Low Frequency Content Research Articles

On the optimization of n-sub-step composite time integration methods

A family of n-sub-step composite time integration methods, which employs the trapezoidal rule in the first n-1 sub-steps and a general formula in the last one, is discussed in this paper. A universal approach to optimize the parameters is provided for any cases of nge 2, and two optimal sub-families of the method are given for different purposes. From linear analysis, the first sub-family can achieve nth-order accuracy and unconditional stability with controllable algorithmic dissipation, so it is recommended for high-accuracy purposes. The second sub-family has second-order accuracy, unconditional stability with controllable algorithmic dissipation, and it is designed for heuristic energy-conserving purposes, by preserving as much low-frequency content as possible. Finally, some illustrative examples are solved to check the performance in linear and nonlinear systems.

Suppression durations for facial expressions under breaking continuous flash suppression: effects of faces\u2019 low-level image properties

Perceptual biases for fearful facial expressions are observed across many studies. According to the low-level, visual-based account of these biases, fear expressions are advantaged in some way due to their image properties, such as low spatial frequency content. However, there is a degree of empirical disagreement regarding the range of spatial frequency information responsible for perceptual biases. Breaking continuous flash suppression (b. CFS) has explored these effects, showing similar biases for detecting fearful facial expressions. Recent findings from a b. CFS study highlight the role of high, rather than low spatial frequency content in determining faces’ visibility. The present study contributes to ongoing discussions regarding the efficacy of b. CFS, and shows that the visibility of facial expressions vary according to how they are normalised for physical contrast and spatially filtered. Findings show that physical contrast normalisation facilitates fear’s detectability under b. CFS more than when normalised for apparent contrast, and that this effect is most pronounced when faces are high frequency filtered. Moreover, normalising faces’ perceived contrast does not guarantee equality between expressions’ visibility under b. CFS. Findings have important implications for the use of contrast normalisation, particularly regarding the extent to which contrast normalisation facilitates fear bias effects.

Ultra-low frequency (ULF) magnetic field emissions associated with some major earthquakes occurred in Indian Subcontinent

We have analyzed ULF magnetic field emissions data obtained from identical 3- component search coil magnetometers installed at Indian stations of Agra (Geograph. lat. 27.2°N, long. 78°E), Shillong (25.92oN, 91.88oE), and Kachchh (23.78oN, 71oE) for the months of April 2012 and September 2013 in which three major earthquakes occurred, one of magnitude M = 8.5 in Indonesian region and two of magnitudes M = 7.4 and 6.8 in Pakistan. The results of the analysis show the occurrence of ULF bursts 09-16 days prior to the occurrence of these earthquakes at Agra and Shillong stations. Such bursts are not observed at Kachchh station. Analysis of ULF data obtained from a flux gate magnetometer at a remote station Kakioka in Japan (Geograph. lat. 36.2oN, long. 140.2oE) does not show such burst also. The bursts contain electromagnetic pulses of periods 10–15s (f = 0.06–0.1 Hz). The origins of these bursts in lithospheric sources (earthquakes) are confirmed by polarization parameter analysis of data. These ULF data are not influenced by lightning and magnetic storms. A long period analysis of the ULF data for the same months between the years 2010 and 2017 shows that occurrence of such bursts are characterized by either large magnitude shallow depth earthquakes (M > 6) or multiple shallow depth earthquakes of low magnitudes (M < 6). The precursory periods of the recorded ULF bursts match very well with those reported by Singh et al. (2018) who analyzed very low frequency (VLF) and ionospheric total electron content (TEC) data recorded at Agra station.

Effect of Frequency Content of Earthquake on the Seismic Response of Interconnected Electrical Equipment

To ensure the stable operation of safety-related nuclear power plant (NPP) equipment, they are tested by following the seismic qualification procedures. The in-cabinet response spectrum (ICRS) is used to test the mounted components. However, the ICRS varies significantly with the number of uncertainties that include (1) loaded and unloaded condition of the cabinets, (2) the number of connected cabinets (grouping effects), and (3) higher frequency contents in the seismic inputs. This study focuses on the ICRS generation and alteration induced due to the listed uncertainties. A prototype of an electrical cabinet was experimentally examined. Followed by the numerical modeling of the cabinet, the seismic analysis for the group of cabinets was performed using artificial ground motion compatible with the standard design spectrum and the real accelerograms of high and low frequency contents. The seismic response using finite element (FE) analysis manifests (1) natural frequency of loaded cabinets reduced due to the in-cabinet components while for the unloaded cabinets it increased significantly, (2) a consistent reduction in ICRS due to the grouping effect was recorded when excited by the lower-frequency motion, while it was amplified dramatically due to high-frequency pulses. Interconnected cabinets under the low-frequency input motions have a significant reduction of 50% in the ICRS that corresponds to the higher stiffness of the cabinets, while a 100% increase under the high frequency of ground motion was obtained. High frequency of ground motion, usually above 10 Hz, can cause the interconnected cabinets to resonate as the natural frequency of these equipment lies in this range.

Investigation into the turbulence statistics of installed jets using hot-wire anemometry

This work presents a detailed study of the turbulence flow statistics of a jet mounted with its axis parallel to a rigid flat plate. Hot-wire constant temperature anemometry has been used to measure the single-point and two-point statistics of the axial velocity component at several locations within the jet flow field. Results show that the jet mean flow near the plate surface is subjected to a local acceleration and redirection due to a Coandă-type effect. The propagation of these effects downstream of the plate trailing edge is strongly dependent on the plate position. Regarding the velocity fluctuations, the mean turbulence intensity levels are seen to decrease as the radial distance between the jet and surface decreases. Analysis of the single-point power spectral density data on the shear layer close to the plate shows that the reduction in magnitude of the low-frequency content of the energy spectrum is responsible for the decrease in turbulence intensity. Additionally, the characteristic time and length scales computed from two-point measurements reduce as the plate is mounted closer to the jet centre-line. The axial eddy convection velocity is seen to increase in the region of high turbulent kinetic energy in the shear layer adjacent to the surface. Empirical models for turbulence characteristic scales and eddy convection velocity are presented. These findings suggest that both the amplitude and distribution of the jet mixing noise sources are affected when closely installed next to a surface. This paper is a continuation of a recent investigation on the turbulence statistics of isolated jets presented in Proença (Exp Fluids 60(4):63, 2019).Graphic abstract

Reinforcement of Low-Frequency Sound by Using a Panel Speaker Attached to the Roof Panel of a Passenger Car

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The woofer in a car should be large to cover the low frequencies, so it is heavy and needs an ample space to be installed in a passenger car. The geometry of the woofer should conform to the limited available space and layout in general. In many cases, the passengers feel that the low-frequency contents are not satisfactory although the speaker specification covers the low frequencies. In this work, a thin panel is installed between the roof liner and the roof panel, and it is used as the woofer. The vibration field is controlled by many small actuators to create the speaker and baffle zones to avoid the sound distortion due to the modal interaction. The generation of speaker and baffle zones follows the inverse vibro-acoustic rendering technique. In the actual implementation, a thin acrylic plate of 0.53x0.2 m&lt;sup&gt;2&lt;/sup&gt; is used as the radiator panel, and the control actuator array is composed of 16 moving-coil actuators. The shape of the desired speaker zone is an ellipse, and the required amplitude of this piston source is pre-calculated to satisfy the desired sound radiation at the ear position. The gain of the actuator array to properly generate the desired vibration field is obtained by solving an inverse problem constructed by the transfer mobility between each actuator and field point on the plate. For the recruitment of the low-frequency deficiency of human auditory characteristics, the desired sound spectrum is set to follow the equal-loudness contour of 40 phons. It is confirmed that the woofer in a car can be replaced by the developed panel speaker.&lt;/div&gt;&lt;/div&gt;

Wide-Band Current Transformers for Traveling-Waves-Based Protection Applications

Owing to its system independence and unmatched speed, Traveling-wave (TW)-based relaying has become a top contender for transmission and distribution systems. However, existing measurement devices, such as Current Transformers (CTs), are incapable of capturing the full frequency content of TWs, as their bandwidths are limited. Consequently, existing TW-based schemes rely on distorted waveforms and partial-frequency spectra, i.e., low-frequency content, of TWs to perform protection tasks. Such incomplete information can lead to misoperation of protection schemes. This article investigates the impacts of the limited bandwidth of CTs on the shape and frequency content of TWs under faults in transmission and distribution lines. Additionally, it proposes a compensation technique, which utilizes an Unknown Input Kalman Filter (UIKF), to accurately estimate the primary current of a CT from the measured secondary one, i.e., to capture the waveshape and full frequency-content of TWs. The proposed method removes the bandwidth barrier from the measurement of current-TWs, and thus allows relays to access the High-Frequency (HF) spectra of TWs in order to devise more-effective TW-based schemes. Simulation results corroborate that the proposed method accurately compensates for low frequency-content and distorted waveform of TWs in the secondary currents of CTs under various fault types, resistances, inception angles, and locations.

Dynamic behavior of vertically irregular sloping ground buildings

This paper examines the level of vertical irregularity and the influence of the frequency content of earthquakes on the vertically irregular reinforced concrete step-back, step-back–setback building frames. The regularity indices of these frames determined first to assess the level of irregularity and then the dynamic responses of the buildings under three distinct earthquakes of low, intermediate, and high-frequency contents are evaluated. It is noted that the irregularity of step-back frames is decreasing as the number of storeys increasing, whereas the irregularity of step-back–setback frames is increasing as the number of storeys and bays increasing. And the step-back frames are seismically responsible for low and medium frequency content earthquakes. In contrast, the step-back–setback frames are responsible for medium and high-frequency content earthquakes. The seismic vulnerability indices of these building frames are also determined and it is observed that there is rarely any correlation existed between the seismic vulnerability index, regularity index, and the influence of the earthquake frequency content.

Acoustic analysis to monitor implant seating and early detect fractures in cementless THA: An in vivo study.

The initial stability of cementless total hip arthroplasty (THA) implants is obtained by an interference fit that allows osseointegration for a long term secondary stability of the implant. Yet, finding the insertion endpoint that corresponds to an appropriate initial stability is currently often based on a number of subjective experiences of the orthopedic surgeon, which can be challenging. In order to assist the orthopedic surgeons in their pursuit to find this optimal initial stability, this study aims to determine whether the analysis of sound that results from the implant insertion hammer blows can be used to objectively monitor the insertion process of cementless THA implants. An in vivo study was conducted. The experimental results revealed vibro-acoustic behavior sensitive to implant seating, related to the low frequency content of the response spectra. This sensitive low-frequency behavior was quantified by a set of specific vibro-acoustic features and metrics that reflected the power and similarity of the low-frequency response. These features and metrics allowed monitoring the implant seating and their convergence agreed well with the endpoint of insertion as determined by the orthopedic surgeon. Intraoperative fractures caused an abrupt and opposite change of the vibro-acoustic behavior prior to the notification of the fracture by the orthopedic surgeon. The observation of such an abrupt change in the vibro-acoustic behavior can be an important early warning for loss of implant stability. The presented vibro-acoustic measurement method shows potential to serve as a decision supporting source of information as it showed to reflect the implant seating.

A Hybrid Approach Calculating Lateral Spreading Induced by Seismic Liquefaction

Liquefaction-induced lateral spreading has caused severe damages to the infrastructures. To predict the liquefaction-induced lateral spreading, a hybrid approach was proposed based on the Newmark sliding-block model. One-dimensional effective stress analysis based on the borehole investigation of the site was conducted to obtain the triggering time of liquefaction and acceleration time history. Shear wave velocity of the liquefiable soil was used to estimate the residual shear strength of liquefiable soil. The limit equilibrium analysis was conducted to determine the yield acceleration corresponding with the residual shear strength of liquefied soil. The liquefaction-induced lateral spreading was calculated based on the Newmark sliding-block model. A case study based on Wildlife Site Array during the 1987 Superstition Hills earthquake was conducted to evaluate the performance of the hybrid approach. The results showed that the hybrid approach was capable of predicting liquefaction-induced lateral spreading and the calculated lateral spreading was 1.5 times the observed displacement in terms of Wildlife Site Array. Numerical simulations with two other constitutive models of liquefiable sand were conducted in terms of effective stress analyses to reproduce the change of lateral spreading and excess pore water ratio over the dynamic time of Wildlife Site Array. Results of numerical simulations indicated that the lateral spreading varied with the triggering time of liquefaction when different constitutive models were used. The simulations using PM4sand and UBC3D-PLM constitutive models predicted 5.2 times and 4 times the observed lateral spreading, respectively. To obtain the site response, the motions recorded at and below the ground surface were analyzed using the Hilbert–Huang transform. The low-frequency content of the motion below the ground surface was amplified at the ground surface, and the liquefaction effect resulted in a shift of the frequency content. By comparing the response spectra of the entire ground surface motion and the ground surface motion from the beginning to the triggering time of liquefaction, the liquefaction effect at the site was confirmed.

Frequency Analysis of Solar PV Power to Enable Optimal Building Load Control

In this paper, we present a flexibility estimation mechanism for buildings’ thermostatically controlled loads (TCLs) to enable the distribution level consumption of the majority of solar photovoltaic (PV) generation by local building TCLs. The local consumption of PV generation provides several advantages to the grid operation as well as the consumers, such as reducing the stress on the distribution network, minimizing voltage fluctuations and two-way power flows in the distribution network, and reducing the required battery storage capacity for PV integration. This would result in increasing the solar PV generation penetration levels. The aims of this study are twofold. First, spectral (frequency) analyses of solar PV power generation together with the power consumption of multiple building TCLs (such as heating, ventilation, and air conditioning (HVAC) systems, water heaters, and refrigerators) are performed. These analyses define the bandwidth over which these TCLs can operate and also describe the PV generation frequency bandwidth. Such spectral analyses, in frequency domain, can help identify the flexible components of PV generation that can be consumed by the various TCLs through optimal building load utilization. Second, a quadratic optimization problem based on model predictive control is formulated to allow consuming most of the low and medium frequency content of the PV power locally by building TCLs, while maintaining occupants’ comfort and TCLs’ physical constraints. The solution to the proposed optimization problem is achieved using optimal control strategies. Numerical results show that most of the low and medium frequency content of the PV generation can be consumed locally by building TCLs. The remaining high-frequency content of the PV generation can then be stored/offset using energy storage systems.

Delaying the source ghost to improve the ultralow-frequency response of a marine air-gun source

The competing effect between the fundamental bubble and its source-ghost response results in a strong attenuation of the lowest frequencies (below 7 Hz). This loss cannot be compensated easily by adjusting the source depth. Consequently, the low-frequency content in marine seismic data is not optimal, degrading the performance of low-frequency dependent processing approaches, such as full-waveform inversion. To overcome this, we have developed an additional source to counteract the ghost from the main source. In this situation, the fundamental bubble is characterized by the depth of the main source, whereas the ghost response is characterized by the summed depth of the main and additional sources. This source setup mitigates the competing effect and reduces the suppression of ultralow frequencies. Compared with a conventional horizontal source, our source design will reduce the mid- to high-frequency output, which may be beneficial in situations in which environmental constraints limit the maximum allowed output of a marine source.

Without low spatial frequencies, high resolution vision would be detrimental to motion perception.

A normally sighted person can see a grating of 30 cycles per degree or higher, but spatial frequencies needed for motion perception are much lower than that. It is unknown for natural images with a wide spectrum how all the visible spatial frequencies contribute to motion speed perception. In this work, we studied the effect of spatial frequency content on motion speed estimation for sequences of natural and stochastic pixel images by simulating different visual conditions, including normal vision, low vision (low-pass filtering), and complementary vision (high-pass filtering at the same cutoff frequencies of the corresponding low-vision conditions) conditions. Speed was computed using a biological motion energy-based computational model. In natural sequences, there was no difference in speed estimation error between normal vision and low vision conditions, but it was significantly higher for complementary vision conditions (containing only high-frequency components) at higher speeds. In stochastic sequences that had a flat frequency distribution, the error in normal vision condition was significantly larger compared with low vision conditions at high speeds. On the contrary, such a detrimental effect on speed estimation accuracy was not found for low spatial frequencies. The simulation results were consistent with the motion direction detection task performed by human observers viewing stochastic sequences. Together, these results (i) reiterate the importance of low frequencies in motion perception, and (ii) indicate that high frequencies may be detrimental for speed estimation when low frequency content is weak or not present.

Bayesian Restoration of Audio Degraded by Low-Frequency Pulses Modeled via Gaussian Process

A common defect found when reproducing old vinyl and gramophone recordings with mechanical devices are the long pulses with significant low-frequency content caused by the interaction of the arm-needle system with deep scratches or even breakages on the media surface. Previous approaches to their suppression on digital counterparts of the recordings depend on a prior estimation of the pulse location, usually performed via heuristic methods. This paper proposes a novel Bayesian approach capable of jointly estimating the pulse location; interpolating the almost annihilated signal underlying the strong discontinuity that initiates the pulse; and also estimating the long pulse tail by a simple Gaussian Process, allowing its suppression from the corrupted signal. The posterior distribution for the model parameters as well for the pulse is explored via Markov-Chain Monte Carlo (MCMC) algorithms. Controlled experiments indicate that the proposed method, while requiring significantly less user intervention, achieves perceptual results similar to those of previous approaches and performs well when dealing with naturally degraded signals.

Measurement of high-frequency milling forces using piezoelectric dynamometers with dynamic compensation

Piezoelectric dynamometers are widely used to measure machining forces during milling operations. While dynamometers are precise in measuring the low-frequency content of machining forces, their electromechanical dynamics distort the high-frequency content of the forces, resulting in critical measurement errors particularly in high-speed or highly intermittent milling processes. Existing methods (e.g. Augmented Kalman Filter) that are used to remediate the high-frequency content of the forces measured by dynamometers require tuning multiple parameters based on prior knowledge of the measurement noise and accurate models of the dynamometer dynamics, which continuously change during the process as material is machined away from the workpiece. Two new methods are presented in this paper to address this issue. The first method uses regularized deconvolution to estimate machining forces from the output signal of the dynamometer. In this method, regularization does not require prior knowledge of the measurement noise or variations of the system dynamics, but it cannot be implemented in online force monitoring and control systems. The second method designs a Sliding Mode Observer (SMO) to estimate milling forces recursively at each measurement timestep. The SMO can be implemented in online force estimation, is robust against the variations of system dynamics, and requires tuning only one gain that is independent from the system dynamics or measurement noise. The presented experimental results verify the effectiveness of these methods in accurately estimating high-frequency milling forces from dynamometer measurements.

Vector-acoustic full-waveform inversion: Taking advantage of wavefield separation and dealiasing

Vector-acoustic full-waveform inversion (VAFWI) directly inverts vector-acoustic (VA) data, which consist of pressure and particle displacement components, at the cost of conventional acoustic full-waveform inversion (FWI). VA data contain information about the direction of arrival of the recorded seismic waves. In VAFWI, this directional information is taken into account by introducing an appropriate data weighting. With this weighting, in the geometry of a marine seismic experiment, the VAFWI adjoint calculation approximates inverse wavefield extrapolation, resulting in the natural separation of up- and downgoing recorded waves. If the free-surface effects are modeled during the inversion, the wave separation leads to (1) suppression of surface-related artifacts, (2) constructive interference of receiver ghosts with their primaries leading to preservation of the low-frequency content in the adjoint fields, and (3) compensation for insufficient spatial wavefield sampling on the receiver side. The horizontal displacement component helps interpolate the missing data. Synthetic examples demonstrate that for undersampled data, VAFWI consistently recovers the subsurface properties with higher resolution and fewer artifacts than conventional FWI.

Study on mechanism and filter efficacy of AGO/IAGO in the frequency domain

PurposeThe purpose of this paper is to analyze the mechanism and filter efficacy of accumulation generation operator (AGO)/inverse accumulation generation operator (IAGO) in the frequency domain.Design/methodology/approachThe AGO/IAGO in time domain will be transferred to the frequency domain by the Fourier transform. Based on the consistency of the mathematical expressions of the AGO/IAGO in the gray system and the digital filter in digital signal processing, the equivalent filter model of the AGO/IAGO is established. The unique methods in digital signal processing systems “spectrum analysis” of AGO/IAGO are carried out in the frequency domain.FindingsThrough the theoretical study and practical example, benefit of spectrum analysis is explained, and the mechanism and filter efficacy of AGO/IAGO are quantitatively analyzed. The study indicated that the AGO is particularly suitable to act on the system's behavior time series in which the long period parts is the main factor. The acted sequence has good effect of noise immunity.Practical implicationsThe AGO/IAGO has a wonderful effect on the processing of some statistical data, e.g. most of the statistical data related to economic growth, crop production, climate and atmospheric changes are mainly affected by long period factors (i.e. low-frequency data), and most of the disturbances are short-period factors (high-frequency data). After processing by the 1-AGO, its high frequency content is suppressed, and its low frequency content is amplified. In terms of information theory, this two-way effect improves the signal-to-noise ratio greatly and reduces the proportion of noise/interference in the new sequence. Based on 1-AGO acting, the information mining and extrapolation prediction will have a good effect.Originality/valueThe authors find that 1-AGO has a wonderful effect on the processing of data sequence. When the 1-AGO acts on a data sequence X, its low-pass filtering effect will benefit the information fluctuations removing and high-frequency noise/interference reduction, so the data shows a clear exponential change trends. However, it is not suitable for excessive use because its equivalent filter has poles at the non-periodic content. But, because of pol effect at zero frequency, the 1-AGO will greatly amplify the low-frequency information parts and suppress the high-frequency parts in the information at the same time.

The frequency domain of ground reaction forces during running in patients with low back pain: comparing with healthy control group

Background: Walking and running frequency domain analysis of ground reaction forces are associated with injuries. Comparison of frequency domain analysis of ground reaction forces could be useful in rehabilitation of low back patients. The objective of this study was to investigate frequency domain of ground reaction forces during running in patients with low back pain compared to healthy control ones. Methods: This study was a semi-experimental. The statistical sample of the present study include forty males that divided into a healthy control group (n=20) and a group of patients with low back pain (n=20). A foot scan system (sampling rate: 300 Hz) was used for measuring ground reaction forces in ten regions of the foot during running. Independent sample t-test was used for statistical analysis. Alpha level was set at p<0.05. Results: The frequency content with power 99.5% in the Toe 1 region in low back patients group was significantly greater that that healthy group by 50% (P=0.017). Also, the frequency content with power 99.5% in the Toe 1-5 region in low back patients group was significantly lower than that healthy group (P=0.007). Number of essential harmonics in forth metatarsal in low back patients group was greater than that healthy ones (P=0.049). Conclusion: Patients with low back pain had lower running speed than that healthy ones and therefore lower values of most of the components of the frequency may be due to the guarded gait mechanism in these patients. This altered running speed and lower frequency content is suggestive of a lower mechanical efficiency of running in patients with low back pain.

Seismic study of soda straws exposed to nearby blasting vibrations

This research study has been developed to preserve a remarkable soda straw population from vibrations emitted by nearby rock blasting. The Choranche stalactite cave (Vercors, France) contains thousands of exceptionally long soda straws reaching a few meters in length for ~ 0.5 cm in diameter. These slender speleothems are very vulnerable to vibrations not only because of their structural fragility but also because of their dynamic amplification. We found that soda straws’ first natural frequencies lie within the frequency range emitted by nearby rock blasting works (ten to hundreds of Hz). We used in situ blast records, laboratory characterization, and a dynamic 2D finite element code to simulate the load experienced by the soda straw population. We show that induced loads may be increased by a factor 5 due to resonance. Consequently, short soda straws (0.1–1 m) were found more vulnerable to nearby blasting vibrations than longer speleothems (> 1 m), despite greater own weight and inertial forces for the latter. Simulations made on several blast tests yielded an admissible 2.4 mm/s peak particle velocity along the cave. Subsequent blasting works were carried out in compliance with this threshold with no harm done to the soda straw population. We also computed the dynamic response of soda straws exposed to a regional earthquake. In this case, longer speleothems (> 1 m) are primarily affected by resonance because of ground motion lower frequency content. We show that the omission of dynamic resonance or its simplification as made in previous studies may significantly underestimate the induced load in speleothems.

Comparison of Dispersion Image Resolution Acquired Using Multichannel Analysis of Surface Waves with Different Source Energy and Stacking

Active MASW method is a promising method to determine the stiffness characteristics of the subsurface strata. The accumulation energy to obtain high-resolution dispersion image depends largely on the energy generated from the active source. From the results obtained, the influence of different source energy, source plate and number of stacking on the dispersion image resolution were significant. Heavier source weight (e.g.7 kg sledgehammer) produced highamplitude energy band and provides even lower frequencies. However, if lighter source weight (e.g. small steel hammer) was used, it needs to be coupled with rubber plate to increase the low frequency content, thus generate deeper depth of investigation. Finally, the use of rubber plate and stacking increased the resolution of the dispersion image especially at lower frequencies by significantly suppressing the ambient noise.