Duct Noise Research Articles

Hydro-acoustic numerical methodology to compute noise generated by perforated plates in duct systems

The paper presents an innovative numerical methodology to determine hydro-acoustic behavior of perforated plates. One of the main objectives is to have a methodology that ensures a good prediction of radiated noise in ducts with a limited computational cost. The initial assessment is carried out considering the entire perforated plate. The acoustic source is determined by applying Lightill Analogy to the CFD results from LES for the whole perforated plate. An acoustic simulation is then performed to determine the dynamic pressure in the duct on either side of the perforated plate. An experimental set-up is built in order to obtain the noise generated by the perforated plate. It also allows to assess the validity of the established method. A good correlation is observed in terms of dynamic pressure upstream and downstream the perforated plate. An optimization of the method is then carried out to reduce the computational time by determining the acoustic source generated by a unit hole. This source is then applied in the acoustic simulation to each hole of the perforated plate. Comparison with measurements is also satisfactory for this simplified method.

Noise control in ducts using Herschel-Quincke acoustic metamaterial array

Acoustic resonators, like Helmholtz, side branch tubes, expansion chambers, Herschel-Quincke, and so on, are usually used to control duct noise in acoustic systems. Noise attenuation in local resonators occurs due to impedance mismatch of incident, transmitted, and reflected waves induced by the addition of resonators. The frequency band and the attenuation level generated by the resonator depend on its geometry and behave as a narrow-band filter. However, a much larger frequency band, called bandgap, is obtained if they are appropriately distributed (periodically or not) along the duct length. This paper investigates acoustic noise attenuation for an acoustic metamaterial using a Herschel-Quincke (HQ) resonator. It is modeled by the transfer matrix method and the finite element method. Also, a study about the arrangements of the HQ resonators (serial and parallel) is conducted to verify their accuracy and efficiency related to bandwidth enlargement and sound transmission loss. Simulated results are obtained for different examples and compared to those from the literature.

Time-Resolved Flowfield and Acoustic Measurements of a Ducted and Unducted Rotor

The impact of adding a duct to an 18.5″ diameter, triple-bladed, electrically driven rotor operating up to Mtip=0.5 at static conditions is investigated experimentally to identify the source mechanisms governing the measured acoustic differences. An unducted rotor is characterized by strong, coherent tip vorticity that interacts with each blade passing, generating high-amplitude harmonic tones. The addition of the duct results in tonal broadening and an increase of the primary and harmonic tone amplitudes. Up to a 15 dB increase in broadband noise across the entire frequency range is measured. High-speed particle imaging velocimetry up to 6.4 kHz is employed to acquire phase-locked and ensemble-averaged measurements of the velocity, vorticity, and turbulence fields. While it is often theorized that a duct can shield lateral acoustic radiation, this study finds that the ducted rotor is louder across all operating regimes, and the noise increases with tip Mach number. The ducted rotor dissipates the coherent tip vorticity into incoherent turbulence intensity due to the interaction of the vortices with the duct internal walls, contributing to a strong broadband noise increase and tonal broadening. The ducted rotor broadband noise increase is due to the rotor interacting with a more turbulent inflow condition and increased turbulence intensity throughout the plume shear layer.

Deep-learning-based generative design for optimal reactive silencers

A deep-learning-based generative design method is proposed to improve the frequency-dependent characteristics of a reactive silencer, and it has been validated both numerically and experimentally. The noise attenuation performance of the reactive silencer is evaluated with its transmission loss (TL), which varies with frequency and strongly depends on the partition layout inside the reactive silencer. The artificial neural network model for the generative design of the reactive silencer consists of three subnetwork models: the generator, predictor, and converter. The generator model created numerous partition layouts, and their TL curves were estimated using the predictor model. A converter model was developed to identify the frequency-dependent characteristics of the TL curves in a low-dimensional latent space. The latent space was extensively investigated to successfully select the optimal partition layouts satisfying given design requirements, including the target shape of the TL curve and its averaged target TL value. The effectiveness of the proposed method was demonstrated by applying it to three reactive silencer design problems with different design requirements. Among the three optimal silencers, one was physically investigated, and its noise attenuation performance was validated with an acoustic experiment. Because the artificial neural network model of the proposed method was developed for a normalized silencer and requires no prior knowledge of acoustics, it can be easily applied to reduce duct noise in the industry.

Research on tunable bandgap characteristics of the phononic crystal duct with piezoelectric thin plates for the broad noise reduction

To achieve adjustable mid-low frequency noise reduction characteristics in pipeline systems, one hybrid phononic crystal duct with piezoelectric thin plates is proposed. Firstly, phononic crystal duct model with Helmholtz mufflers and built-in piezoelectric thin plates is established to reduce low-to-mid frequency noise in the duct. The theoretical transmission loss is verified using simulation methods, revealing the sound attenuation law and bandgap mechanism. Under the shunting effect of the RLC circuit, three locally resonant bandgaps are generated: the mechanical locally resonant band gap introduced by the thin plate, acoustic locally resonant band gaps caused by the Helmholtz muffler cavity structure, and the electromechanical locally resonant band gaps dominated by the shunting circuit. By adjusting the shunting circuit, the acoustic impedance of the piezoelectric thin plate can be controlled, and then the acoustic characteristics of low and medium frequencies will change. For the shunting circuit composed of inductance, capacitance and resistance in series, the inductance and capacitance can adjust the position of the three band gaps; the negative capacitance can effectively enhance the tunability of the electromechanical locally resonant band gap, which is helpful to obtain the low-frequency band gap; while resistance primarily affects the magnitude of acoustic attenuation within the bandgap. Subsequently, based on the analysis of the noise characteristics of the air-assisted system of one fuel cell vehicle, one hybrid phononic crystal duct with compact structures is designed to achieve a broadband duct noise reduction of 330 Hz–4000 Hz.

Development of a compact device for duct noises

Noise radiated from ducts associated with air conditioning equipment is one of the most important noises that must be addressed to create a quiet space. While it is relatively easy to reduce this noise if the equipment works at a constant frequency, it is difficult to reduce noises of various frequencies being generated depending on changes in its output. Although a side branch is one of the solutions for duct noises, a single side branch can only be effective for a specific frequency. On the other hand, multiple side branches can broaden the effect, However, the space required for side branches becomes very large and is not practical for use. In this study, a compact duct noise reduction device based on the side branch concept is proposed. The side branches of multiple lengths filled with porous materials are considered to achieve both a wide-band reduction and space saving. The design process of the device which can be installed easily is illustrated and the reduction performance are estimated by numerical studies.

Low frequency noise control using coupled loudspeaker and negative impedance circuit

It remains a technical challenge for low-frequency duct noise control devices to combine compact and broadband features at same time. A semi-active noise control approach is introduced to solve this problem. The device consists of a main pipe and two loudspeakers connected by a circuit containing a negative impedance network. Due to the effect of the negative impedance circuit, the virtual bypass is composed of the coupled loudspeaker and the circuit has sound transmission characteristics close to a physical duct in a certain frequency range. The effect of the negative impedance network and circuit on the performance of the silencer is studied by using the transfer matrix method. Through the design of the circuit, the sound transmission process of the virtual bypass can be realized. If the proper circuit network and parameters are selected, the virtual bypass HQ tube can achieve compact broadband noise control at low frequencies. A semi-active noise control device has 10dB noise reduction in the range of 100-300Hz.

Design of sinusoidal-shaped inlet duct for acoustic mode modulation noise reduction of cooling fan

The noise reduction method of installing a sinusoidal-shaped inlet duct on a cooling fan is analyzed theoretically and experimentally in terms of the acoustic mode modulation. Based on Lowson's point force model, the correlation between the free field noise and acoustic mode of the fan rotor and the unsteady forces on the rotor blade surface is established. The azimuthal acoustic mode is demonstrated to partially reflect the composition and intensity of the acoustic source. The theoretical variation in azimuthal acoustic mode indicates that it is possible to completely cancel each mode simultaneously when the phase differences of forces of each order in the secondary source are controlled. A sinusoidal-shaped inlet duct noise reduction structure controlled by a parameter equation is proposed, and the preferred parameter selection is given by the far-field noise measurements. The results of azimuthal mode decomposition exhibit a similar trend to the theoretical analysis. Compared with the straight duct, the chosen sinusoidal-shaped inlet duct achieved greater noise reduction at the blade passing frequency and its first harmonic. The average total sound pressure level in the far field was 6.0 dBA lower than that of the prototype fan, and 1.4 dBA lower than that of the straight duct model.

Advancing flow-duct noise control: Examples of innovative silencers

Two examples of innovative silencers for advanced duct noise control are presented. The first example is related to using metamaterials and the so-called ‘slow sound' to create a compact virtual Herschel-Quincke tube [1]. The second example revolves around a concept called the ‘modal filter' [2], which is designed to selectively damp a single propagating mode. For plane waves, optimal propagational damping is achieved by a wall impedance equal to the Cremer impedance. We will demonstrate how to compute the Cremer impedance, even for low frequencies, at which the classical solution is invalid [3]. Such an arrangement efficiently dampens both the plane (0,0) and the first radial mode (0,1). To further dampen the first two circumferential modes (1,0) and (2,0), a section with the Cremer impedance can be combined with a modal filter made of micro-perforated plates. These plates are inserted parallel to the flow direction and positioned at modal velocity maxima in the duct cross-section. We anticipate that both examples can be useful in industries requiring compact noise mitigation solutions in flow ducts. [1] https://doi.org/10.1016/j.jsv.2019.115045. [2] https://doi.org/10.1121/2.0000473. [3] https://doi.org/10.1121/1.5136952

Broadband sound attenuation and absorption by duct silencers based on the acoustic black hole effect: Simulations and experiments

Wideband reduction of both sound reflection and transmission using compact non-intrusive liners remains a challenging issue for duct noise control applications. This work focuses on the acoustical performance limitations of cylindrical silencers made up of annular ring resonators with axial gradient of their cavity depths. It is shown from theoretical, numerical and experimental studies that a sub-wavelength silencer with optimized acoustic black hole (ABH) properties can be designed through which incident sound waves are retarded and fully dissipated within the activated resonant cavities. Such ABH-type silencer requires a suitable interplay between the wall impedance axial variations and the visco-thermal losses within the cavities. Key parameters that influence this balance are the axial growth of cavity depths and the wall porosity. A causal-based criterion has been proposed that maximizes the total integrated dissipated power, thereby leading to the optimal value of the wall porosity and to the ultimate bandwidth-to-length ratio that can be achieved by an ABH-type silencer given a target dissipation value. A space-frequency region has been numerically and experimentally identified over which the ABH effect is prominent. The causal-based criterion has been extended to account for the low-frequency ultimate performance of ABH silencers with coiled cavities and for the effects of a low-speed grazing flow.

Optimal duct length of ducted fans for minimizing first blade-passing frequency noise

Fans generate tone noise and broadband noise. The first blade-passing frequency noise is a tone noise and is the most dominant; attaching a duct and a stator can reduce this noise. However, there has been no theoretical development in using structural characteristics to reduce the noise generated by ducted fans. In this study, noise predictions using Lighthill's equation and Green's function were performed to determine the optimal duct length for a ducted fan's noise reduction. New formulations for predicting the length were derived using a modal noise analysis. Noise sources were obtained from computational fluid dynamics data and wall pressure spectra. The formulations deduce a linear relationship between a duct's radius and length and can be used as a tool to indicate fan size at the design stage under space and weight constraints without high computational costs.

Development of flow-vibroacoustic coupled numerical methods for prediction of noise radiation due to flow-born vibration of compressor discharge piping system

The compressor and fan in air conditioner outdoor unit are the core component which deter-mines noise performance as well as cooling or heating performance. Among these, the com-pressor has larger contribution to the noise of outdoor units. For high efficiency air condition-er, the size and operating speed of compressor are smaller and faster. The high-speed com-pressor causes loud noise which is complaints for the customer. Traditionally, it is well-known that the vibration of compressor and connected duct is main noise source. However, as the compressor speed increases, the refrigerant flow in the compressor discharge duct also emerged as a major noise source. To reduce the compressor noise operating at high speed, it is necessary to analyze vibrational and acoustic characteristics of compressor discharge duct. This duct noise has the two-types source: structure born noise and flow induced noise. The structure born noise is generated by the duct vibration caused by compressor movement. For the flow induced noise, the static pressure field of refrigerant flow in the duct is vibrational source. In this paper, the compressor discharge duct noise considering two mechanisms was investigated. The refrigerant flow is solved using CFD and duct vibration and noise radiation are computed by FE-BE method.

Acoustic analysis of coupled loudspeakers for low frequency duct noise reflection

The passive control of low-frequency duct noise remains a technical challenge. Inspired by the research of Wang and Huang [Investigation of a broadband duct noise control system inspired by the middle ear mechanism, Mech. Syst. Signal Process. 31 (2012) 284-297.], an electro-mechanical coupling approach is introduced to solve this problem. The device consists of the main duct segment and multiple sets of connected loudspeakers, which functions as a side-by tube in traditional Herschel-Quincke (HQ) tube. The acoustic waves imposed on the upstream loudspeaker can be transmitted to the other loudspeaker via the connecting circuit immediately, which represents a fast track when compared with the wave transmission via the fluid medium in the main duct. A periodic silencer array is developed to broaden the bandwidth and increase the magnitude of noise attenuation. Transfer matrix method is adopted to investigate its acoustic performance. If proper parameters are selected, the proposed silencer can function effectively as wave reflector in low frequency range. The TL spectrum exhibits multiple peaks, which are mainly contributed by the HQ-tube-like resonance and mechanical resonance of the loudspeaker itself.

Programmable time-serial resonances for broadband spectrum matching

Spatially parallel and cascading acoustic liners are commonly used to treat duct noise. Their performance depends on the spectral match with source characteristics. Such liners rely on subtle geometrical and mechanical designs which are not difficult to adapt to different noise sources in a passive device. Here, we propose a temporal analogy, which is a shunted electromechanical diaphragm (SEMD) having different resonant frequencies in different time segments, namely, a time-serial resonator. Its sound absorption spectrum can be easily adapted by a program and yet the device functions passively. The acoustic impedance of the SEMD is determined by the shunt circuit, hence its resonant frequency and absorption peak. A multiple set of branch circuits are used to shunt the diaphragm. A MOSFET, which is a voltage-controlled ultrafast electronic switch, is introduced to cascade and switch each circuit branch. By activating circuit branches in a pre-defined time sequence, we obtain a series of resonance and the absorption which are effective for specific frequency bands at different time segments. When viewed over a long period, the averaged effective sound absorption spectrum is broadened and can be easily shaped by the working duty cycle of each circuit branch. Note that the switching voltage changes the circuit states without supplying power to the SEMD. Both numerical results and experimental demonstrations are presented. This study would open the new era of temporal design of acoustic liners.

Broadband noise attenuation in the flow duct using metamaterial-based acoustic liners

An acoustic liner design based on metamaterial is proposed for broadband duct noise reduction. The material is constructed by four periodically arranged partitions-embedded units covered by a perforated plate, forming a linear reflected phase-shifting within 0 to 2π in a wide target frequency range. The sound absorption performance under normal incident waves is firstly examined by both numerical simulation and impedance tube measurement, leading to close agreement to the target design. Then, the meta-liner is installed in a flow tube to assess its capability of reducing broadband noise in a duct with aerodynamic flows. The experiments investigations are performed in an advanced grazing flow tube developed at the Hong Kong University of Science and Technology. The effect of sound levels (ranging from 120dB to 140dB), flow speeds with the Mach number up to 0.3, and the position of sound source (at upstream and downstream sides of meta-liner) are investigated. Results show that a nearly flat transmission loss is achieved in the target frequency range by the metamaterial-based acoustic liner in various conditions, showing its potential for practical industrial applications.

Effect of the microstructure on the acoustic performance of porous material liner in the duct

Acoustic liners can efficiently reduce the sound pressure level in the duct. The porous material liner has recently received wide attention for the excellent attenuation performance at mid- to high- frequencies. However, more attention was paid to the sound attenuation of the given porous material liner in the duct rather than the design of the porous material liner for a duct noise problem. In this work, from a micro perspective, the influences of the average fiber radius and gap of the porous material liner on the acoustic field in a duct are systematically analyzed, based on the bottom-up method and Johnson-Champoux-Allard model. The multimodal method is utilized to evaluate the duct acoustic field. The work may guide the selection and design of the porous material during the application of the porous material in the duct noise problem.

Low-frequency duct noise control using coupled loudspeakers.

A duct noise control device is introduced based on a Herschel-Quincke (HQ) tube and electro-mechanical coupling. The device consists of the main duct segment and one set of connected loudspeakers, which functions as a side-by tube in a traditional HQ tube. The acoustic waves imposed on the upstream loudspeaker can be transmitted to the other loudspeaker via the connecting circuit immediately, which represents a fast track when compared with the wave transmission via the fluid medium in the main duct. It is the core noise attenuation mechanism in this silencer. The transfer matrix method is used to investigate the silencer performance. A periodic silencer array is also developed to broaden the bandwidth and increase the magnitude of noise attenuation. The results predicted by the plane wave theory fit well with the simulation using a three-dimensional finite element method. A simplified experiment was conducted to verify the silencing effect of the silencer and the accuracy of theoretical prediction method. Contrasting with a traditional reactive silencer, low-frequency noise attenuation can be achieved in a more compact and flexible way, which can be seen as an improvement in low frequency noise control.

Air Duct Optimization Design Based on Local Turbulence Loss Analysis and IMOCS Algorithm

Considering the complex flow state of the duct flow field in the exhaust system, the structural parameters can significantly impact the internal flow field and noise. This paper takes the noise generated by the duct system under operating conditions as the research object, studies the mechanism of duct noise generation through theoretical analysis, numerical simulation and experimental test, and proposes an optimization design method, that is, to improve the duct structure by adding duct guide vanes. In order to maximize the optimization effect of the guide vane, a multiobjective optimization design of its profile is required, including the parametric expression of the guide vane profile, establishing the design variables and optimization objectives, and establishing the Kriging approximation model. The IMOCS algorithm is used to accurately and efficiently calculate the Pareto front solution to obtain the optimal profile of the duct guide vane and finally improve the noise-reduction performance of the duct system. This paper applies this design method to an integrated stove head duct to verify its accuracy, and prototype tests are conducted according to the optimization results. The test results show that the optimized integrated cooker has improved the outlet flow rate of the whole machine by 1.2 m3/min and reduced the noise by 2.3 dB.

Numerical Study on the Duct Noise Control of Single-Stage Axial Flow Fan with Wave Leading Edge Stator Blade Configurations

Numerical Study on the Duct Noise Control of Single-Stage Axial Flow Fan with Wave Leading Edge Stator Blade Configurations

Compact broadband acoustic meta-silencer based on synergy between reactive and resistive units

Acoustic silencers are the most effective solution to control noise in ducts. In this paper, we propose a compact acoustic meta-silencer (AMS) based on the synergy between reactive and resistive units that enables the reduction of low-frequency and broadband noise. We first propose a conceptual AMS comprising simple reactive and resistive units to verify its unique sound attenuation performance. To explore its potential for application, we then propose an advanced AMS unit consisting of two independent annular chambers that represent reactive and resistive units, respectively. The synergistic mechanism between reactive and resistive units to achieve superior sound attenuation is revealed. Next, the band structures of the infinite periodic advanced AMS are discussed, and three different types of advanced AMS containing six units are examined. It is demonstrated numerically and experimentally that the optimized AMS with a compact size can achieve a transmission loss higher than 15 dB over a super-wide low-frequency range (290–1344 Hz). The work here provides a new avenue for the design of low-frequency and broadband meta-silencers to control the noise in ducts.