Multi-tone Noise Research Articles

Tonal noise reduction of an electric ducted fan using over-the-rotor acoustic treatment

This study is concerned with reducing the tonal noise emitted by electric ducted fans which serve as the propulsion system for a variety of unmanned aerial vehicles (UAVs). The underlying concept is utilizing the shroud pressure fluctuation caused by the rotating blades to drive the over-the-rotor (OTR) acoustic treatment and generates a secondary sound field that cancels the primary ducted fan sound field. Experiment shows a considerable destructive effect can be made by meticulous tuning of the strength and phase of the secondary source. In view of the space constraints of a real electric ducted fan, the acoustic treatment is reconfigured as a labyrinth-type acoustic metamaterial that reduces the structure size while maintaining the noise reduction performance at the same time. An advantage of the proposed technique is that each tone can be dealt with by a specifically designed acoustic treatment occupying a small patch on the fan shroud. Hence, a compact multi-tonal noise control solution is possible by circumferentially deploying several OTR acoustic treatments.

On eigenvalue shaping for two-channel decentralized active noise control systems

Recent works show that two-channel decentralized active noise control (DANC) systems are able to achieve optimal noise reduction performance with guaranteed convergence by proper matrix eigenvalue shaping for each frequency. In this paper, we study the impact of three eigenvalue shaping approaches on the performance of a time-domain two-channel DANC system, where the first two approaches are from the literature and the third one is newly proposed as an extension of one of the two approaches. By theoretical analysis and experimental investigation of the three approaches, it is found that the eigenvalues of the 2 × 2 so-called characteristic matrices in the frequency-domain should be shaped by considering two aspects. Firstly, the two eigenvalues for each matrix need to be pushed towards the positive real axis to ensure stability. Secondly, the eigenvalues inherently affect the two auxiliary filters in the time-domain. They should be shaped so that the two filters have roughly the same magnitudes to facilitate implementation. Simulation results using the measured acoustic paths demonstrate the efficacy of the proposed eigenvalue shaping approach and the adaptive filtering technique for controlling sinusoidal noise, multitone noise, white noise and traffic noise. Experimental result shows the efficacy of the proposed approach for controlling white noise in three dimensional space.

Numerical design of Helmholtz resonators with multiple necks for multi-tonal noise control

The reduction of multi-tonal noise at multiple frequencies simultaneously is a challenge in many industrial fields. Different solutions such as metamaterials consisting of periodic Helmholtz resonators embedded into a porous layer have been studied in the literature. Generally, a classical resonator made of a cavity connected to a neck provides only one resonant transmission loss peak. In this study, a design of acoustic metamaterials is proposed numerically using the finite element method for multi-tonal noise reduction. The resonator is made of multiple necks extended into the cavity and is periodically distributed within a porous material. The cylindrical global cavity of the resonator is partitioned into several sub-cavities, which are separated from one another by a rigid wall, and each sub-cavity is connected to one neck. Helmholtz resonators with 2, 3, 4, and 5 necks are presented, they exhibit multiple resonance transmission loss peaks which correspond respectively to the number of the resonator necks. The proposed acoustic metamaterial designs can be used to reduce multi-total noise at several frequencies simultaneously.

Analysis of the Frequency Interference in the Narrowband Active Noise Control System

The narrowband active noise control (ANC) algorithm works well in the multi-tone noise reduction system. However, the performance of this algorithm degenerates significantly when the noisy frequencies become very close (e. g. frequency gap is 3Hz where the sampling frequency is 5000Hz), which can be seen as frequency interference effects. Up to now, however, this effect has not been researched in detail. In this work, the frequency interference effects are analyzed and discussed. The convergence process, the noise reduction value, the stability condition and the optimum step size are obtained by extensive math formulation derivation, provided two frequencies are close and fixed. Theoretical results show that, tones with large frequency difference can be approximated to independent convergence while the system with close frequencies suffers from poor convergence rate. The value of step size plays an important role in convergence, but the interference effects could never be eliminated by adjusting step size. Simulations are conducted to verify the analysis and the results match well with the analysis.

Development and experimental verification of a new computationally efficient parallel narrowband active noise control system

The filtered-x least mean square based narrowband active noise control (FX-NANC) system with a parallel structure is typically used to cancel low-frequency multi-tonal noise. The computational efficiency and system performance may, however, be greatly degraded due to an increase in the number of controlled frequencies or the length of estimated secondary path. To alleviate this problem, a cost-saving FE-NANC system has been recently proposed, but the system suffers from slow convergence. This paper presents a local secondary path estimation based filtered-error least mean square (LFELMS) algorithm and applies it to a narrowband active noise control (NANC) system. By filtering a single error signal with a set of low-order local secondary path (LSP) models to obtain filtered signals for weight updating, the proposed LFELMS based NANC (LFE-NANC) system requires significantly less computational cost compared to the FX-NANC system and has considerably faster convergence speed than the FE-NANC system. Two new LSP modeling methods are also proposed. In addition, a computational complexity analysis of some typical NANC systems and the proposed LFE-NANC system is provided. Numerical simulations are performed to investigate the accuracy of the LSP modeling methods and compare the performance of the proposed system with other NANC systems. Finally, real-time experiments are conducted to confirm the effectiveness of the proposed system in single-tone, multi-tone and non-stationary situations.

Global feedforward active noise control in vibro-acoustic cavities without increasing structural vibrations.

Interior noise in vibro-acoustic cavities may be generated due to acoustic and structural disturbances. Earlier studies have shown that for global control, the maximum reduction in acoustic potential energy can be realised by using an optimum combination of acoustic and structural actuators. However, it is observed that this reduction in interior noise may also be accompanied with an increase in kinetic energy of the cavity structure. This paper presents the development of a feedforward technique for active noise control in vibro-acoustic cavities ensuring that the noise reduction does not lead to an increase in kinetic energy. The problem is formulated as a constrained minimisation problem to minimise the acoustic potential energy subject to a constraint that the kinetic energy does not increase. Through a numerical study, it is shown that the optimum solution of the above problem indeed is favourable in terms of reduction in acoustic potential energy in the cavity and kinetic energy of the structure. The paper further proposes a method for solution of this constrained minimisation problem using a penalty function method and solution of sequential unconstrained problems. The proposed method is validated through a numerical study on a car-like cavity for single- and multi-tonal noise.

PSO based adaptive narrowband ANC algorithm without the use of synchronization signal and secondary path estimate

Abstract Narrowband active noise control (ANC) conventionally uses the waveform synthesis method in which a synchronization signal is essential. In addition to this, to update the parameters of the controller, prior estimation of the secondary path is required. Any mismatch in the frequency of the synchronization signal and the secondary path estimate compared to actual leads to a degraded performance in noise control. In this paper, a new type of narrowband ANC algorithm is proposed which requires neither the synchronization signal nor the secondary path estimate for controller tuning. Therefore, the algorithm is free from the effect of the change in the source frequency and the secondary path transfer function. The proposed adaptive algorithm is based on a non-gradient type optimization algorithm named as particle swarm optimization (PSO) algorithm. Through exhaustive computer based simulation, it has been demonstrated that the algorithm is not only capable to control a monotone noise but also efficiently control a multi-tone noise. The algorithm, due to its adaptive nature, is also capable of tracking the change in amplitude and frequency of the noise as well as, the change in the characteristics of primary and secondary paths.

Effects of low-frequency noise on human cognitive performances in laboratory

Indoor working and living environments are increasingly exposed to low-frequency noise sources. The well-known relationship between noise conditions and effects on human health requires the development of a proper procedure to evaluate the stress due to acoustical factors. For this purpose, an experiment, based on Soft Metrology principles, was designed to measure the changes of cognitive and physiological parameters (response time and heart rate) on a sample of 25 male and female volunteers, aged 19–29 years, exposed to three types of noise in a hemi-anechoic room. Participants were involved in a cognitive task (Stroop effect) for 10 min in four different conditions: silence, stochastic broadband multi-tonal noise (BBN), stochastic low-frequency multi-tonal noise (LFN1), and low-frequency stationary noise with regular amplitude modulation (LFN2). All sounds were reproduced by two loudspeakers at equivalent sound pressure level of 93 dB. Results showed that in noise conditions, subjects reduced their response times. This is an evidence of growing stress, according to arousal theory. In particular, LFN1 and LFN2 produced cognitive stress comparable to stochastic broadband multi-tonal noise. Furthermore, subdividing the subjects in extroverts and introverts through the Eysenck Personality Questionnaire–Revised psychological test, it was shown that LFN1 and LFN2 produced higher stress effects than stochastic broadband multi-tonal noise on the cognitive performances and a physiological stress comparable to stochastic broadband multi-tonal noise in introverts, whereas no effects were observed in extroverts, as hypothesized by Eysenck. This result highlights the necessity in the future to consider the personality parameter as a key factor in the evaluation of the effects of noise on humans.

Speech Understanding With Various Maskers in Cochlear-Implant and Simulated Cochlear-Implant Hearing: Effects of Spectral Resolution and Implications for Masking Release.

The purpose of this study was to investigate the relationship between psychophysical spectral resolution and sentence reception in various types of interfering backgrounds for listeners with cochlear implants and normal-hearing subjects listening to vocoded speech. Spectral resolution was measured with a spectral modulation detection (SMD) task. For speech testing, maskers included stationary speech-shaped noise (SSN), four-talker babble, multitone noise, and a competing talker. To explore the possible trade-offs between spectral resolution and susceptibility to different types of maskers, the degree of simulated current spread was varied within the vocoder group, achieving a range of performance for SMD and speech tasks. Greater simulated current spread was detrimental to both spectral resolution and speech recognition, suggesting that interventions that decrease current spread may improve performance for both tasks. Better SMD sensitivity was significantly correlated with improved sentence reception. In addition, differences in sentence reception across the four maskers were significantly associated with SMD across the combined group of cochlear-implant and vocoder subjects. Masking release (MR) was quantified as the signal-to-noise ratio difference in speech reception threshold between the SSN and competing talker. Several individual cochlear-implant subjects demonstrated substantial MR, in contrast to previous studies, and the degree of MR increased with better SMD thresholds across subjects. The results of this study suggest that alternative masker types, particularly competing talkers, are more sensitive than stationary SSN to differences in spectral resolution in the cochlear-implant population.

Annoyance perception of complex multi-tone noise signals in both harmonic and inharmonic structures within the built environment

Assorted building mechanical systems generate tonal components within the background noise of built environments. In most cases, this type of noise includes multiple tones in harmonic or inharmonic structures rather than a single tone. However, there is limited information on the comprehensive annoyance caused by multiple tones as perceived by human occupants. Two current standards, ISO 1996-2 and ANSI S1.13, propose calculation methods to address tones in noise, but those methods only analyze the tones individually. This paper aims to investigate how each tone contributes to overall annoyance perception when complex tones are present in background noise. Noise stimuli with five-tone complexes between 125 Hz to 2 kHz were artificially generated for subjective testing. The levels of each tone were randomly adjusted for every trial, and both harmonic and inharmonic structured tone complexes were utilized. Ten musically trained subjects participated in the subjective test involving paired comparisons. Each participant was asked to choose which noise stimulus is more annoying between two noise signals. Perceptual weighting analysis is applied to the results to compute a spectral weighting function for overall annoyance. The performance of the derived spectral weighting function is examined against annoyance ratings of actual building mechanical noises.

Multi-tone noise elimination in a space-constrained room lined with hybrid sound absorbers using a particle swarm method

High noise levels can be harmful to workers and lead to psychological and physiological ailments. A broadband noise hybridized with multiple pure tones has a high peak value and is detrimental to human hearing in an enclosed machine room. To efficiently reduce the reverberant part of the peak noise level, interest in shape optimization of a Helmholtz resonator attached to the internal wall is coming to the forefront. Research on Helmholtz resonators in dealing with a pure tone has been well-addressed. However, the development of a hybrid sound absorber used in simultaneously reducing both the pure tone and the broadband noise is rare. Therefore, we propose two kinds of hybrid sound absorbers (sound absorber A: a piece of perforate plate + a piece of sound absorbing material + air space + a Helmholtz resonator; sound absorber B: a piece of perforate plate + a piece of sound absorbing material + air space + two Helmholtz resonators) used to deal with a broadband noise hybridized with multiple tones. Here, two case studies for dealing with broadband noise hybridized with one tone and two tones will be introduced. Moreover, a numerical assessment of a sound absorption coefficient and the best optimized design for two kinds of hybrid sound absorbers under a space-constrained situation will be performed. Consequently, a successful approach in eliminating a broadband noise hybridized with multiple tones by using an optimally shaped hybrid sound absorber as well as a particle swarm method within a constrained space has been demonstrated.

Estimation of optimal sound-absorbing cover for premises with multi-tone noise

Despite the wide spread of computing devices and mathematical optimization methods in various fields of knowledge, little attention is paid to optimization of solving the problems of labor safety. In the author’s opinion the promising solution is developing the methods of sound-absorbing cover optimization based on non-linear programming, one of which has been the objective of the work. The method for estimation of the optimal cover in premises with multi-tone noise conditions is proposed in the paper. Different noise tones can be absorbed by different materials with the most suitable properties. The method is based on random selection of surface area values of different materials, with further selection of the optimal cover. The paper gives the estimation algorithm and the obtained results using the Mathcad computer program. The estimation results indicate the regularities of achieving the optimum and a significant economic effect. The proposed estimation method and algorithm can be used for taking labor safety measures for noise reduction, namely, sound-absorbing covers design for production facilities. The paper has a scientific novelty, based on random selection of materials surface area values, and practical significance based on economic effect and working conditions improvement

A Diagnosis Testbench of Analog IP Cores for Characterization of Substrate Coupling Strength

A diagnosis testbench of analog IP cores characterizes their coupling strengths against on-chip environmental disturbances, specifically with regard to substrate voltage variations. The testbench incorporates multi-tone digital noise generators and a precision waveform capture with multiple probing channels. A prototype test bench fabricated in a 90-nm CMOS technology demonstrates the diagnosis of substrate coupling up to 400MHz with dynamic range of more than 60dB. The coefficients of noise propagation as well as noise coupling on a silicon substrate are quantitatively derived for analog IP cores processed in a target technology, and further linked with noise awared EDA tooling for the successful adoption of such IP cores in SoC integration.

Performance study of fast frequency-hopped/M-ary frequency-shift keying systems with timing and frequency offsets over Rician-fading channels with both multitone jamming and partial-band noise jamming

In this study, the effects of timing and frequency offsets on bit-error rate (BER) performance of fast frequency-hopped M-ary frequency-shift keying communication systems over Rician-fading channels with both multitone jamming (MTJ) and partial-band noise jamming (PBNJ) are investigated. Analytical BER expressions are derived for both linear-combining and product-combining receivers. Numerical results show that under both MTJ and PBNJ conditions, the BER performance falls between the two extreme cases, in which either MTJ or PBNJ presents. It is found that the BER performance is severely degraded as the timing or frequency offset increases. The product-combining receiver is found to be more sensitive to the timing and frequency offsets than the linear-combining receiver. It is also observed that for the linear-combining receiver, the optimum diversity order increases as the timing and frequency offset increases over both Rician-fading and Rayleigh-fading channels; however, the reverse is true for the product-combining receiver.

Analysis of the filtered-X LMS algorithm and a related new algorithm for active control of multitonal noise

In the presence of tonal noise generated by periodic noise source like rotating machines, the filtered-X LMS (FXLMS) algorithm is used for active control of such noises. However, the algorithm is derived under the assumption of slow adaptation limit and the exact analysis of the algorithm is restricted to the case of one real sinusoid in the literature. In this paper, for the general case of arbitrary number of sources, the characteristic polynomial of the equivalent linear system describing the FXLMS algorithm is derived and a method for calculating the stability limit is presented. Also, a related new algorithm free from the above assumption, which is nonlinear with respect to the tap weights, is proposed. Simulation results show that in the early stage of adaptation the new algorithm gives faster decay of errors.

Convergence Analysis of a Complex LMS Algorithm for Active Control of Multitonal Noise

Convergence Analysis of a Complex LMS Algorithm for Active Control of Multitonal Noise

Multichannel active noise control system for local spectral reshaping of multifrequency noise

This paper discusses the development of a multichannel active system for local spectral reshaping of multitone noise. The aim of this work is to design a real practical system that performs well in local active noise control (ANC) applications, and so improving the comfort sensation produced by enclosed sound fields. The adaptive algorithm implemented in the controller is a multichannel extension of the multifrequency adaptive equalizer developed by Kuo. The philosophy behind these equalizers lies in independently controlling some given frequencies of a primary signal. Moreover, the algorithm should manage to generate usefully sized zones of equalization in order to allow for the head motion of somebody with restricted mobility; for example, a passenger seated in a car. To verify the successful implementation of the multichannel system, experiments were carried out under listening room conditions. The developed prototype consists of an array of up to four microphones used as error sensors and two secondary sources. A 4100-type Bruel and Kjaer mannequin with two calibrated microphones at the ear canals was used to measure sound levels in a hypothetical listener's head. Different synthesized repetitive noises were used, as reference signals, specifically repetitive noise with harmonics of 15, 20, and 28 Hz , as well as an 80 Hz single tone. The equalized points include an area around the error sensor positions, and these are measured using the mannequin and an x– y moving platform. The extent to which the experimental equalization zones obtained favourable results validates the multichannel local ANC equalization system. Different error sensor positions around the listener head were also tested. The residual field inside the equalization zone was measured in all cases.

ON-LINE FUNDAMENTAL FREQUENCY TRACKING METHOD FOR HARMONIC SIGNAL AND APPLICATION TO ANC

In this paper, a new indirect feedback active noise control (ANC) scheme based on the fundamental frequency estimation is proposed for systems with a harmonic noise. When reference signals necessary for feedforward ANC configuration are difficult to obtain, the conventional ANC algorithms for multi-tonal noise do not measure the reference signals but generate them with the estimated frequencies [1]. However, the beating phenomena, in which certain frequency components of the noise vanish intermittently, may make the adaptive frequency estimation difficult. The confusion in the estimated frequencies due to the beating phenomena makes the generated reference signals worthless. The proposed algorithm consists of two parts. The first part is a reference generator using the fundamental frequency estimation and the second one is the conventional feedforward control. We propose the fundamental frequency estimation algorithm using decision rules, which is insensitive to the beating phenomena. In addition, the proposed fundamental frequency estimation algorithm has good tracking capability and lower variance of frequency estimation error than that of the conventional cascade ANF method. We are also able to control all interested modes of the noise, even which cannot be estimated by the conventional frequency estimation method because of the poorS /N ratio. We verify the performance of the proposed ANC method through simulations for the measured cabin noise of a passenger ship and the measured time-varying engine booming noise of a passenger vehicle.

HYBRID NOISE CONTROL IN DUCTS

Presented in this paper is a hybrid noise control system used to attenuate multi-tonal noise in ducts. The hybrid system is built by integrating active feedback noise control with adaptive–passive noise control. The adaptive–passive part of the hybrid noise control system attenuates the dominant tonal noise within the duct while the active noise control system attenuates the noise at other frequencies. To achieve such a hybrid system, the constituent technology for each noise control solution is extended. In particular, an algorithm is described for tuning adaptive–passive noise control devices in the presence of multi-tone noise sources. Also, a feedback controller design methodology is presented. This methodology, adapted from the robust control literature, is easily applied to uncertain acoustic duct systems possessing significant time delays. In the system studied, the time delay arises from the non-collocation of the microphone and the control speaker. The uncertain dynamics of the duct are due to the on-line tuning of the adaptive–passive element. The extended adaptive–passive and robust noise control technologies are combined into a hybrid system. Experimental results are included as are extensions to broadband noise sources.

ACTIVE CONTROL OF MULTI-TONAL NOISE WITH REFERENCE GENERATOR BASED ON ON-LINE FREQUENCY ESTIMATION

In this paper, a novel active noise control (ANC) structure with a frequency estimator is proposed for systems with multi-tonal noise. The conventional feedforward ANC algorithms need a measured reference signal to calculate the gradient of the squared error and filter coefficients. For ANC systems applied to aircraft or passenger ships, which reference signals are usually measured are so far from seats where engines from the main part of controllers is placed that the scheme might be difficult to implement or very costly. Feedback ANC algorithms which do not require a measure of reference signals, use error signals alone to update the filter and are usually sensitive to measurement noise and unexpected transient noise such as a sneeze, clapping of hands and so on.The proposed algorithm, which estimates frequencies of the multi-tonal noise in real time using adaptive notch filter (ANF), improves convergence rate, threshold SNR and computational efficiency compared with the conventional ones. The reference signal needed for the feedforward control is not measured directly, but is generated with the estimated frequencies. It has a strong similarity to the conventional IMC-based feedback control because the reference is generated from the error signal in both cases. The proposed ANC algorithm is compared with the conventional IMC-based feedback control algorithm.Cascade ANF, which has a low computational burden, is used to implement the ANC system in real time. Experiments for verifying efficacy of the proposed algorithm are carried out in the laboratory.