Duct Noise Research Articles

Optimizing the acoustic performance of turbochargers

Turbocharger usage is predicted to increase in passenger vehicles for reasons such as enabling engine downsizing and improving thermodynamic efficiency. In order to achieve customer expectations, the acoustic performance of the turbo needs to be engineered for a wide range of operating load conditions. This requires the turbo supplier to set component targets in CAE and identify undesirable noises early in the development process using test cell acoustic measurements. Noise from a turbocharger can be separated into two main categories: (1) aeroacoustic sources including blade pass, pulsation, surge and broadband flow noise (2) structureborne from vibration sources such as subsynchronous, first order imbalance, and component resonances. One technique for comparing compressor acoustic performance is noise mapping, where the in duct noise data are collected across the full turbo operating range and post processed into 3D plots. Audible noises that can be generated in a centrifugal turbocharger and methods of identifying sources during development are presented in detail.

Effects of secondary loudspeaker properties on broadband feedforward active duct noise control

Dependence of the performance of feedforward active duct noise control on secondary loudspeaker parameters is investigated. Noise reduction performance can be improved if the force factor of the secondary loudspeaker is higher. For example, broadband noise reduction improvement up to 1.6 dB is predicted by increasing the force factor by 50%. In addition, a secondary loudspeaker with a larger force factor was found to have quicker convergence in the adaptive algorithm in experiment. In simulations, noise reduction is improved in using an adaptive algorithm by using a secondary loudspeaker with a heavier moving mass. It is predicted that an extra broadband noise reduction of more than 7 dB can be gained using an adaptive filter if the force factor, moving mass and coil inductance of a commercially available loudspeaker are doubled. Methods to increase the force factor beyond those of commercially available loudspeakers are proposed.

Shape optimization of a general bypass duct for tone noise reduction using continuous adjoint method

A novel continuous adjoint-based acoustic propagation method is proposed for low-noise turbofan duct design. A fan bypass duct tonal noise propagation model that is verified by comparison with an analytical solution of the modal radiation from a semi-infinite duct with the shear layer is enhanced with its continuous adjoint formulation, having been applied to design the bypass duct. First, this article presents the complete formulation of the time-dependent optimal design problem. Second, a continuous adjoint-based acoustic propagation method for two-dimensional bypass duct configurations is derived and presented. This article aims at describing the potential of the adjoint technique for aeroacoustic shape optimization. The implementation of the unsteady aeroacoustic adjoint method is validated by comparing the sensitivity derivative with that obtained by finite differences. Using a continuous adjoint formulation, the necessary aerodynamic gradient information is obtained with large computational savings over traditional finite-difference methods. The examples presented demonstrate that the combination of a continuous-adjoint algorithm with a noise prediction method can be an efficient design tool in the bypass duct noise design problem.

LOUDSPEAKER SLOTTED DUCT PORT

Provided herein is a loudspeaker system utilizing one or more ducted slot ports. In various embodiments, a ducted slot port may incorporate an acoustic low pass filter, such as a bend in the airflow path, to control midrange leakage. A ducted slot port may also minimize standing waves within the port duct and control turbulent port noise, such as by varying its cross-sectional area substantially continuously and symmetrically along the port duct's entrance-exit axis.

Attenuation Performance of a Semi-Active Helmholtz Resonator in a Grazing Flow Duct

Side branch Helmholtz resonators (HRs) are widely used to control low frequency tonal noise in air duct system. The passive Helmholtz resonator only works effectively over a narrow frequency range around resonance frequency. Changes in the exciting frequency and temperature will decrease the noise reduction performance. Many studies have been conducted on incorporating a Helmholtz resonator with active noise control to tuning the resonance frequency of HRs. The objective of this study is to study the effect of flow on the semi active Helmholtz resonator for duct noise control. Owing to a low Mach number air flow, the discontinuity condition at the joint is analytically formulated according to the conservation of the momentum and mass of air flow. Based on the transfer function at the junction, a controller function is proposed to tune the semi-active Helmholtz resonator under flow condition.

Narrow sidebranch arrays for low frequency duct noise control

The present study investigates the sound transmission loss across a section of an infinitely long duct where one or more narrow sidebranch tubes are installed flushed with the duct wall. The finite-element method is used to compute the wave propagation characteristics, and a simplified theoretical analysis is carried out at the same time to explain the wave mechanism at frequencies of high sound reduction. Results show that the high sound transmission loss at a particular frequency is due to the concerted actions of three consecutive sidebranch tubes with the most upstream one in the resonant state. The expansion chamber effect of the setup also plays a role in enhancing sound attenuation at non-resonance frequencies. Broadband performance of the device can be greatly enhanced by appropriate arrangements of tube lengths and/or by coupling arrays on the two sides of the duct.

A nonlinear active noise control algorithm for virtual microphones controlling chaotic noise

In active noise control (ANC) systems, virtual microphones provide a means of projecting the zone of quiet away from the physical microphone to a remote location. To date, linear ANC algorithms, such as the filtered-x least mean square (FXLMS) algorithm, have been used with virtual sensing techniques. In this paper, a nonlinear ANC algorithm is developed for a virtual microphone by integrating the remote microphone technique with the filtered-s least mean square (FSLMS) algorithm. The proposed algorithm is evaluated experimentally in the cancellation of chaotic noise in a one-dimensional duct. The secondary paths evaluated experimentally exhibit non-minimum phase response and hence poor performance is obtained with the conventional FXLMS algorithm compared to the proposed FSLMS based algorithm. This is because the latter is capable of predicting the chaotic signal found in many physical processes responsible for noise. In addition, the proposed algorithm is shown to outperform the FXLMS based remote microphone technique under the causality constraint (when the propagation delay of the secondary path is greater than the primary path). A number of experimental results are presented in this paper to compare the performance of the FSLMS algorithm based virtual ANC algorithm with the FXLMS based virtual ANC algorithm.

Time-domain simulation of acoustic wave propagation and interaction with flexible structures using Chebyshev collocation method

A time-domain Chebyshev collocation (ChC) method is used to simulate acoustic wave propagation and its interaction with flexible structures in ducts. The numerical formulation is described using a two-dimensional duct noise control system, which consists of an expansion chamber and a tensioned membrane covering the side-branch cavity. Full coupling between the acoustic wave and the structural vibration of the tensioned membrane is considered in the modelling. A systematic method of solution is developed for the discretized differential equations over multiple physical domains. The time-domain ChC model is tested against analytical solutions under two conditions: one with an initial state of wave motion; the other with a time-dependent acoustic source. Comparisons with the finite-difference time-domain (FDTD) method are also made. Results show that the time-domain ChC method is highly accurate and computationally efficient for the time-dependent solution of duct acoustic problems. For illustrative purposes, the time-domain ChC method is applied to investigate the acoustic performance of three typical duct noise control devices: the expansion chamber, the quarter wavelength resonator and the drum silencer. The time-dependent simulation of the sound-structure interaction in the drum silencer reveals the delicate role of the membrane mass and tension in its sound reflection capability.

Narrow sidebranches for duct silencing

A narrow sidebranch attached to the rigid wall of a duct will result in high sound transmission loss across it at its resonance frequencies. Coupling narrow sidebranches together will therefore produce a broadband silencing device for duct noise control. However, the sidebranch length variation will affect the broadband performance. Numerical investigation was carried out in this study to understand the effects of the sidebranch length variation and the sidebranch width on the overall sound attenuation spectrum of the coupled sidebranches. It is found that broadband sound attenuation below the first higher mode cut-off frequency of the main duct of over 20dB across the working bandwidth can be achieved if the length variation and widths of the sidebranches are appropriately chosen.

Investigation of a broadband duct noise control system inspired by the middle ear mechanism

A new duct noise control device is introduced based on the mechanism of human middle ear which functions as a compact, broadband impedance transformer between the air motion in the outer ear and the liquid motion in the inner ear. The system consists of two rigid endplates, simulating the tympanic membrane and the stapes footplate, and they are connected by a single rigid rod, simulating the overall action of the ossicular chain. These three pieces are placed in a side-branch cavity, and the whole device is called an ossicular silencer. A specific configuration is investigated numerically with a two-dimensional finite element model. Results show that broadband noise attenuation can be achieved in the very low frequency regime. Typically, two or more resonance peaks are found and the transmission loss between two neighbouring peaks is maintained at a high level. The cavity length is found to be the most crucial parameter that determines the effective frequency range of the ossicular silencer. The total cavity volume, which is a major controlling factor in most existing noise control devices, becomes less influential. The fluid medium in the enclosed cavity mainly acts like an added mass, while its stiffness effect is negligible. Simplified plane wave analysis is also conducted to reveal the mechanisms of the system resonances. The first resonance is identified as of the mass-spring system with mass contributions from both fluid and the plates, while the second one is of the Herschel–Quincke (HQ) tube resonance.

Wave propagation in a duct with a periodic Helmholtz resonators array

Helmholtz resonator is often used to reduce noise in a narrow frequency range. To obtain a broader noise attenuation band, combing several resonators is a possible way. This paper presents a theoretical study of sound propagation in a one-dimensional duct with identical side-branch resonators mounted periodically. The analysis of each resonator was based on a distributed-parameter model that considered multi-dimensional wave propagation in its neck-cavity interface. This model provided a more accurate prediction of the resonant frequency of the resonator than traditional lumped-parameter model. Bloch wave theory and the transfer matrix method were used to investigate wave propagation in these spatially periodic resonators. The results predicted by the theory fit well with the computer simulation using a three-dimensional finite element method and the experimental results. This study indicates that the wave coupling in this periodic system results in the dispersion of the frequency band into the stop and the pass bands. The long-term significance is that periodic resonators may more effectively control noise in ducts by broadening the bandwidth they attenuate and increasing the magnitude of sound attenuation.

529 ダクト内騒音に対するサブバンド適応フィルタを用いたANCシステムの適用

The present paper describes the numerical investigation of characteristics of adaptive feedforward active control systems of broadband noise in ducts. Active control systems using delayless subband adaptive filters have been proposed to improve the performance of the active control. Proposed delayless subband system can achieve relatively large attenuation in wide frequency range. The load of processing for active control may be able to further reduce by a low order adaptive filter with a low sampling rate.

Duct Noise Control by Using Very Light Composite Plate

A compact flow-through plate silencer is constructed for low frequency noise control by using new reinforced composite plates. The concept comes from the previous theoretical study [1] that in a duct, a clamped supported plate covered with a rigid cavites. The structural property of the very light plate with high bending stiffness is very crucial element in such plate silencer. In this study, an approach to fabricate new reinforced composite panel with light weight and high bending stiffness is developed in order to realize the function of this plate silencer practically. The performance of two plate silencer with the stopband from 229 to 618Hz in which the transmission loss is higher than 10 dB over the whole frequency band can be achieved.

A Secondary Source Configuration for Control of a Ventilation Fan Noise in Ducts

The main noise source in heating, ventilation, and air conditioning systems is usually a ventilating fan. Noise, generated by the ventilating fan is transmitted through the duct into the living and working environment. A typical fan noise spectrum consists of a broadband noise, which is superimposed with pure tones. Different methods are available to reduce a transmission of such noise from the ventilating fan into the living and working environment. In this article it is demonstrated how a feedforward active noise control system can be implemented together with a side branch resonator. Effectiveness of the feedforward active noise control system depends on the quality of a reference signal, which should be in a perfect correlation with the primary noise. An acoustic feedback is the main problem of feedforward active noise control systems in ducts. A combined method uses a single loudspeaker to work as a dipole source and a side branch resonator to reduce the acoustic feedback. A side branch resonator reduces noise transmission in a narrowband frequency range as well. In this article, a theoretical background of a dipole source with a side branch resonator is presented, along with some measurement results and simulations of active noise control.

Enhancing the Characterisation of Ducted Noise Sources Using a Spectral Formulation

The characterization of ducted sound sources is very useful to the aeronautical and automotive industries. It provides inputs to models used for predicting flow-generated noise at the design stage. Conventional inverse approaches, such as the Equivalent Source Method, offer accurate reconstruction results. But they require either a large number of in-duct field pressure measurements or a suitable location of a few number of equivalent sources. This research proposes an alternative approach formulated in the spectral domain: the Spectral Decomposition Method. It directly provides a cross-sectional imaging of the source velocity distribution from pressure data measured at small or large stand-off distance to the source. Well-resolved imaging results of the axial acoustic velocity generated by wall-mounted drivers have been obtained from either induct or wall-pressure measurements acquired respectively without or with flow.

A Study of the Herschel-Quincke Tube Concept

A study of the Herschel-Quincke (HQ) tube concept for the reduction of noise in circular ducts is presented here, which contain the models, the attenuation conditions and the acoustic characteristics of HQ tube without flow and with mean flow. Then the simulations have been done by using GT-POWER, which explain the influences on the acoustic characteristics of the angle between the main duct and the bypass duct and the changes of the diameters. The results show that the angle has an impact with flow, no effect without flow, and both of them are affected by the changes of the diameters.

The theoretical fundamentals of an adaptive active control using periodic Helmholtz resonators for duct-borne transmission noise in ventilation systems

This article theoretically proposes an innovative semi-active noise control method for the control of duct-borne transmission noise. This method is designed to be a more efficient, robust and broad-band adaptive attenuating effect than the traditional pass noise control methods used in duct-borne sound control. Based on the fundamental theory on the pass-stop band structure of sound waves propagating in a one-dimensional periodic duct system, the proposed control method combines band-gap attenuation using the side-branch resonators array with the auto-adaptive system of noise control. Using this method in the simulation and analysis, the transmission noise in an air duct can be effectively controlled in relatively wide bands by periodic alternate resonators of small variable parameters. The developed semi-active control method is numerically designed to efficiently reduce the duct-borne transmission noise at frequencies around a variable major tone in various working conditions. Practical applications: The proposed semi-active noise control method with band-gap attenuation of a periodic adaptive resonator array is generally an efficient, robust and broad-band noise control system. It has the potential to be used to control duct-borne transmission noise in order to satisfy stringent indoor acoustical requirements such as those of the indoor air-conditioned environments of a hospital, a scientific laboratory, a council hall and a living room.

The use of axial splitters inside ducts for active noise control of higher order acoustic modes

Studies of noise reduction in ducts have become, over the years, a field of intense application mainly because of the industrial potential and feasibility of practical systems. The traditional approach to noise control in ducts uses passive techniques such as dissipative and reactive silencers. However, with the advance of the technology and limitations of conventional noise control treatments, active noise control (ANC) has become a practical and economic method due to powerful hardware for the digital signal processing in real time. However, in industrial applications, the design and employment of active noise control not always is simple and practicable. In exhaust systems, the ducts have geometry which allows high order acoustic modes to propagate at the frequencies to be controlled. These characteristics make it difficult to implement an ANC system. Higher order modes in duct sometimes result in lower ANC performance. In this paper, a methodology based on splitting the duct into parallel axial sections is presented in order to ensure that only plane waves are present in the region of the ANC system. The aim of this work is to show that this artifice can be a viable and cost effective solution. Numerical acoustic simulations were carried out in order to demonstrate the viability of the suggested technique. A finite elements method is used. An optimization using evolutionary algorithms is also used to select optimal actuators positions. Attenuation of 26 dB was obtained for tonal noise in a duct with splitters, while simulations with the conventional duct had 3 dB of mitigation

Active noise reduction in ventilation ducts.

ANC is commonly used in several noise exposed areas. The basic idea has been extensively analyzed in literature, especially for the cancellation of sinusoidal noise in ducts. Here, the challenge for an ANC-system is the attenuation of undesired broadband noise in a ventilation duct with high air flow. In this case special consideration must be given toward the performance of the acoustic sensors while exposed to turbulent flow. The concept of a Swinbank-source in combination with an adaptive filter appears to fit well for the given problem. Furthermore, causality constraints concerning the feed forward adaptive controller are analyzed with respect to sensor and actuator placement. In an experimental setup with a duct of 3 m length and a diameter of 350 mm, noise attenuation of 14 dB in a frequency range from 50<f<450 H can be achieved downstream of the control source.

Modified-filtered-u LMS algorithm for active noise control and its application to a short acoustic duct

This paper presents a new adaptive algorithm for active noise control (ANC) that can be effectively applicable to a short acoustic duct, such as the intake system of an automobile engine, where the stability and fast convergence of the ANC system is particularly important. The new algorithm, called the modified-filtered-u LMS algorithm (MFU-LMS), is developed based on the recursive filtered-u LMS algorithm (FU-LMS) incorporating the simple hyper-stable adaptive recursive filter (SHARF) to ensure the control stability and the variable step size to enhance the convergence rate. The MFU-LMS algorithm is implemented by purely experimental ways, and is applied to active control of noise in a short acoustic duct, and is validated using two experimental cases of which the primary noise sources are a sinusoidal signal embedded in white noise and a chirp signal. The experimental results demonstrate that the proposed MFU-LMS algorithm gives a considerably better performance than other conventional algorithms, such as the filtered-x LMS (FX-LMS) and the FU-LMS algorithms.