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05.01 Aircraft Interior Noise

Prediction and Improvement of Aircraft Cabin Acoustics using Statistical Energy Analysis and Sound Quality Evaluation
Time: 12:00 pm

Author: Nurkan Turkdogru Gurun

Abstract ID: 1382

Aircraft interior acoustic design is a key influencer for cabin comfort. An essential part of the design is optimization of acoustic insulation systems under weight restrictions to create a pleasant environment for human ear. Considering the complexity of aircraft geometry, noise sources, and transfer paths, computational prediction techniques become invaluable tools for increasing the accuracy in material selection while reducing design time and costs. In this study, a procedure that integrates sound quality evaluation with Statistical Energy Analysis (SEA) to design aircraft acoustic insulation systems is described. SEA is employed to predict the cabin sound pressure levels of a narrow body aircraft insulated with sound absorption and vibration damping materials. Aircraft cabin including under-floor sections is modelled based on 3D airframe and VIP style interior design and the model is validated with flight test data. Transfer functions obtained from SEA model for selected transfer paths are utilized to filter the noise signal recorded with a binaural recording system during flight. Sound quality metrics are computed in order to map perceptive response. An iterative process is introduced to improve acoustic design by investigating the effects of different sound insulation systems and room absorption values on noise levels and sound quality metrics.

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Aircraft interior acoustics – background noise contamination
Time: 12:20 pm

Author: Ramana Kappagantu

Abstract ID: 1882

Aircraft interior noise is an important factor to be considered for cabin comfort.  In a cruising condition this noise source is mostly broadband in nature and is coming from the exterior, primarily the turbulent boundary layer (TBL) of the flow around the moving aircraft.  Capturing this noise to a high frequency is critical for designing the sound packaging. Also, this becomes important in the design of public announcement (PA) system for the aircraft cabin, i.e. the correct placement of speakers.   One of the metrics used for this acoustic design is speech transmission index.  Deterministic techniques like finite or boundary element techniques for low frequencies and ray tracing method to reach higher frequencies are better suited for getting the narrow band responses.  On the other hand, to characterize the background noise due to the TBL loads, statistical energy analysis (SEA) route is pursued.  In this paper the authors combine different techniques to capture the background noise and use them with PA sources and eventually capture the sound perceived at points of interest.  The articulation metrics are compared for different operating conditions of the aircraft.   In the presentation attempts will be made to play the auralized sounds.

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Multilayer treatment for subwavelength and broad absorption
Time: 5:00 pm

Author: Josué Costa Baptista

Abstract ID: 2076

Single layer optimized microchannels (268µm channels size) present high absorption at the quarter-wave resonance frequency (2460Hz for 30mm-thick treatment) but cannot provide significant absorption at lower frequencies. In this work, the absorption coefficient of multilayer treatments with 2, 5, 10- and 30-layers of channels with size varying from 50µm to 15mm was numerically optimized. The equivalent fluid wave number and characteristic impedance of each layer were predicted using the JCAL model. The Double-scale Asymptotic Method (DAM) was used to obtain the JCAL parameters. The multilayer treatment absorption was modelled with the Transfer Matrix Method (TMM). It was shown that multilayer treatments present superior absorption than single layer. For instance, bilayer treatment made of a 1mm-thick top layer (facing incident wave) of channels of 58µm and a 29mm-thick bottom layer of channels with 8.1mm provides perfect absorption around 1200Hz (i.e. 1260Hz below the quarter-wave resonance frequency of 30mm-thick single layer treatment). Alternatively, a 30-layer treatment with channels size varying from 100µm to 9.6mm provides absorption higher than 0.8 between 1350 and 6270Hz (i.e. 54% higher than single layer treatment with same thickness). These results pave the way to the fabrication of new multilayer treatments with interesting subwavelength and broadband absorption capabilities.

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MEMS microphone intensity array for cabin noise measurements
Time: 8:20 am

Author: Carsten Spehr

Abstract ID: 2288

Aircraft cabin noise measurements in flight are used toto quantify the noise level, and to identify the entry point of acoustic energy into the cabin. Sound intensity probes are the state-of-the-art measurement technique for this task. During measurements, additional sound absorbing material is used to ease the rather harsh acoustic measurement environment inside the cabin. In order to decrease the expensive in-flight measurement time, an intensity array approach was chosen. This intensity probe consists of 512 MEMS-Microphones. Depending on the frequency, these microphones can be combined as an array of hundreds of 3D- intensity probes. The acoustic velocity is estimated using a high order 3D finite difference stencil.   At  low frequencies, a larger spacing is used to reduce the requirement of accurate phase match of the microphone sensors. Measurements were conducted in the ground-based Dornier 728 cabin noise simulation as well as in-flight.

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Influence of mobility completeness and source behavior on the robust-ness of Transfer Path Analysis and Source Characterization methods: A numerical study.
Time: 11:40 am

Author: Simon Prenant

Abstract ID: 2334

Structure borne noise is considered a major contribution to the noise generated inside aircrafts. In order to analyze it, engineering methods have been developed such as Transfer Path Analysis (TPA) and Source Characterisation (SC). These methods are based on active and passive properties of the source and the receiving structure being coupled or decoupled. The theoretical formulation requires mobility according to all Degrees Of Freedom (DOFs) and rotational DOFs represent a challenge for experimental application. To fulfill the mobility matrix, indirect method have been developed and specific sensors have been proposed, resulting in a more complex experimental set-up and an increase in measurement uncertainties. The necessity of assessing the full matrix completeness is thus still questionable. The robustness of these methods with respect to the matrix completeness and the source behavior is investigated numerically in this work. A numerical model has been developed to simulate vibrating sources with simple or complex vibratory behavior and to assess the mobility matrices for any completenesses. Velocity on the receiving structure is used as a target indicator. The influence of source behavior and completeness are discussed and the results show that the required mobility completeness depends on the source behavior.

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Vibro-acoustic modeling of aircraft structures using Finite Element- informed Statistical Energy Analysis
Time: 12:40 pm

Author: abderrazak mejdi

Abstract ID: 2417

This paper addresses the problem of predicting the structure born and airborne sound transmission in aircraft using Statistical Energy Analysis (SEA). Often analytical formulations are used to approximate the SEA parameters.  In the present prediction method, a finite element (FE)-informed SEA approach is employed. To compute the coupling coefficient, the structure is represented with a repetition of unit cell and an FE model of the unit cell is assigned to evaluate the direct field dynamic stiffness matrix of the SEA subsystems at the connections. An efficient strategy is employed to determine the equivalent material properties of the FE model. Thus, a two-dimensional unit cells of different constructions such as composite, sandwich, visco-elastic laminate and ribbed section sections can be used. To evaluate the equivalent properties of multilayers structures, each layer is assumed as thick laminate with orthotropic orientation. Moreover, rotational inertia and transversal shearing, membrane and bending deformations are accounted for. First order shear deformation theory is employed. The developed approach handles symmetrical layouts of unlimited number of transversal compressible or incompressible layers. The accuracy of this modeling approach is confirmed through comparison to alternate validated theoretical approaches. Representative examples of spacecraft structural response and interior noise predictions for typical load cases are shown and the use of SEA models as a tool for guiding construction of complex structures to meet acoustic performance targets and optimize designs are presented.  Conclusions about the application and advantages of this approach is presented.

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Design, development, testing and application of digital MEMS pressure sensor array for full-scale vibroacoustic measurements
Time: 11:00 am

Author: Pankaj Joshi

Abstract ID: 2671

This manuscript addresses design, development, and application of micro-electro-mechanical systems (MEMS) based digital pressure sensor array for vibroacoustic measurements. These vibroacoustic measurements were conducted on a A320 type single aisle aircraft demonstrator subjected to broadband as well as tonal excitations. Cabin noise levels were measured with both condenser microphones as well as digital MEMS pressure sensor array. The measured cabin noise shows strong qualitative as well as quantitative agreement between both type of measurement devises for full scale cabin noise measurements inside an aircraft demonstrator. The observed strong agreement is valid for both single wall (fuselage with thermal insulation) and double wall (fuselage with thermal insulation and trim panel) cabin noise measurements. Such strong agreement within 1.0 dB tolerance is significantly motivating for further development of reliable but low-cost MEMS based measurement devises and corresponding efficient data post-processing algorithms for full scale vibroacoustic measurements in general. Additionally, it is also demonstrated that the large number of MEMS based digital pressure sensors can be used in areas where the physical space constraints are high. This demonstration shows strong potential to derive additional vibroacoustic indicator for the development and the testing of future noise control solutions in a non-traditional way.

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SEA model for structural acoustic coupling by means of periodic finite element models of the structural subsystems
Time: 3:00 pm

Author: Luca ALIMONTI

Abstract ID: 3044

Statistical Energy Analysis (SEA) often relies on simplified analytical models to compute the parameters required to build the power balance equations of a coupled vibro-acoustic system. However, the vibro-acoustic of modern structural components, such as thick sandwich composites, ribbed panels, isogrids and metamaterials, is often too complex to be amenable to analytical developments without introducing further approximations. To overcome this limitation, a more general numerical approach is considered. It was shown in previous publications that, under the assumption that the structure is made of repetitions of a representative unit cell, a detailed Finite Element (FE) model of the unit cell can be used within a general and accurate numerical SEA framework. In this work, such framework is extended to account for structural-acoustic coupling. Resonant as well as non-resonant acoustic and structural paths are formulated. The effect of any acoustic treatment applied to coupling areas is considered by means of a Generalized Transfer Matrix (TM) approach. Moreover, the formulation employs a definition of pressure loads based on the wavenumber-frequency spectrum, hence allowing for general sources to be fully represented without simplifications. Validations cases are presented to show the effectiveness and generality of the approach.

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Noise measurements in the AS 350 helicopter under the specific operating conditions of SAR mission
Time: 11:20 am

Author: Felipe Gelain

Abstract ID: 3228

The AS 350 helicopter is a small and versatile helicopter used in many countries by private and public operators. In some countries the model is also often used for SAR missions. In the Brazilian public rescue services, AS 350 helicopters account for around 80 percent of the whole fleet. SAR missions often require operating conditions different from those for usual passenger transport. This includes flying and hovering with open doors or at low altitudes and low speeds. Noise exposure during SAR missions is of concern not only for the crew but also for the passengers, i.e. injured patients or newborn. While very important, data on acoustic conditions in SAR aircraft are scarce and do not exist for the AS 350 yet. Therefore, to investigate the noise in the AS 350 during SAR specific operating conditions different aircraft of the same model were equipped with recording equipment (microphone and SQuadriga II stand-alone front end ) and flown in the specific operating conditions of SAR missions. Recordings were analyzed using not only 1/n-octave and or FFT analysis, but also variable frequency resolution Fourier analysis for better low frequency analysis and psychoacoustic models available in ArtemiS Suite v12. Results show how noise characteristics vary depending on the operation of the aircraft, and which operating conditions are the most critical and should be avoided. The data can be used by SAR operators to build noise optimized mission profiles for critical missions, such as transportation of newborn and maintaining operational safety.

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