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  • 1. Auregan, Yves
    et al.
    Farooqui, Maaz
    Groby, Jean-Phillipe
    Low frequency sound attenuation in a flow duct using a thin slow sound material2016In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 139, no 5, p. EL149-EL153Article in journal (Refereed)
    Abstract [en]

    A thin subwavelength material that can be flush mounted in a duct and that gives an attenuation band at low frequencies in air flow channels is presented. To decrease the material thickness, the sound is slowed in the material using folded side branch tubes. The impedance of the material is compared to the optimal value given by the Cremer condition, which can differ greatly from the air characteristic impedance. Grazing flow on this material increases the losses at the interface between the flow and the material.

  • 2.
    Farooqui, Maaz
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Innovative noise control in ducts2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The objective of this doctoral thesis is to study three different innovative noise control techniques in ducts namely: acoustic metamaterials, porous absorbers and microperforates. There has been a lot of research done on all these three topics in the context of duct acoustics. This research will assess the potential of the acoustic metamaterial technique and compare to the use of conventional methods using microperforated plates and/or porous materials. 

    The objective of the metamaterials part is to develop a physical approach to model and synthesize bulk moduli and densities to feasibly control the wave propagation pattern, creating quiet zones in the targeted fluid domain. This is achieved using an array of locally resonant metallic patches. In addition to this, a novel thin slow sound material is also proposed in the acoustic metamaterial part of this thesis. This slow sound material is a quasi-labyrinthine structure flush mounted to a duct, comprising of coplanar quarter wavelength resonators that aims to slow the speed of sound at selective resonance frequencies. A good agreement between theoretical analysis and experimental measurements is demonstrated.

    The second technique is based on acoustic porous foam and it is about modeling and characterization of a novel porous metallic foam absorber inside ducts. This material proved to be a similar or better sound absorber compared to the conventional porous absorbers, but with robust and less degradable properties. Material characterization of this porous absorber from a simple transfer matrix measurement is proposed.The last part of this research is focused on impedance of perforates with grazing flow on both sides. Modeling of the double sided grazing flow impedance is done using a modified version of an inverse semi-analytical technique. A minimization scheme is used to find the liner impedance value in the complex plane to match the calculated sound field to the measured one at the microphone positions.

  • 3.
    Farooqui, Maaz
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Elnady, Tamer
    Akl, Wael
    Sound attenuation in ducts using locally resonant periodic aluminum patches2016In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 139, no 6, p. 3276-3286Article in journal (Refereed)
    Abstract [en]

    In recent years, the control of low frequency noise has received a lot of attention for several applications. Traditional passive noise control techniques using Helmholtz resonators have size limitations in the low frequency range because of the long wavelength. Promising noise reductions, with flush mounted aluminum patches with no size problems can be obtained using local resonance phenomenon implemented in acoustic metamaterial techniques. The objective of this work is to introduce locally resonant thin aluminum patches flush mounted to a duct walls aiming at creating frequency stop bands in a specific frequency range. Green's function is used within the framework of interface response theory to predict the amount of attenuation of the local resonant patches. The two-port theory and finite elements are also used to predict the acoustic performance of these patches. No flow measurements were conducted and show good agreement with the models. The effect of varying the damping and the masses of the patches are used to expand the stop bandwidth and the effect of both Bragg scattering and the locally resonant mechanisms was demonstrated using mathematical models. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated.

  • 4. Farooqui, Maaz
    et al.
    Elnady, Tamer
    Akl, Wael
    Validation of low frequency noise attenuation using locally resonant patches2016In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524Article in journal (Refereed)
    Abstract [en]

    Since conventional silencers in acoustic ducts have problems of size limitations at low frequencies and being prone to high backpressure, locally resonant aluminum patches are introduced in acoustic duct walls aiming at creating frequency stop bands in the low frequency region (below 1 KHz). With these flush mounted patches, promising noise reductions, with no such drawbacks, can be obtained, building on local resonance phenomenon implemented in acoustic metamaterials techniques. The objective of the current paper is to experimentally validate the performance of an array of flexible side-wall-mounted patches inside ducts. The experimental results are compared with Analytical Green's function method as well as Numerical Finite Element Method and a close agreement was found. The results show that the presence of the patches singly or periodically can play a prominent role in designing any acousticbandgap materials. The effect of the arrays of patches on the effective dynamic density and bulk modulushas also been investigated.

  • 5.
    Farooqui, Maaz
    et al.
    Ain Shams University, Egypt.
    Elnady, Tamer
    Glav, Ragnar
    Karlsson, Tony
    Modeling and characterization of a porous metallic foam inside ducts2015In: SAE International Journal of Materials & Manufacturing, ISSN 1946-3979, E-ISSN 1946-3987, Vol. 8, no 3, p. 937-945Article in journal (Refereed)
    Abstract [en]

    A novel porous metallic foam has been studied in this work. This composite material is a mixture of resin and hollow spheres. It is lightweight, highly resistive to contamination and heat, and is capable of providing similar or better sound absorption compared to the conventional porous absorbers, but with a robust and less degradable properties. Several configurations of the material have been tested inside an expansion chamber with spatially periodic area changes. Bragg scattering was observed in some configurations with certain lattice constants. The acoustic properties of this material have been characterized from the measurement of the two-port matrix across a cylindrical sample. The complex density and speed of sound can be extracted from the transfer matrix using an optimization technique. Several models were developed to validate the effect of this metallic foam using Finite Elements and the Two-port Theory. There was a good agreement between both models and the measurement results.

  • 6.
    Farooqui, Maaz
    et al.
    KTH.
    Elnady, Tamer
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Measurement of perforate impedance with grazing flow on both sides2016In: 22nd AIAA/CEAS Aeroacoustics Conference. American Institute of Aeronautics and Astronautics., American Institute of Aeronautics and Astronautics, 2016Conference paper (Refereed)
    Abstract [en]

    In this paper, the acoustic impedance of a microperforated plate (MPP) with two-sided grazing flow is educed by a modified semi-analytical inverse technique. The MPP sample is placed in the middle of a rectangular duct with grazing flow. The mode matching formulation is derived in terms of pressure for a MPP with two-sided grazing flow. The inputs to this mode matching technique are complex acoustic pressure measured at twelve positions at the wall of the duct, upstream and downstream of the MPP section. First the ability of the code to reproduce the pressure field for a given impedance is tested. Second the ability to educe the correct impedance for a given pressure distribution is tested. The results of the mode matching code are in good agreement with the simulations for no flow cases.

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