Maaz Farooqui

Low frequency sound attenuation in a flow duct using a thin slow sound material

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.

Measurement of perforate impedance with grazing flow on both sides

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 ...

Validation of low frequency noise attenuation using locally resonant patches

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 Greens 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 acoustic bandgap materials. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated.

Sound attenuation in ducts using locally resonant periodic aluminum patches.

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. Greens 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.

Modeling and Characterization of a Novel Porous Metallic Foam Inside Ducts

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 c ...

Investigation of the impedance of a micro-perforated plate with two-sided grazing flow

Acoustic impedance for perforates is mainly quantified by semi or totally empirical formulas, which depend on the experimental set-up used and are specific for each type of tested sample. In this work, the acoustic impedance of a micro-perforated plate (MPP) with two-sided low Mach grazing flow is educed by a modified semi-analytical inverse technique. The inputs to this technique are complex acoustic pressure measured at eight positions at the wall of the duct, upstream and downstream of the MPP section. Several measurements on the two sided grazing flow rig are conducted and trends in the impedance shift are observed. These trends serve to quantify the change in the resistance and reactance of a MPP when it is subjected to two sided grazing flow. An empirical formula is also proposed which is based on these trends. This formula helps in approximating the behavior of MPP’s in applications such as perforated cooling fans, perforated guide vanes, and turbine nozzles. It was observed that with increased flo...

Sound attenuation in ducts using locally resonant periodic flush mounted flexible silicon aerogel patches

In recent years, low frequency noise has become an important factor especially in the Aircraft, HVAC, and Automotive industries. In order to reduce this low frequency noise, noise attenuation by the classical Helmholtz resonators has size limitations due to the large wavelengths. Promising noise reductions, with flush mounted Silicon Aerogel patches, can be obtained implementing attenuation due to local resonance and that too without any size constraints. The objective of the current paper is to introduce locally resonant Silicon Aerogel patches flush mounted to an acoustic duct walls aiming at creating frequency stop bands at the low frequency zone (below 500 Hz). Green’s Function is used under the framework of interface response theory to predict the degree of attenuation of the local resonant patches. Realistic techniques for expanding the stop bandwidth have been introduced and difference between the Bragg scattering and the locally resonant mechanism was demonstrated using mathematical models. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated. It is also shown that the numbers and periodicity of these local resonators also plays role in determining the depth and width of the acoustic band gap.

Noise reduction in pipelines using Helmholtz resonators

Compressors are at the heart of most petrochemical and industrial power plants. They are usually the source of noise generation transmitted to pipelines. This noise is undesirable for people living close to installation and can also potentially cause structural failures in the piping. Particular attention is given to the modified solution using the Helmholtz resonator concept to reduce noise. An assessment of the noise reduction results using various types of resonators is carried out. A comparison between analytical, numerical simulation using COMSOL and experimental test is presented and discussed using a known patch-test.