Low-frequency Noise Reduction Research Articles

WAVE PROPAGATION THROUGH HOLLOW BODIES AND NOISE REDUCTION

Low frequency noise (2–200 Hz) reduction in the passenger compartment has emerged in the past few years as a crucial subject of research in the car industry. This kind of noise is mainly due to the panels' vibrations, therefore our aim is to decrease the part of structural energy that reach them, i.e., we want to increase the part of energy that dissipates while propagating in the car body frame. This approach requires the understanding of structural wave propagation through the beam like structure (pillar, cross members…) as well as reflection and transmission at the structural joints. This is the physical problem that we want to address in this paper. Since car body frames are much too complex for physical understanding, we focused on simpler representative academic structures. We developed a numerical tool for the prediction and visualization of wave propagation, based on finite element models (FEM). Our FEM are first validated by comparison with experimental modal analysis, and then used for transient analysis. In both cases, the good agreement between calculations and experiments shows the reliability of our model and allows us to use it for wave propagation visualization. We illustrate our results by making a movie that helps to understand how waves propagate through a two hollow bodies junction.

Low-frequency noise reduction in Y-Ba-Cu-O SQUIDs by artificial defects

We demonstrate, that extremely simple arrangements of few antidots in HTS rf-SQUIDs can significantly reduce the 1/f noise in ambient field down to the level of zero-field noise. The onset field B/sub on/ at which the low-frequency noise starts to increase, is shifted from B/sub on//spl ap/8 /spl mu/T without antidots to B/sub on//spl ap/40 /spl mu/T with antidots for field cooled measurements and to B/sub on//spl ap/25 /spl mu/T for zero field cooled experiments. The geometric arrangements of the antidots are obtained from the analysis of the current distribution in the washer and the position of penetrating vortices in the case of zero field cooled experiments.

Adaptive Volterra filters for active control of nonlinear noise processes

This paper presents a Volterra filtered-X least mean square (LMS) algorithm for feedforward active noise control. The research has demonstrated that linear active noise control (ANC) systems can be successfully applied to reduce the broadband noise and narrowband noise, specifically, such linear ANC systems are very efficient in reduction of low-frequency noise. However, in some situations, the noise that comes from a dynamic system may he a nonlinear and deterministic noise process rather than a stochastic, white, or tonal noise process, and the primary noise at the canceling point may exhibit nonlinear distortion. Furthermore, the secondary path estimate in the ANC system, which denotes the transfer function between the secondary source (secondary speaker) and the error microphone, may have nonminimum phase, and hence, the causality constraint is violated. If such situations exist, the linear ANC system will suffer performance degradation. An implementation of a Volterra filtered-X LMS (VFXLMS) algorithm based on a multichannel structure is described for feedforward active noise control. Numerical simulation results show that the developed algorithm achieves performance improvement over the standard filtered-X LMS algorithm for the following two situations: (1) the reference noise is a nonlinear noise process, and at the same time, the secondary path estimate is of nonminimum phase; (2) the primary path exhibits the nonlinear behavior. In addition, the developed VFXLMS algorithm can also be employed as an alternative in the case where the standard filtered-X LMS algorithm does not perform well.

Low-frequency aircraft noise and its effects on residential land use

Low-frequency aircraft noise is most noticeable to the rear and side of aircraft that are taking off and are on or near to the ground. Recently concluded at Minneapolis–St. Paul International Airport (MSP), this study investigated levels and effects of low-frequency noise in residential areas. The study built on work conducted at other airports. It reached five significant findings: (1) the primary effect of low-frequency noise annoyance caused by noise-induced rattling of windows, doors and furnishings; (2) the best predictor of annoyance was the average of maximum levels in the 25–80-Hz bands (called the low-frequency sound level, LFSL); (3) the existing residential sound insulation program at MSP does not measurably change the low-frequency noise reduction (LFNR), but it reduces the annoyance as much as a 5-dB reduction in low-frequency noise level; (4) the maximum practical level of noise in new single-family residences is approximately 25 dB, about 10 dB higher than the LFNR found in the typical untreated residence, and (5) the maximum exterior LFSL compatible with residential use is 87 dB.

Gas turbine noise mitigation for a residential development

A residential development was proposed adjacent to a gas turbine electrical power production peaking facility. To determine compliance with local standards, noise levels and frequency content were measured at reference locations under summer and winter conditions. The Environ. Noise Model (ENM) was calibrated on the basis of the reference noise level data, and noise measurement results at other locations were compared to the ENM predictions. The measurement results were also used to determine the effective noise source height for calculations of shielding. The ENM was then used to predict noise levels at the development site. To evaluate methods of obtaining compliance with the standards, the ENM was used to assess the effects of berms, walls, and grading on the development site, and an enclosure for the gas turbines. Recommendations were then prepared for noise abatement and mitigation measures, including the use of an enclosure, an onsite berm, and reduction of low frequency noise.

MULTIPLE REFERENCE FEEDFORWARD ACTIVE NOISE CONTROL PART II: REFERENCE PREPROCESSING AND EXPERIMENTAL RESULTS

Multiple reference active noise control (MRANC) has been applied to acoustical fields with multiple noise sources to achieve low frequency noise reduction. The traditional control configuration feeds each reference signal into a different control filter. This configuration has been widely adopted due to its potential performance in a general multiple noise source environment. However, it entails the problem of ill conditioning when the reference signals are correlated. In this paper time domain analysis has been carried out to investigate the problem of ill conditioning for MRANC. To cope with the problem of ill conditioning, a reference signal preprocessing step is added to the conventional active noise control process. This preprocessing step essentially constructs a new set of reference signals, which preserve all the information of the original reference, but are uncorrelated with each other. An adaptive decorrelation filter based on the Wiener filter theory and Gram–Schmidt orthogonalization theorem is constructed to implement the reference signal preprocessing stip. Experiments based on sound transmission through a vibrating plate have been conducted and the results presented are consistent with the theoretical analysis.

Low-frequency noise reduction in YBa2Cu3O7−δ superconducting quantum interference devices by antidots

It is demonstrated that the low-frequency noise due to vortex motion in high-temperature superconducting quantum interference devices (SQUIDs) in ambient magnetic fields can strongly be reduced by a simple arrangement of antidots patterned into the SQUID. Sputter-deposited YBa2Cu3O7−δ radio-frequency SQUIDs (rf-SQUIDs) with step edge junctions are characterized before and after patterning of antidots in the vicinity of the Josephson junction. No deterioration of the rf-SQUIDs due to the introduction of the antidots can be detected. In contrary, the onset of the increase of the low-frequency noise in an applied magnetic field is shifted from 10 μT for the bare SQUID to 40 (field cooled) and 18 μT (zero-field cooled) for the rf-SQUIDs with antidots. The reduction of low-frequency noise in ambient field is explained by trapping of vortices by the antidots. The comparison of zero-field and field-cooled experiments demonstrates that flux penetrating the washer does not affect the low-frequency noise as long as the vortices are homogeneously distributed and the flux is properly pinned in the vicinity of the junction.

A theoretical study of duct noise control by flexible panels

Theoretical exploration is undertaken for passive noise control by flush-mounted panels in an otherwise rigid duct. For a plane sound wave traveling in the flexible segment, the wall compliance renders a wave speed less than the isentropic speed of sound in air. Scattering and reflection occur at the upstream edge of the panel while the energy flux of the transmitted wave is partitioned between the wall flexural waves and the sound in air. For a lossless panel these waves are scattered and reflected again by the downstream edge forming standing waves responsible for the undesirable passbands. For panels with substantial structural damping, however, both flexural and sound waves diminish with distance, eliminating the passbands. It is estimated that the wave dissipation by panel materials like rubber could outperform typical fibrous duct lining. The combination of wave reflection, dissipation, and slowing down allows broadband, low-frequency noise reduction over a short distance.

Hole confinement and low-frequency noise in SiGe pFETs on silicon-on-sapphire

We present the dc, ac, and low-frequency noise characteristics of SiGe channel pFETs on silicon-on-sapphire (SOS). The SiGe pFETs show higher mobility, transconductance, and cutoff frequency compared to the Si control devices. A significant reduction in low-frequency (1/f) noise is observed in the SiGe pFETs, and understood to be the result of a lower border trap density sampled at the Fermi-level due to the valence band offset. The linear g/sub m/ of the SiGe pFET's at 85 K shows a secondary peak which is attributed to the turn-on of the surface channel.

Reduced low-frequency noise Schottky barrier diodes for terahertz applications

The Schottky barrier diode remains an important device for detecting and mixing at terahertz frequencies, particularly in plasma diagnostics and atmospheric remote sensing. In plasma diagnostics, plasma density is monitored using a receiver with an IF typically below 10 MHz, thus it is critical to reduce low-frequency excess noise to improve sensitivity. We present here a submicrometer Pt/GaAs diode fabrication process that has resulted in a significant reduction in low-frequency noise over that of commercially available devices. Noise performance is optimized by establishing an even distribution of gallium and arsenic (stoichiometric surface) at the contact surface. The effects of different SiO/sub 2/ deposition and etching techniques are also compared. Diodes fabricated with the optimized procedure have exhibited a signal-to-noise ratio that is three times higher than that of commercially available diodes at 0.9 THz using an IF of 80 kHz, as well as competitive performance at 1.4 THz with an IF in the gigahertz range.

HTSC devices fabricated by selective epitaxial growth

The use of a selective epitaxial growth technique for fabricating YBCO thin-film microstructures is described. No film post-deposition processing is required; hence damage to the structure is minimized. The technique is compatible with a passivation process to protect the structure without exposure to air. The microbridges, Josephson junctions and rf SQUIDs protected by an amorphous YBCO passivation have long lifetime even after severe accelerated aging tests. Rf SQUIDs fabricated by this technique show a significant reduction of low-frequency noise when operating in weak magnetic fields compared with SQUIDs fabricated by the conventional ion beam etching technique.

Multistage active vibration isolation system

The major obstacle to the detection of low-frequency gravitational waves with an earth-based interferometer is seismic noise. The current design of the initial Laser Interferometer Gravitational-Wave Observatory (LIGO) receiver, now under construction, projects that ground noise will limit the operating band to frequencies above 40 Hz. In this article, we describe recent progress on the JILA active vibration isolation system. This device is being constructed to demonstrate the technology needed for useful reduction of low-frequency seismic noise in a gravitational wave interferometer. It consists of three spring-mounted stages, each of which provides both active and passive isolation. To date, all of the control loops on the first two of the three stages have been closed. Together they can reduce large vibrations by at least 70 dB in both vertical and horizontal directions at 1.5 Hz and above.

Phase noise and a low-frequency noise reduction method in bipolar transistors

A low-frequency amplitude and phase-noise measuring system was set up and both noises were measured. It is noted they were correlated in a bipolar transistor. Both noises were from the same origin: diffusion coefficient fluctuation in the base. A demonstration was carried out on the amplitude noise reduction by using the correlation between both noises.

A smart sensor system for carbon monoxide detection

This paper illustrates a smart sensor system for carbon monoxide detection. An innovative technological approach has been pursued to fabricate gas sensors on silicon substrate, compatible with IC fabrication. A mixed analog-digital electronic interface processes the outputs of three sensors to compensate relative humidity and interfering gases. Sigma-delta signal processing and low-frequency noise reduction techniques are used to minimize silicon area and to meet the required performance in a standard CMOS technology.

A formal presentation and comparative study of low-frequency noise reduction techniques for direct current superconducting quantum interference devices

We present a comparative study of existing low-frequency noise reduction techniques for dc superconducting quantum interference devices (SQUIDS). These methods are essential to applications involving high-Tc SQUIDS because they dramatically reduce the 1/f noise to useful low-frequency noise levels. We propose a formal presentation of the problem and explain mathematically the working principles of these noise reductions methods, particularly for fluctuations arising from the critical current of the junctions. An increase in the white noise level is generally observed when a sinusoidal lock-in detection is used for these methods and we give here a mathematical explanation for this peculiar effect. Finally, noise measurements made on high-Tc grain boundary dc SQUIDS are presented; the experimental results are in close agreement with theory.

Reduction of low-frequency excess noise in superconducting quantum interference devices by applying high-frequency magnetic fields

We have investigated a possible reduction of low-frequency excess noise and the dependence of the flux signal on temperature changes in direct current superconducting quantum interference devices, dc SQUIDs, generated by trapped flux. Niobium thin film dc SQUIDs, carefully cooled down in a well shielded environment, have been exposed to switching transients. The thereby generated low-frequency excess noise and in some case an excess white noise, could substantially be reduced by applying a rf magnetic field of sufficiently high frequency to the SQUID. The temperature dependence of the flux signal of the SQUID and the magnetic field dependence of the drift could also be reduced in these cases, where the drift is caused by movement of trapped vortices.

Reciprocity Technique for Analysis of Road Noise of Vehicles. 1st Report. Measurement of Mechanical-Acoustic Transfer Functions.

A technique based on the reciprocity principle has been developed for measuring mechanical-acoustic transfer functions for vehicle road noise induced by forced displacement inputs from the suspension. With this technique, acoustic excitation is applied to the car body and the resulting constraint force of the tires are measured by means of tri-axial load cells with a loudspeaker installed at the driver's ear position. This paper will first explain the mechanical-acoustic reciprocity principle corresponding to forced displacement excitation. Empirical examples will then be given of applications of this technique to the reduction of low-frequency road noise of a passenger car. The results indicated that the reciprocity technique is applicable to a fully trimmed vehicle that includes an engine and suspension and is often cheaper and more convenient than the equivalent shaker-based excitation method for measuring mechanical-acoustic transfer functions.

Active Noise Control Using Fast Kalman Filter Estimation.

Active noise control (ANC) which is very effective for reduction of low-frequency noise has received much attention. However, the performance of the ANC system is generally not of a sufficiently high level for practical applications. In order to improve the performance, we propose a new feedforward ANC system which uses the fast Kalman filter algorithm. First, computer simulations are carried out to compare the conventional ANC systems which use the LMS and the LIM algorithm with the proposed ANC system. Based on the comparisons, the proposed ANC system is superior to the conventional ANC systems. Finally, experiments are carried out in an anechoic room to verify the superiority in practical use.

The Effects of Coupled Source/cavity modes on the Acoustic Insertion Loss of Close-Fitting Enclosures

This paper reports a theoretical and experimental study of the prediction of the insertion loss for low frequency noise from close-fitting enclosures. Experimental work has been carried out for various small panel designs with several common structural materials using a special test rig. The theoretical model considers not only the modal response of the enclosure plate but also the 3 dimensional acoustic modes in the air gap and the modal component of the source plate. The theory and the experiments show two important points; 1) the contribution of the non-fundamental mode of the source plate is important for the insertion loss of the enclosure; and 2) the cavity modes have significant effects on insertion loss not just at the acoustic resonances but they also affect the coupling between the source and enclosure plates. The effects of stiffness and damping on the higher structural and acoustic modes are also studied in the experiments. Comparisons of various tested plates suggest that wood is potentially a good material for low frequency noise reduction due to the high stiffness-mass ratio. The attachment of stiffeners can improve the insertion loss at some frequencies, but may decrease the insertion loss at other frequencies.

軸流ファンにおける低周波吸音について

The acoustic treatment with a rather large cavity was discussed from both aspects of analysis and experiment for low-frequency noise reduction in a low-speed axial flow fan. The rectangular and annular ducts with treatments were used. It is tentatively concluded that the axial modes of sound wave travelling in the cavity played an important role in low-frequency noise reduction mechanism. The acoustic treatment with a biased cavity showed a promise from a view point of weight reduction in aeroengines and fluid machinery.