Low Frequency Broadband Noise Research Articles

Maze-like acoustic metamaterial for low-frequency broadband noise suppression

We propose the maze-like acoustic metamaterial (MLAMM) unit suitable for regulating the transformer noise. The influence of the length and width of the acoustic waveguide on the sound transmission loss (TL) is studied, and the TL and resonance frequencies of the MLAMM unit are calculated. The visco-thermal effect weakens the resonance transmission peak of the MLAMM unit. Finally, experiments verify that the proposed MLAMM unit has low-frequency broadband sound insulation performance, and the TL is about 25 dB in the frequency band of 100 Hz–500 Hz. The research provides a reasonable and effective solution for noise suppression of transformers.

Tip leakage flow and aeroacoustics analysis of a low-speed axial fan

Axial fans are widely used in the aerospace field, and new regulations and environmental concerns are prompting manufacturers to design efficient low-noise axial fans. The aerodynamic performance and acoustic emissions of axial fans are substantially affected by the unavoidable tip clearance. Herein, a systematic analysis is performed on axial fans with different tip-clearance sizes to gain a clear understanding of the characteristics of tip leakage flow and investigate the generation mechanism of aeroacoustics. The reliability and accuracy of the numerical predictions are successfully validated through a comparison with experimental data. The unsteady pressure information on the blade surface is examined to clarify the main noise sources. The results show that the enhanced intensity of the blade tip vortex and thereby the enhanced interaction with the blade surface are the main drivers for the extra broadband noise when the tip clearance is increased. For the low-speed axial fans, the low-frequency broadband noise below 600 Hz is mainly related to the blade tip vortex and the interaction of the boundary layer instabilities with the leading edge and trailing edge, whereas the broadband noise above 1200 Hz is mainly due to the turbulent boundary layer fluctuations.

Study of a Nonlinear Membrane Absorber Applied to 3D Acoustic Cavity for Low Frequency Broadband Noise Control.

As a new approach to passive noise control in low frequency domain, the targeted energy transfer (TET) technique has been applied to the 3D fields of acoustics. The nonlinear membrane absorber based on the TET can reduce the low frequency noise inside the 3D acoustic cavity. The TET phenomenon inside the 3D acoustic cavity has firstly investigated by a two degrees-of-freedom (DOF) system, which is comprised by an acoustic mode and a nonlinear membrane without the pre-stress. In order to control the low frequency broadband noise inside 3D acoustic cavity and consider the influence of the pre-stress for the TET, a general model of the system with several acoustic modes of 3D acoustic cavity and one nonlinear membrane is built and studied in this paper. By using the harmonic balance method and the numerical method, the nonlinear normal modes and the forced responses are analyzed. Meanwhile, the influence of the pre-stress of the nonlinear membrane for the TET is investigated. The desired working zones of the nonlinear membrane absorber for the broadband noise are investigated. It can be helpful to design the nonlinear membrane according the dimension of 3D acoustic cavity to control the low frequency broadband noise.

Monitoring bubble production in a seagrass meadow using a source of opportunity

Under high irradiance, the photosynthetic activity of dense seagrass meadows saturates the water forming oxygen bubbles. The diel cycle of bubble production peaks at mid-day, following light intensity pattern. It is well known that bubbles strongly affect the acoustic propagation, increasing signal attenuation and decreasing the effective water sound speed, noticeable at low frequencies. Thus, the diurnal variability of bubbles may show an interference pattern in the spectrograms of low frequency acoustic signals. In an experiment conducted in July 2016 at the Aquaculture Research Station of the Portuguese Institute for the Sea and Atmosphere in Olhão, Portugal, the spectrograms of low frequency (<20 kHz) broadband noise produced by water pumps in a pond of 0.48 ha covered by the seagrass Cymodocea nodosa showed interference patterns that can be ascribed to the variability of the sound speed in the water. Preliminary analysis suggests that the daily cycle of bubble concentration can be inferred from these interference patterns.

Low-frequency broadband noise correlation processing in deep-water

The Comprehensive Nuclear-Test-Ban Treaty Organization operates an International Monitoring System (IMS). The IMS includes hydroacoustic stations composed of hydrophones deployed in the ocean deep-sound-channel in a two-kilometers-side triangular configuration(referred to as triad) in the horizontal plane. Data are continuously recorded on hydrophone triads (with a sampling frequency of 250 Hz) and have been archived during the last decade. Previous experimental studies have demonstrated that coherent waveform can be extracted from broadband coherent processing of ocean ambient noise, typically above f &amp;gt; 100 Hz [e.g., see Roux et al., J. Acoust. Soc. Am. 116(4), 1995–2003 (2004)] . We investigated here the emergence of coherent arrivals from the correlation processing of the low-frequency broadband ambient noise recorded during the years 2006–2007 on IMS hydrophones located in the Southern Hemisphere. This low-frequency acoustic ambient noise includes various components from anthropogenic and biological sources as well as from seismic origin (e.g., earthquakes and microseims) and also significant ice-breaking noise originating from Antarctica especially during the Austral summer period. The feasibility of passive basin scale tomography using long-term monitoring of ocean noise will be discussed.

Control of noise at the entrance or exit of parking garages

The low-frequency broadband noise generated by vehicles may be disturbing for the neighboring residents when vehicles enter or leave underground (or semi-underground) parking garages. A special structural ramp was designed to reduce noise emissions at the entrance or exit of garages. Two-layer steel grilles with rectangular slots evenly distributed, on which the tires of vehicles run, were installed on the original cement ramp. Damping perforated plates sandwiched between the upper and under grille layers fixed on the original cement ramp, formed a resonance sound absorption structure together with the cavity in the layer under the grille. In addition, aluminum foams were embedded in the upper grille layer, and waterproof porous sound absorption materials filled the cavity of the under layer. Field tests using light cars with less than 8 seats were performed to evaluate the effect of the special structural ramp on noise reduction compared to the original concrete ramp. Results show that, when all seats were taken, noise reduction in the A-weighted level of 8 dB on the uphill ramp and 6 dB at the downhill ramp were achieved at the distance of 0.5 m away from the tires outside the vehicle, and a 13 dB and a 8 dB reduction at ears of the passenger respectively. Meanwhile, only a reduction of 2 dB or less could be achieved with further treatment of sound absorption on the middle of the pavement.

Identification and Attenuation of a Tonal-Noise Source on an Aircraft's Landing Gear

Aircraft landing gear noise is a major contributor to the overall airframe noise during the landing approach of a commercial aircraft. Fairings covering geometrically complex areas of the gears have proved to be very effective in noise reduction. However, in tests on an A340’s main landing gear, the overall benefit of the fairings was offset by both a slight increase in low-frequency broadband noise and a strong tonal noise. In this study, the identification and attenuation of the tonal-noise source has been carried out using a one-quarter-scale A340 main-landing-gear model. Aeroacoustic and aerodynamic tests were conducted in a closed-section wind tunnel, using a phased microphone array on the ceiling of the test section, flush-mounted pressure transducers on the model surface, and the particleimage- velocimetry technique. Far-field noise tests were then taken in an open jet aeroacoustic facility, using far-field microphones to verify the wind-tunnel test results. The experiments demonstrated the tonal-noise mechanism and a number of different control methods.

Chaos and plasticity in superconductor vortices: Low-dimensional dynamics

We present results of numerical simulations for driven vortex lattices in the presence of a random disorder at zero temperature. We show that the plastic dynamics of vortices display dissipative chaos. Intermittency ``routes to chaos'' have clearly been identified below the differential resistance peak. The peak region is characterized by positive Lyapunov exponents, which are characteristic of chaos, and low frequency broad-band noise. Furthermore, we find a low fractal dimension of the strange attractor, which suggests that only a few dynamical variables are sufficient to model the complex plastic dynamics of vortices.

A local active noise control system based on a virtual-microphone technique for railway sleeping vehicle applications

Low-frequency broadband noise generated on a railway vehicle by the wheel–rail interaction could be a big annoyance for passengers in sleeping cars. Low-frequency acoustic radiation is extremely difficult to attenuate by using passive devices. In this article, an active noise control (ANC) technique has been proposed for this purpose. A three-dimensional cabin was built in the laboratory to carry out the experiments. The proposed scheme is based on a Filtered-X Least Mean Square (FXLMS) control algorithm, particularised for a virtual-microphone technique. Control algorithms were designed with the Matlab-Simulink tool, and the Real Time Windows Target toolbox of Matlab was used to run in real time the ANC system. Referring to the results, different simulations and experimental performances were analysed to enlarge the silence zone around the passenger's ear zone and along the bed headboard. Attenuations of up to 20 and 15 dB(A) (re:20 μPa) were achieved at the passenger's ear in simulations and in experimental results, respectively.

Dissipative Feedback Control of a Reverberant Enclosure Using a Constant Volume Velocity Source

Loudspeakers are commonly used as actuators in active noise control applications. In this work, a loudspeaker is compensated so that it approximates a constant volume velocity source over the piston mode frequency range (up to approximately 600 Hz). This moves the speaker dynamics out of the desired control bandwidth (20 Hz–500 Hz). A static gain feedback controller using the compensated speaker is applied to a rigid-wall enclosure to reduce the acoustic potential energy of the system while being subjected to low frequency broadband noise. The results are compared to the performance of a similar control law using an uncompensated speaker, and a model based H2 optimal controller. The results indicate that the controller using the compensated speaker offers definite advantages in terms of stability and performance, while requiring a minimal amount of control effort.

Flux flow noise and dynamical transitions in a flux line lattice.

Generation of low frequency broadband noise is studied in the low- ${T}_{c}$ superconductor 2 $H$- ${\mathrm{NbSe}}_{2}$ near the ``peak effect.'' A striking dependence on both current and magnetic field is observed in the power, spectral shape, and non-Gaussian nature of the noise. The noise is very large in the plastic flow regime of the dynamics, and small in both the elastic flow and fluid flow regimes. The results show a rapid decrease in the velocity correlation length, a measure of the spatial homogeneity of the moving flux line assembly, as the upper critical field is approached.

Static and Dynamical Noises Due to the Sliding Charge Density Waves in K0.3MoO3

The low-frequency broadband noise of K 0.3 MoO 3 associated with the sliding motion of charge-density waves (CDW) was measured in the frequency range between 1 mHz and 1 MHz. The power spectrum of voltage fluctuation consists of several peaks and the 1/ f α background spectrum. As a result of an analysis, the former was found to be directly related to the energy distribution of the hopping among metastable states of the CDW's, and the energy distribution was estimated from the experimental data. On the other hand, the latter is considered to come from the interaction among semi-macroscopic moving CDW domains, which are weakly coupled with each other.

The effect of strong impurities on the low-frequency broadband noise in NbSe 3

The low-frequency broadband noise was measured for various NbSe 3 samples in which Ta and Zr is intentionally doped. Although the doping of Ta hardly changes the noise features, the doping of Zr changes the noise spectrum drastically. In Zr doped samples, the noise spectrum is represented as S(f) αf − α (α ranges between 0.5 and 0.6). From this result, we propose that the origin of f − α noise in the sliding CDW systems, including 1/f noise in pure samples, is closely related to the competition between the interaction of the CDWs with impurities and the interdomain interaction. Moreover, the coherent volume of the moving CDWs was found to be unchanged by the doping of the impurities.

Broadband noise in blue bronze K 0.3MoO 3

Low-frequency broadband noise was measured in the frequency range between 10 −3 Hz and 1 MHz in K 0.3MoO 3. It was found that the measured noise spectrum cannot be represented only by a simple power law, and it is rather rugged. From this result, the energy and its distribution associated with the transition among the metastable states of the charge-density wave in K 0.3MoO 3 were deduced.

Intermittent impulsive oscillations in K 0.3MoO 3

The undamped oscillations observed just above the switching threshold was investigated in K 0.3MoO 3. Each oscillation is impulsive, and possibly represents the elementary process of the motion of the charge density waves (CDW's). Furthermore, intermittency is found in these impulsive oscillations, which is comfirmed to be the direct origin of the low-frequency broadband noise. The analysis of the noise indicates that the degree of freedom of the motion is almost unity which means that the motion of the CDW's is spatially coherent.

Active noise control in finite length ducts

A simple technique for the active control of sound in ducts, initially suggested by Olson and May [1], is investigated in detail. A simple, “virtual earth” principle, feedback loop is used to drive the sound pressure to a minimum at a microphone placed close to a loudspeaker in the duct wall. This produces a reflection of downstream travelling plane waves. A detailed investigation of the loudspeaker near field has enabled the optimum position of the microphone to be identified. The system is shown to be especially effective at the frequencies of the longitudinal duct resonances, where the acoustic response of the duct produces a high loop gain. Results are presented which show a reduction of up to 20 dB in the amplitude of low frequency broadband noise at a position downstream of the cancelling source.

Broad band noise associated with the current carrying charge density wave state in TaS 3

We report the observation of low frequency broad band noise in the current carrying charge density wave (CDW) state of orthorhombic TaS 3. The noise amplitude is proportional to the number of condensed electrons, and we suggest that the broad band noise reflects the dynamics of the internal deformations of the CDW condensate.

A comparison of the overall and broadband noise characteristics of full-scale and model helicopter rotors

The broadband noise generated by full-scale and model rotors is compared in terms of spectral content and the dependence on tip speed and rotor thrust/pitch angle. Low frequency broadband noise and high frequency broadband noise are studied separately and blade 'scaling' effects are outlined. The degree of agreement between measurements and theoretical and semi-empirical prediction methods is reviewed together with the directionality patterns. The parameters relating to the overall noise are also discussed. It is shown that in general good agreement is obtained between the full-scale and model rotors when considering spectral content and the dependency of the noise levels on tip speed and thrust. The scaling factors usually considered applicable to the low frequency broadband noise do not, however, appear to apply to either the model or full scale rotors.

The noise characteristics of a large “clean” rotor

A two bladed 56 ft diameter rotor was run on a tower in an inverted mode so that the problem of recirculation and the difficulties of measuring noise directivity characteristics could be overcome. This paper outlines the analysis procedure used and presents the detailed results obtained. From a practical point of view rotor noise can be considered to consist of rotational or discrete frequency noise, low frequency broadband noise and high frequency broadband noise. The spectrum characteristics and the directivity patterns of each of these sources have been examined as a function of the blade tip speed, the total rotor thrust and the measurement angle relative to the rotor disc plane. The trends associated with the overall noise, which is dependent on the relative magnitude of the individual sources, have also been studied. These results have been compared, where possible, with the trends given by theoretical and semi-empirical prediction methods. Time history traces are also included ; these show that even under ideal conditions rotor noise is impulsive in nature.