Development and testing of cabin sidewall acoustic resonators for the reduction of cabin tone levels in propfan-powered aircraft

The use of Helmholtz resonators to increase the sidewall transmission loss (TL) in aircraft cabin sidewalls is evaluated. Development, construction, and test of an aircraft cabin acoustic enclosure, built in support of the Propfan Test Assessment (PTA) program, is described. Laboratory and flight test results are discussed. Resonators (448) were located between the enclosure trim panels and the fuselage shell. In addition, 152 resonators were placed between the enclosure and aircraft floors. The 600 resonators were each tuned to a propfan fundamental blade passage frequency (235 Hz). After flight testing on the PTA aircraft, noise reduction (NR) tests were performed with the enclosure in the Kelly Johnson Research and Development Center Acoustics Laboratory. Broadband and tonal excitations were used in the laboratory. Tonal excitation simulated the propfan flight test excitation. The resonators increase the NR of the cabin walls around the resonance frequency of the resonator array. Increases in NR of up to 11 dB were measured. The effects of flanking, sidewall absorption, cabin absorption, resonator loading of trim panels, and panel vibrations are presented. Resonator and sidewall panel design and test are discussed

VIBROACOUSTIC BEHAVIOR AND NOISE CONTROL STUDIES OFADVANCED COMPOSITE STRUCTURES

The research presented in this thesis is devoted to the problems of sound transmission and noise transmission control for advanced composite payload fairings. There are two advanced composite fairings under study. The first is a tapered, cylindrical advanced grid-stiffened composite fairing, and the second is a cylindrical ChamberCore composite fairing. A fully coupled mathematical model for characterizing noise transmission into a finite elastic cylindrical structure with application to the ChamberCore fairing is developed. It combines advantages of wave radiation principles and structural-acoustic modal interaction, and provides an ideal noise transmission model that can be extended to other finite cylindrical structures. Structural-acoustic dynamic parameters of the two fairings are obtained using a combination of numerical, analytical, and experimental approaches. An in-situ method for experimentally characterizing sound transmission into the fairings called noise reduction spectrum (NRS) is developed based on noise reduction. The regions of interest in the NRS curves are identified and verified during a passive control investigation, where various fill materials are added into wall-chambers of the ChamberCore fairing. Both Helmholtz resonators (HRs) and long T-shaped acoustic resonators (ARs) are also used to successfully control noise transmission into the ChamberCore fairing. In the process, an accurate model for the resonant frequency calculation and design of cylindrical HRs is derived. Further, a novel and more general model for the design of multi-modal, long, T-shaped ARs is developed, including three new end-correction equations that are validated experimentally. The control results show that noise attenuation is significant in the controlled modes, and the control is also observed in some modes that are not targeted, due to acoustic modal coupling via the structure. Helmholtz resonators are found to produce between 2.0 and 7.7 dB increase in NRS in the targeted cavity modes while ARs produced 4.7 to 5.3 dB of control. Relative positioning between the matched resonators is heuristically optimized experimentally, and demonstrates that spacing should be maximized for best performance. Once the feasibility and optimization of resonator control were established, six integral acoustic resonators are fabricated directly into the wall-chambers of the ChamberCore structure. Performance is found to be as well as for the non-integrated resonators

Sonic Crystal Noise Barriers

An alternative road traffic noise barrier using an array of periodically arranged vertical cylinders known as a Sonic Crystal (SC) is investigated. As a result of multiple (Bragg) scattering, SCs exhibit a selective sound attenuation in frequency bands called band gaps or stop bands related to the spacing and size of the cylinders. Theoretical studies using Plane Wave Expansion (PWE), Multiple Scattering Theory (MST) and Finite Element Method (FEM) have enabled study of the performance of SC barriers. Strategies for improving the band gaps by employing the intrinsic acoustic properties of the scatterer are considered. The use of the tube cavity (Helmholtz type) resonances in Split Ring Resonator (SRR) or the breathing mode resonances observed in thin elastic shells is shown to increase Insertion loss (IL) in the low-frequency range below the first Bragg stop band. Subsequently, a novel design of composite scatterer uses these 2 types of cylindrical scatterer in a concentric configuration with multiple symmetrical slits on the outer rigid shell. An array of composite scatterers forms a system of coupled resonators and gives rise to multiple low-frequency resonances. Measurements have been made in an anechoic chamber and also on a full-scale prototypes outdoors under various meteorological conditions. The experimental results are found to confirm the existence of the Bragg band gaps for SC barriers and the predicted significant improvements when locally resonant scatterers are used. The resonant arrays are found to give rise to relatively angle-independent stop bands in a useful range of frequencies. Good agreement between computational modelling and experimental work is obtained. Studies have been made also of the acoustical performances of regular arrays of cylindrical elements, with their axes aligned and parallel to a ground plane including predictions and laboratory experiment

Development and Application of Novel Sound Absorbers

The sound absorbers in most common use today are porous materials like fibers and foams. This work examines three alternatives to porous absorbers: microperforated panels, acoustic fabrics, and additively manufactured absorbers. The research is a combination of design, measurement and characterization of their properties, and analysis. Microperforated panels are thin metallic or plastic panels with sub-millimeter size holes or perforations. Sound absorption can be tuned by adjusting the spacing between the panel and a cavity behind the panel. Several different configurations were considered where the geometry behind the panel was divided up into channels of varying length and cross-sectional area. Results showed that the sound absorption effectiveness could be improved at low frequencies and that the absorber was more effective over a broader range of frequencies. This was demonstrated using both impedance tube measurements and diffuse field sound absorption measurements in a small reverberant room. Sound absorptive fabrics are similar to microperforated panel absorbers and function using the same principle. Acoustic resistance is high through the fabric due to small holes or the tight weave. If the particle velocity is high in the fabric, the fabric will effectively attenuate sound. The sound absorption is easily tuned by adjusting the distance between the fabric and a hard backing. It is demonstrated that the transfer impedance can be simulated using theory similar to that typically used for characterizing microperforated panel absorbers. Recently, there has been a great interest in using additive manufacturing to develop sound absorbers. The design space was partially explored by designing absorbers using long perforations, lightweight panels, and Helmholtz resonators with long necks. The sound absorbers are shown to be very effective at low frequencies where conventional sound absorbers like fibers and foams are ineffective

A measurement based study of the acoustics of pipe systems with flow

The focus of this thesis is the measurement of specific aeroacoustic properties in ducts at frequencies below the cut-on frequency of the first higher order mode. A body of measurement results are presented which highlight the effect of flow on some of the aeroacoustic characteristics in ducts as well as describe the aeroacoustic sources of an in-duct orifice and a simple expansion chamber. The results have been compared with published theory where appropriate. Important developments from measurements of the acoustic characteristics of a simple duct with flow include a new experimental method to determine the viscothermal attenuation coefficient. In addition, pressure reflection coefficient measurements of an unflanged duct with flow with two different edge conditions are used in conjunction with a numerical model developed by Gabard [1] to determine the extent of vorticity shed from the duct termination. A novel method is presented for the measurement of aeroacoustic source strengths in ducts with flow. The source is defined in terms of acoustic power and is determined by measuring the acoustic power flux both upstream and downstream of the source region in a duct. The method adopts a plane wave approximation and was assessed experimentally by creating a source in a duct at a number of known frequencies and modifying its magnitude by a known amount. The source measurement technique is applied to an in-duct orifice. The results are used to determine the spectral characteristic and velocity dependence of the source. The results indicate that the duct-to-orifice area ratio has a important effect on the spectral characteristics and velocity dependence of the source. New measurements of the aeroacoustic source strength of a simple flow excited expansion chamber are presented. The results indicate that lock-on flow tones occur when hydrodynamic modes which form in the chamber match the tailpipe resonant frequencies. The results are compared with predictions of a model based on describing function theor

Terahertz Photoacoustic Spectroscopy Using an MEMS Cantilever Sensor

In this paper, a microelectromechanical systems cantilever sensor was designed, modeled, and fabricated to measure the photoacoustic (PA) response of gases under very low vacuum conditions. The micromachined devices were fabricated using silicon-on-insulator wafers and then tested in a custom-built, miniature, vacuum chamber during this first-ever demonstration. Terahertz radiation was amplitude modulated to excite the gas under test and perform PA molecular spectroscopy. Experimental data show a predominantly linear response that directly correlates measured cantilever deflection to PA signals. Excellent low pressure (i.e., 2-40 mTorr) methyl cyanide PA spectral data were collected resulting in a system sensitivity of 1.97 × 10 -5 cm -1 and a normalized noise equivalent absorption coefficient of 1.39 × 10 -9 cm -1 W Hz -1/2

Towards a quieter world : three-dimensional printed acoustic metamaterials for noise control

Environmental noise impacts the everyday life of millions of people and it represents a growing concern for the health of the world's population. To mitigate this impact, noise reducing materials such as foam or barriers are employed extensively with effective results. However, the efficacy of such materials is limited by the inverse relationship between the frequency of the attenuated waves and materials characteristics like thickness and density, as described by the mass-law. In order to overcome this fundamental limitation, a new challenge in acoustic engineering has emerged to design and manufacture lightweight and subwavelength materials that can break the mass-law. A potential solution to this challenge is represented by a recently discovered family of materials, called acoustic metamaterials, which show properties typically not found in nature. These materials are made of resonant building blocks that are smaller than the wavelength of the attenuated acoustic wave. When these building blocks are combined to form a metamaterial, they lead to the formation of band gaps - near their resonance frequency - that deeply attenuate the incident sound. The manufacturing of noise reducing acoustic metamaterials could also largely benefit from recent advances in three-dimensional printing technologies, as they offer the possibility to fabricate abstract shapes and to carefully choose some properties of the printed materials. The work presented in this thesis describes the modelling, fabrication and measurement of noise reducing acoustic metamaterials based on Helmholtz resonators, thin plates and active piezoelectric plates. These materials have been produced through original and innovative three-dimensional printing techniques. The results of this thesis can be applied to noise control in audio applications such as headphones, hearing aids and smart speakers. Similarly, other fields like aerospace and automotive industry or architectural acoustics could also greatly benefit from lightweight subwavelength noise reduction.Environmental noise impacts the everyday life of millions of people and it represents a growing concern for the health of the world's population. To mitigate this impact, noise reducing materials such as foam or barriers are employed extensively with effective results. However, the efficacy of such materials is limited by the inverse relationship between the frequency of the attenuated waves and materials characteristics like thickness and density, as described by the mass-law. In order to overcome this fundamental limitation, a new challenge in acoustic engineering has emerged to design and manufacture lightweight and subwavelength materials that can break the mass-law. A potential solution to this challenge is represented by a recently discovered family of materials, called acoustic metamaterials, which show properties typically not found in nature. These materials are made of resonant building blocks that are smaller than the wavelength of the attenuated acoustic wave. When these building blocks are combined to form a metamaterial, they lead to the formation of band gaps - near their resonance frequency - that deeply attenuate the incident sound. The manufacturing of noise reducing acoustic metamaterials could also largely benefit from recent advances in three-dimensional printing technologies, as they offer the possibility to fabricate abstract shapes and to carefully choose some properties of the printed materials. The work presented in this thesis describes the modelling, fabrication and measurement of noise reducing acoustic metamaterials based on Helmholtz resonators, thin plates and active piezoelectric plates. These materials have been produced through original and innovative three-dimensional printing techniques. The results of this thesis can be applied to noise control in audio applications such as headphones, hearing aids and smart speakers. Similarly, other fields like aerospace and automotive industry or architectural acoustics could also greatly benefit from lightweight subwavelength noise reduction