restoration considering visibility

Abstract: A new algorithm for iterative image restoration based on the least-squares criterion, which can provide a restored image with a good appearance for a human observer, is proposed. The introduction of a nonlinear constraint matrix, which is determined by taking account of features of an input image or an intermediate processing result, solves the problem in image restoration of obtaining a result without serious deterioration of local space details and discernible noise for human eyes. It is also evident in the course of derivation of the algorithm that the procedure defined as reblur serves to suppress noise amplification in the sense of the least-squares criterion as well as to ensure convergence of the solution. Suitability of the proposed algorithm is confirmed through experiments of iterative restoration for an image degraded with a Gaussian point-spread function with noise.

New technique for the direct measurement of core noise from aircraft engines

Abstract: A new technique is presented for directly measuring the core noise levels from gas turbine aircraft engines. The technique requires that fluctuating pressures be measured in the far-field and at two locations within the engine core. The cross-spectra of these measurements are used to determine the levels of the far-field noise that propagated from the engine core. The technique makes it possible to measure core noise levels even when other noise sources dominate. The technique was applied to signals measured from an AVCO Lycoming YF102 turbofan engine. Core noise levels as a function of frequency and radiation angle were measured and are presented over a range of power settings.

Disc brake noise reduction

Abstract: A disc brake pad assembly has clench tabs extending through rubber-like grommets in openings in a caliper housing leg so that the grommets are retained in the openings and the brake pad assembly is retained on the housing leg. The grommets provide a noise dampening action during braking to reduce brake noise.

Noise reduction circuit

Abstract: PURPOSE:To sharply reduce noise with a simple circuit configuration by constituting a closed loop for feeding back a reproduced luminance signal to be outputted to an output terminal through an 1H delay circuit CONSTITUTION:For example a reproduced luminance signal with noises which has been frequency-modulated, recorded in a magnetic recording medium, reproduced, and taken out through an FM demodulator is inputted to an input terminal 8 and applied to a subtractor 9 to subtract a reproduced luminance signal from an 1H delay circuit 10 from the said input signal, so that noise free from line correlation are mainly taken out The subtractor 13 outputs noise reduced reproduced luminance signal obtained by subtracting a signal from an attemuator 12 from an reproduced luminance signal inputted from the input terminal 8 to an output terminal 14

Design guide for shipboard airborne noise control

Abstract: After discussing noise criteria, an outline of a shipboard noise control plan is given to assist the designer in formulating approaches to meet the criteria. Guidelines on acoustical design practices are then presented. The major part of the Guide deals with noise prediction procedures, using a source-path-receiver approach. Information on noise control treatments is provided.

A Comprehensive Bibliography of Literature on Helicopter Noise Technology

Abstract: : This bibliography, covering the period 1975 through calendar 1980, also provides, abstracts on literature that appear to make a significant contribution to the field of helicopter noise technology. The helicopter is recognized as a complex noise generator, with significant contributions from the rotors, the engine and the gearbox. Progress continues to be made in the noise areas of: (a) Formulations, math models and analytical procedures; (b) Noise prediction methodology; (c) Noise reduction techniques; and (d) Subjective response to helicopter noise. The body of information, data and knowledge has use in many applications, including the reduction of helicopter noise in a cost effective manner and in minimizing annoyance to the civil populace.

Linear versus logarithmic spatial filtering in the removal of multiplicative noises.

Abstract: Linear and logarithmic coherent spatial-filtering techniques are compared in the separation of multiplicative noise from an image in a coherent-optical image-processing system It is demonstrated that both techniques are effective in the removal of simple grating-type high-frequency noises, but that linear filtering cannot remove noise of spatial frequencies lower than 5 lines/cm, whereas logarithmic filtering can

Field Study of Sound Radiation by Power Transformers

Abstract: Information needed for the acoustic design of power transformer installations required to meet specific property line noise criteria was obtained through a study, in the field, of 40 in-service power transformers. This paper reports on the unique method of data acquisition and of data reduction; summarizes the information gained by the analyses of the nearfield and farfield noise measurements regarding (1) the tonal composition of the core noise, (2) the circumferential statistics of the sound radiation pattern, (3) the variation of the noise level with distance, and (4) the relation between noise levels measured at NEMA positions and in the far field; and recommends improvements in the standard transformer noise measurement method that would yield information about the tonal characteristics of the transformer noise.

Reduction of FM noise in impatt oscillators by optical illumination

Abstract: Experiments are described in which laser illumination of the active device has reduced the FM noise output by up to 5 dB for a GaAs impatt oscillator and 2 dB for a Si impatt oscillator. A qualitative explanation of the observed results is given, and the effect of laser amplitude noise on the maximum noise reduction obtainable is shown.

A unified analysis of roundoff noise reduction in digital filters

Abstract: Two seemingly unrelated roundoff noise reduction approaches, which lead to two distinct digital filter structures, i.e., the error-cancellation structure and the minimum roundoff-noise state-space structure, can be shown to share a common idea. This common idea is the following. The transfer function for the round-off errors is to be altered in such a way that the output noise is reduced in some sense, while the signal transfer function should remain intact. This idea is evident in the error-cancellation structure, however, it is disguised as the similarity transform in the state-space structure. This common idea can be formulated to relate the two approaches and form a new combined approach which seems to offer further insight and other low-noise structures.

Application of a Structural-Acoustic Diagnostic Technique to Reduce Boom Noise in a Passenger Vehicle

Abstract: This paper is No. 81011. Body "boom" is a structurally generated noise that can be excited in a passenger vehicle by road roughness or powertrain vibration. It typically occurs in a frequency band below 200 hz and results when the input forces to the body structure are transmitted to the wall panels around the passenger compartment which, in turn, vibrate and excite the acoustic modes of the compartment cavity. This paper concerns the diagnoiss of boom noise which is excited by engine vibration. Several approaches have been described in the literature for reducing engine induced boom noise. These include reducing the input excitation of the engine to the body structure by use of a vibration absorber, adjusting the engine mounts to tune the engine vibration for minimum acoustic participation, and the use of a Helmholtz resonator to modify the acoustic response of the compartment cavity. Another approach, which is the topic of this paper, is to identify and alter the structural wall panels which excite the boom noise. T his approach has been applied successfully to reduce boom noise generated by road input and is based on the use of a finite element model of the compartment cavity to determine the panel participations in exciting the acoustic modes. A diagnostic technique for noise reduction based on the use of the finite element model together with experimental panel vibration data has been developed, and its application is described in this paper. (Author/TRRL)

Video signal processing circuit for noise reduction

Abstract: In a noise suppression circuit for a video signal the number of elements required can be reduced when a separation circuit (3) for a high-frequency and a low-frequency component is used and the noise suppression is effected in the low-frequency component by means of a comb filter (19) which, in order to realize a still further savings in components, may comprise a delay circuit having a delay (103, 105, 107) which is switchable between a field period plus half a line period and a field period minus half a line period. The noise suppression circuit is therefore particularly suitable for use in television receivers.

Noise rejection system in corona detection apparatus

Abstract: PURPOSE:To increase the effect of noise reduction, by digitizing the 1st and the 2nd pulse through peak detection, and performing the discrimination of signal and noise taking consideration into the combination of the polarity given to the digital signal CONSTITUTION:The 1st and 2nd analog pulses are introduced as the input of the 1st peak detector 6 and the 2nd peak detector 7 respectively from the analog pulse the 1st and 2nd signal lines L1, L2 of the sensor 3 If the polarity of peak of the 1st and 2nd analog pulses is positive, the 1st and 2nd peak detectors 6, 7 output the 1st and 2nd digital pulse outputs +PD1, +PD2, and if the peak polarity is negative, the digital pulse outputs -PD1, -PD2 are outputted, being introduced to the signal 1 noise discrimination logic circuit 8 at the next stage The logic circuit 8 receives the combination of the output +PD1 and -PD2 or output PD1 and +PD2 as tube signals