Acoustic Enclosure Research Articles

Enhancing subjective spaciousness for stereo reproduction in car-size acoustical enclosures based on audio signal decorrelation

Audio signal decorrelation has been used for enhancing the spaciousness of auditory source width (ASW) in a stereo reproduction. However, the acoustic environment in car-size acoustical enclosures is quite different from that in a home theater or listening room. Due to the small cabin size and the substantial presence of absorptive surfaces, the reproduced sound field in car-size acoustical enclosures is characterized by the interior mode region up to some hundreds or one thousand hertz and fast decay of pressure at higher frequencies which could affect the perceived performance of audio signal decorrelation. Therefore, the present work evaluates the effect of reflections in an acoustical enclosure model with car-size on the ASW enhanced by decorrelated audio signals, which reproduced by a standard stereo system. The decorrelation algorithm is handled by all-pass filters sets based on a pair of reciprocal maximal length sequence (MLS). Last, a subjective assessment experiment of ASW validates that appropriate decorrelation method can improve perceived spaciousness for sound reproduction in car-size acoustical enclosures.

Effect of Acoustic Enclosure on the Sound Transmission Loss of Multi-Layered Micro-Perforated Plates

This study presents an examination of the transmission properties of multilayered partitions made up of multiple micro-perforated plates (MPPs) coupled to acoustic enclosures with general impedance boundaries. Multi-layered MPPs can lower the transmission while minimizing reflection in the source and receiving enclosure. Previous research has mainly focused on the double MPPs or triple MPPs partition itself. However, it is vital to analyze the in-situ sound transmission loss of the multi-layered MPP and their efficiency in a complex vibro-acoustic environment. The case when the multilayered MPPs are coupled to a receiving enclosure or coupled to both a source and receiving enclosure is investigated. The objective is to provide an analytical method to evaluate the transmission properties of multilayered MPPs coupled to acoustic enclosures while being computationally more efficient than the finite element method (FEM). Using the modified Fourier series for the acoustic pressure, a variational form for the acoustic and structure medium yields a completely coupled vibroacoustic system. A comparison between the sound transmission loss of the double MPPs, when mounted on an impedance tube and coupled to acoustics enclosures, shows the modal effect of the enclosures. The effect of enclosure shape, impedance boundary, perforation ratio, air gap thickness on the sound transmission properties of the double MPPs structure is examined for both cases. Finally, in both situations, the performance of triple MPP structure insulation is evaluated.

An Investigation of the Acoustic Enclosure of an Air Conditioning Compressor Using Response Surface Analysis and Topological Rigidity Optimization

A novel split-type air conditioning system is introduced to balance usability and portability. Unlike conventional split-type systems, where the compressor is typically placed outside, this system situates the compressor within the indoor unit, which may expose users to compressor noise. There are prominent peaks in the compressor noise spectrum, particularly at the compressor operating frequency and its harmonics, notably the second and third harmonics. The research presents a multilayered acoustic enclosure specifically designed for air conditioning compressors to address this issue without modifying the compressor or indoor unit casing. In order to get better sound insulation performance, a response surface methodology (RSM) is applied to optimize the thickness ratio, open area ratio, and open area height of the acoustic enclosure with predefined thickness. In addition, topological optimization is employed to strengthen weak areas of the acoustic enclosure. Then, experimental trials using the proposed acoustic enclosure are conducted in a semianechoic chamber. Results demonstrate significant reductions in noise levels, including 7.99 dB(A), 5.69 dB(A), and 5.19 dB(A) reductions in the fundamental frequency, second harmonic, and third harmonic noise of the compressor’s operating frequency, respectively.

Study of a proportional spool valve noise by means of Functional Data Analysis: Cavitation and intensity detection

Cavitation inside hydraulic components is a well-known issue, that arises under specific operating conditions. When this phenomenon occurs in valves, it leads to several issues, such as materials damage, performance reduction, and noise. The latter of them is becoming increasingly important in the transport sector, which is moving towards new technologies (i.e., electrification) that are notoriously quiet. Therefore, cavitation detection is becoming a matter of interest for several applications. The purpose of the work presented is to develop a diagnostic technique to detect the onset of cavitation in proportional valves. A 2-way 2-position proportional spool valve has been placed inside an acoustic enclosure; noise signals have been recorded by means of a high frequency microphone, for different flow rates and valve openings. Sound pressure levels have been analysed by means of statistical techniques; a Functional Data Analysis (FDA) has been performed in the open-source R environment, in which the experimental data, collected under different non-cavitation conditions, have been firstly used to perform a Functional Principal Component Analysis (FPCA) and then to define a threshold. Then, a further experimental dataset has been evaluated through a Hotelling control chart, from which it is possible to distinguish cavitating conditions as they fall outside the limits.

Sound Reduction Control in Acoustic Enclosure with Air Ventilation

Sound Reduction Control in Acoustic Enclosure with Air Ventilation

An experimental study of noise reduction of portable generator using local made acoustic enclosure

Portable generators are now widely used in shops, offices, and homes to provide electrical power during power outages in Iraq. However, these generators produce a lot of noise emitted from rotating mechanical parts and fuel combustion inside engine chamber. This noise has a negative impact on the neuroendocrine, cardiovascular, respiratory, and digestive systems. The reduction of generator noise still attracts the interest of many researchers. In this work, the reduction of generator noise under the use of an acoustic enclosure, made from local materials, is investigated experimentally. Experimental work is conducted using a 2kW generator and compares the sound intensity for two cases: a generator without enclosure and one with enclosure. The enclosure is made from plywood, galvanized sheet, glass wool, cork, and a compressed sponge. The results include the measurement of sound intensity at day and night, at zero to full load, and at two different distances. The findings depicted that generator noise reduction reaches 10.3 dB at night and 9.6 dB during the day. In addition, the engine temperature, in case of using the enclosure, is kept within the allowable range of air-cooled generators which is 300 °C. The maximum temperature at 100% load measured inside the enclosure was 77.86 °C during the night time and 91.75 °C at day time.

Acoustic scattering in lined panel cavities with membrane interfaces.

Noise control strategies are extremely important in industry. Acoustic liners and elastic membranes, which absorb or attenuate sound waves, are key components in ducting systems for active and passive noise reduction. In this article, we design and examine a reactive liner panel cavity with flexible interfaces. The lined panel cavity system is comprised of elastic membranes at the interfaces. The aim is to couple the incoming duct modes with the flexible components and then with the localized modes in the lined region. The governing equations for the lined panel cavity system are solved using a mode-matching technique that assures continuity of the normal velocities at the interfaces and can handle a range of strong coupling and higher-order edge conditions in contrast to the coupled mode theory. The aim is to investigate the effects of reactive liners and elastic membrane interfaces on wave scattering through the proposed acoustic enclosure design. The scattering performance of the proposed lined panel cavity is evaluated in terms of the reflected and transmitted energy flux as well as transmission loss. Analysis shows that the resonances at the membrane interfaces coupled to the linear cavities have a direct impact on power variations and the maximum transmission loss.

Vibroacoustic analysis of multi-layered micro-perforated plates coupled to an acoustic enclosure

A vibroacoustic study of multi-layered micro-perforated plates (MPP) coupled to an enclosure cavity excited by internal and external acoustic excitations is presented. Most previous research centered on the multi-layered MPPs structure itself, the absorption and transmission performances were often calculated or tested in a simple acoustic environment. In a more realistic setting, the surrounding systems may affect the efficiency of MPPs. The proper modeling of the MPPs within acoustic enclosures requires careful study. The break-out sound from an enclosure via multi-layered MPP and the energy transmission of the structure into an acoustic enclosure undergoing exterior excitation are studied with an emphasis on the use of MPP in acoustic enclosures to suppress specific resonances of the enclosure. Therefore, an analytical formulation is proposed to model the behavior of the coupled system to address this issue. Using a modified Fourier series for the acoustic pressure, the continuity of the normal velocities at the interfaces of the plate or the MPP and the acoustic medium is assured accurately. A modified variational form for the acoustic and structure medium yields a completely coupled vibroacoustic system. Comparison with the finite element method (FEM) and available literature findings prove the efficacy and precision of the current approach. The effect of important parameters on the vibroacoustic behavior of multi-layered MPPs structure connected to an acoustic enclosure is examined. These parameters include micro-perforation of the plate, hole diameter, air gap thickness, acoustic enclosure dimensions, and the insertion of absorbent material in the gap.

Energy spectral element method for longitudinal acoustic propagation in ducts

Sound field behavior in an acoustic enclosure is an important part of the design of transportation vehicle passenger cabin, concert halls, conference rooms, and etc. Different analysis methods are available and have strengths and weaknesses. Low frequency band envelopes and negligible absorption in the walls can be modeled by Modal Analysis or Finite Element Method. However, as the frequency band increases, both methods become computationally intensive and Statistical Energy Analysis or the Sabine model can be an efficient approach. However, these methods do not take into account any spatial variation within the enclosure. The Energy Flow Analysis (EFA) solution for acoustic enclosures can be done analytically. In this paper, the Energy Spectral Element Method (ESEM) is formulated and applied to predict the spatial distribution of energy flux and density of acoustic ducts at high frequencies. ESEM is a matrix methodology based on EFA to solve acoustic and structural vibration problems. In this work, numerical models involving simple and coupled one-dimensional acoustic ducts are generated by ESEM, and the results are compared with energy densities calculated from the pressure fields predicted by the Spectral Element Method (SEM).

An audio-tactile interface based on dielectric elastomer actuators

This paper presents a concept of a dielectric elastomer actuator (DEA) user interface (smart button) that can sense a user’s touch and provide multi-sensory tactile and acoustic feedbacks through a single electrical input signal. The DEA relies on a multi-layer layout, in which a layer detects user-driven deformations (touches) via custom-built capacitance sensing electronics, and the remaining layers are used to provide actuation (audio-tactile feedbacks). Building upon a recently presented principle, combined tactile and acoustic feedbacks are produced by concurrently exciting different vibration modes of the same active membrane over different frequency ranges. An integrated demonstrator setup is presented, which includes a DEA, an acoustic enclosure, compact sensing and driving electronics. A characterization of the prototype is conducted, including an analysis of the sound pressure level, the force/stroke output at lower working frequencies, the ability to sense deformations with different profiles and produce combined audio-tactile outputs. Compared to previous works on multi-function DEAs, the system presented in this paper provides largely improved sensing performance (with lower working voltage) and features a deeper level of integration (with small-scale custom sensing electronics, and logics embedded onto scalable microcontrollers) and is thus specifically optimised for user-interaction applications. On this end, tests with users are presented here for the first time, which allowed evaluating the subjective perception of the interface’s feedbacks. By means of further optimisation and miniaturisation of the power/sensing electronics and structural components, the layout and multifunction DEA principle presented here might lead, in the future, to the development of DEA-based smart buttons for active surfaces, or portable/wearable user interfaces and communicators.

Assessment of the effectiveness of ventilation for gas turbine generator set safety

Increasing global warming problems appeal to the employment of clean power generation machineries such as gas turbines. Gas turbine is gas-fired engine which is usually enclosed in an acoustic enclosure for higher reliability and lower operating noise. However, it may face the risk of fuel gas accumulation inside the package which can be extremely dangerous. For the safety of a gas turbine generator set, proper ventilation system is crucial to prevent it from danger by diluting the potential leaked gas. Inspired by the significance of ventilation, this paper aims to present and propose study for the assessment of the effectiveness of ventilation for gas turbine packages in worst-case gas leak. A 2 MW ZK2000 gas turbine was chosen for the study. Method of assessment was introduced and numerical studies by Computational Fluid Dynamics (CFD) were adopted, including both transient and steady state analysis. Aiming to figure out the conditions for worst-case gas leak, the location of leak that may cause the most dangerous gas accumulation was obtained by identifying the poorly ventilated regions within the enclosure. After that, the leak rate was estimated that just to trigger the gas detector at the ventilation outlet according to ISO 21789. Worst-case gas leak simulations were performed in cold engine situation because of the possibility analysis of gas leak. The results at standard conditions show great compliance with the limited value in the code thus indicates that the capability of the current ventilation design. Effect of dilution was studied with various ambient temperatures and ventilation flow rates. One of the key findings is that the flow uniformity has the major impact on enhancing dilution of gas cloud. Structures improvements within enclosure and operation instructions were suggested for the safety of gas turbine unit in the event of gas leak.

Experimental tests and prediction of insertion loss for cubical sound insulating enclosures with single homogeneous walls

Limiting the excessive noise of machines and devices can be performed with the use of screens in the form of baffles or acoustic enclosures. Knowledge of their acoustic properties is essential in the proper construction of baffles. The basic properties include the airborne sound insulation of the baffle, and the insertion loss in the case of the arrangement of several baffles constituting the acoustic enclosure. Both parameters can be determined through experimental tests, including laboratory ones and using theoretical computational models.The article presents the results of both experimental and model tests of the sound insulating effectiveness of enclosures. The proposed calculation model for the insertion loss of sound insulating enclosures, intended for mid- and high-frequency ranges, was determined on the basis of the sound transmission loss of a homogeneous baffle, which was calculated from its material data using a theoretical model developed within previous studies, based on the Davy and Sharp models. For the low-frequency range, the use of a modified Oldham model for close-fitting enclosures was proposed. The new calculation model was validated for six enclosure variants consisting of five identical square plates, made of materials such as steel, aluminium, plexiglass, polypropylene and gypsum. For this purpose, the developed prototype stand for testing acoustic enclosures was used, with the possibility of mounting walls of different thickness. A good correlation between the results of the insertion loss spectral characteristic calculations and the results from experimental tests was obtained using the proposed model, in the entire considered frequency range of 100–5000 Hz.

Photoacoustic optical absorption measurement of aerosols

Atmospheric optical absorption, including the contribution from aerosols, is important in modeling propagation of laser beams and for climate research. Photoacoustic methods have been used in the past for this purpose, as they directly measure absorption, rather than the sum of absorption and scattering. Acoustic operating frequency determines the sensed aerosol size range. The design of a windowless double-Helmholtz photoacoustic aerosol sensing system, operating at an acoustic frequency near 150 Hz, with the capability to measure an optical absorption with a 1/e length of 1 Mm, will be presented. In this design, environmental noise limits the instrument sensitivity. To improve immunity to environmental noise with an open resonator, an aluminum acoustic enclosure was built around the system, with welded vacuum flanges for inputs and outputs. Inlet and outlet acoustic mufflers were designed to limit the in-band noise entering the measurement system. A modulated 10 W 1064 nm fiber laser was used as the excitation source, and a high-sensitivity microphone placed within the open-cell cavity was used to measure the internal photoacoustic pressure fluctuations. The system was modeled using various computational tools, including MATLAB, Simulink, and ANSYS, and then was constructed and tested. Lab measurement results will be presented.

Prediction of sound transmission loss of conical acoustic enclosure using statistical energy analysis and its experimental validation

The conical shape structures are widely employed in many practical applications such as aircraft, rockets, tanks, and submarines. Prediction of sound transmission loss is often required at the design and evaluation stages of such structures. In this paper, an analytical model is proposed using the statistical energy analysis (SEA) technique to predict the sound transmission loss of a conical shape acoustic enclosure in a broad frequency range. The proposed model is verified experimentally using the sound intensity experimental technique. It was found that the analytical predictions are in good agreement with the measured transmission loss, specifically at the ring and critical frequencies. The percentage error between predicted and measured lower ring frequency is 5.1% and that for the upper ring frequency is 0.4%. The percentage error between predicted and measured critical frequency is 2.7%. The results obtained indicate that the developed analytical model can be used as an efficient design tool to predict the acoustic performance of conical shape structures.

Punch press noise control through partial acoustic enclosure: A Field Study

Punch press noise control through partial acoustic enclosure: A Field Study

Finite Volume Method Modeling of Heat Transfer in Acoustic Enclosure for Machinery.

This paper deals with the problem of heat accumulation in acoustic enclosures. Increased noise levels at production sites or manufacturing lines force the application of acoustic enclosures. Effective noise reduction due to enclosures often comes with the additional thermal insulation of the device, which in many cases causes a strong increase in the device operation temperature. This paper presents the methodology of thermal phenomena numerical modeling based on the potential influence of acoustic enclosures on the increase in device operation temperature. The proposed model consists of an original acoustic enclosure concept design, and the numerical modeling is based on the computational fluid dynamics FVM (finite volume method) conducted in Ansys Fluent. The research comprised a set of simulations at different air flow rates of 52.5 m3/h, 105 m3/h, 210 m3/h and 420 m3/h at the enclosure inlet. The analysis carried out on the basis of flow paths and temperature distribution plots inside the enclosure led to the conclusion that the expected, analytically calculated minimum volumetric flow rate is not sufficient to effectively cool the investigated device to the required temperature of 26 °C, and higher air flow rates should be applied. Simulation results indicated that the numerical tools can be useful in the prediction of the heat exchange process, as well as in the selection of an appropriate source and location of cooling.

Prediction of sound transmission loss of cylindrical acoustic enclosure using statistical energy analysis and its experimental validation.

In the present work, the sound transmission through a cylindrical shape acoustic enclosure is predicted analytically and verified experimentally. An analytical model is developed, based on the statistical energy analysis (SEA) approach, to examine the transmission loss of a cylindrical acoustic enclosure in different frequency regions, including the low-, intermediate-, and high-frequency ranges. In the developed model, the nonresonant wave response is included in addition to consideration of the resonant response for obtaining more accurate results. It is demonstrated that the developed SEA formulation in this work can compute the resonant as well as the nonresonant sound transmission of the cylindrical acoustic enclosure separately. To validate the analytical model, an experimental setup was developed, and the sound transmission loss of a cylindrical acoustic enclosure was measured using the sound intensity experimental technique. It was found that the analytical results are in good agreement with the measured transmission loss, especially at the panel ring and critical frequencies. The results obtained indicate that the proposed analytical model is efficient to predict the sound transmission loss of cylindrical acoustic enclosures.

INVESTIGATION OF SIGNAL THRESHOLDING EFFECTS ON THE ACCURACY OF SOUND SOURCE LOCALIZATION

In this article theoretical analysis of the signal thresholding effects on the accuracy of cross-correlation based sound source direction of arrival estimation was presented. The aim of the investigation was to determine the theoretical limits and challenges of the accuracy of the localization of a speaker within an acoustic enclosure by cross-correlation of two microphone signals and to offer means to increase the accuracy of sound source direction of arrival estimation via selection of audio frames based on the time lag estimation realiability measure. For the investigation, audio material from an openly accessible database was used. Presented are the methods for obtaining various features of the microphone signal frames, signal amplitude to minimum error amplitude calculation and experimentation with threshold-based audio frame selection.

Influence of the inclination angle on the sound field in an irregular enclosure containing two elastic walls

The sound field model of an irregular enclosure with two elastic walls and an inclined wall is established by the modal theory. The modal parameters of the irregular enclosure are obtained by the envelope rectangular technique. The influence of the inclination wall angle on the sound field in the irregular enclosure is discussed. When the inclination angle is increased from 0° to 45°, the resonance frequencies of the acoustic enclosure are basically reduced in the frequency range of 0∼250 Hz. When the inclination angle is increased from 0° to 45° every 15° interval, the amplitude of a certain acoustic mode itself decreases, while the amplitudes of the acoustic modes coupled to it basically increase. The acoustic potential energy peaks in the enclosure basically shift to the low frequency with the increase of the inclination angle. Furthermore, the change trend of the acoustic potential energy is regular in the low-order modes, but relatively chaotic in the high-order modes. In addition, in order to verify that the accurate description of the primary sound field is the basis of effective noise control, the two-elastic plate model is deliberately treated as one-elastic plate model, and its influence on the performance of active noise control is discussed. Also, if the multi-elastic plate model is regarded as a simplified automobile cab, the research results can be used for the preliminary acoustic design of the cab; that is, the research results can be used for the acoustic design of similar models.

The effect of audible sound frequency on the growth and beta-carotene production of Dunaliella salina

Sound vibrations as a cutting-edge technology considered for promoting the growth of microalgae. In this experiment, the authors have carried out a pilot study to observe the response of Dunaliella salina to audible sound frequencies (100, 200, 500, 1000 Hz). The growth and beta-carotene concentration as a secondary metabolite in optimal culture medium and sub-optimal medium with nitrate deficiency were investigated. The control was in an acoustic enclosure without any sound frequency exposure and cultured in the N-sufficient medium. The sound frequency of 200 Hz was found to promote biomass concentration and cell division up to 50% compared to control. Interestingly, the frequency of 200 Hz promoted growth when the nitrate concentration decreased by 75% in the culture medium. The simultaneous effect of audible sound frequency of 200 Hz and nitrate deficiency showed the amount of beta-carotene increased to 37% compared to control. In the absence of sound frequency, an increase of 18% in beta-carotene was observed. To the best of our knowledge, the frequency of 200 Hz is a good demonstration of the ability of sound stimuli to improve the growth as well as a secondary metabolite of D.salina.