In the ocean, natural and artificial processes generate clouds of bubbles which scatter and attenuate sound. Measurements have shown that at the individual bubble resonance frequency, sound propagation in this medium is highly attenuated and dispersive. Theory to explain this behavior exists in the literature, and is adequate away from resonance. However, due to excessive attenuation near resonance, little experimental data exists for comparison. An impedance tube was developed specifically for exploring this regime. Using the instrument, unique phase speed and attenuation measurements were made for void fractions ranging from 6.2 × 10^−5 to 2.7 × 10^−3 and bubble sizes centered around 0.62 mm in radius. Improved measurement speed, accuracy and precision is possible with the new instrument, and both instantaneous and time-averaged measurements were obtained. Behavior at resonance was observed to be sensitive to the bubble population statistics and agreed with existing theory, within the uncertainty of the bubble population parameters. Scattering from acoustically compact bubble clouds can be predicted from classical scattering theory by using an effective medium description of the bubbly fluid interior. Experimental verification was previously obtained up to the lowest resonance frequency. A novel bubble production technique has been employed to obtain unique scattering measurements with a bubbly-liquid-filled latex tube in a large indoor tank. The effective scattering model described these measurements up to three times the lowest resonance frequency of the structure. / United States Navy Office of Naval Research Ocean Acoustics Program

This diploma thesis deals with the determination of the material acoustic absorption in an impedance tube. The aim of the work is to propose a measuring experiment and to design measuring device with the intention of reducing costs. The aim is to carry out a measuring experiment on a manufactured measuring device. The results of the experiment are evaluated in terms of meeting the requirements formeasuring device. The results of the tested materials are compared with the values from the supplier.

A new way of adhering woven textile to a nonwoven backing was explored, the end product in mind being a sound absorbent panel. Today a hot melt adhesive is sprayed on the nonwoven with woven decorating textile put on top before compression molding. This method results in an uneven layer of adhesive with the consequence of decorating textile detaching from the nonwoven and thereby a high number of discarded products. The report investigates the possibility of using a bicomponent fiber as the adhesive. This would guarantee an even layer of adhesive and make the manufacturing process one step shorter. A bicomponent fiber of core/sheath construction with a low melting polymer in the sheath was incorporated in the nonwoven upon fabrication. In the main bulk of the nonwoven a smaller ratio of bicomponent to normal polyester fiber was used, enough to stabilize the nonwoven. For the top layer of the nonwoven a higher ratio was chosen. As the nonwoven is then compression molded under heat together with the decorating textile, the low melting sheath of the bicomponent fiber will melt and create bonds within the nonwoven as well as to the textile. The ratio in the top layer was varied as well as the pressure in compression molding. The adhesion strength between nonwoven and decorating textile was tested, and the sound absorbing properties of the different manufactured samples were compared. The sound absorbing parameter of air flow resistance was tested and sound absorption was tested using impedance tube with transfer function method. Further, a way of testing transmission loss was developed by the manufacturing of a custom built impedance tube, which was then compared to the transfer function method. The results showed that a higher bicomponent percentage gave higher sound absorption in lower frequencies, but a lower sound absorption in higher frequencies. The thickness of the samples gave a positive effect on the sound absorption in all frequencies. The conclusion is to recommend a thicker material, and choose bicomponent according to which frequencies that should be absorbed and what adhesion strength is needed for end product. / Program: Textilteknik

This thesis was conducted on behalf of Butong AB, who wanted to test and develop an environmental friendly, so called green sound barrier, which combines both art and science.Different configurations of the product were proposed by the company with various filling materials, as it was predicted that the filling materials would be the main sound absorbent among all parts of the structure.The thesis work started by selecting the best of the proposed fillings which could be of interest - that is those which were expected to have high sound absorption coefficients. The selection process was based on experience, reading and advice. The main idea behind the selection process was saving cost for the company as well as effort.Impedance tube method was used for performing the measurements on samples of the green sound barriers, in order to calculate the acoustical properties of each material and every construction, as it was considerably reliable, cheap and fast to use.The measurements were done according to a combination between standards described in ISO 10534-2:1998 and ASTM E2611-09, for performing test measurements using the impedance tube.This master thesis gives an explanation of the predicted absorption characteristics of the green sound barriers including the usage of different fillings, as well as the advantages and disadvantages of using it in real life applications.

This thesis investigates the impedance of acoustic liners, to attenuate noise originating from jet engines and enable compliance with international standards and regulations regarding noise from airplane jet engines. Experimental tests of two supplied liners were conducted in an impedance tube; one liner with known and predictable properties, and one liner with unknown properties. The tests included tonal excitations in the formats of stepped sine and random noise with frequencies within set boundaries. After post-processing of the captured data, the desired impedance could be analysed in terms of excitated frequencies and sound pressure levels. The conclusions from this project are that both of the liners deviated from their expected behavior, which was that liner 1 should have been unaffected by the alternated sound pressure levels, and liner 3 should have shown bigger affection due to the changed sound pressure level. Since the results were different than expected, there might have been minor sources of error during the measurements. It could be investigated if there is leakage from the mounting of the liners, or if the 3D printing resolution is sufficient. Because of limitations in time, there is more left in this project to investigate. Therefore, conducting similar studies where more frequencies, sound pressure levels, and multi-tonal measurements can be included, is suggested as future work.

<p dir="ltr">In the field of noise control engineering, the development of effective low-frequency sound absorption treatments has long been a challenge, since conventional solutions tend to require impractically thick layers of traditional porous materials, such as fibrous materials and foams. In contrast, high surface area particles, such as granular activated carbon (GAC) particles, milled aerogels, and zeolites, have inner-particle pores at micro and nano scales, which improve the low-frequency absorption by slowing the local sound speed. As a result, a 30 mm thick GAC stack can achieve an absorption coefficient of 0.3 at 100 Hz. Hence, these materials have already been used in various low-frequency applications in place of fibrous or foam layers: e.g., MEMS speaker back cavities, Helmholtz resonator liners, micro-perforated panel absorbers, and membrane absorbers. One major practical goal of this research was to determine how best to model and optimize novel treatments consisting in whole or in part of high surface area granular materials. </p><p dir="ltr">The detailed work presented in this thesis starts with a review of acoustical models of various material types, followed by two approaches to modeling and coupling different types of layers in a general and stable manner. In particular, in the second approach, a large, complicated system is divided into a series of small systems, hence avoiding the direct inverse solution of large systems. As a result, the second approach is more efficient and enables computationally intensive tasks such as multi-layer material characterization and sound package optimization. In addition to the modeling techniques, different types of granular stacks’ acoustical behavior were also experimentally investigated and summarized: i.e., 1. the edge-constraint effect resulting from the friction at the wall of the impedance tube; 2. level-dependent behavior; 3. time-dependent behavior; and 4. other non-linear behavior. To capture the observed acoustical physics of GAC stacks, a triple-porosity poro-elastic model with a depth-dependent modulus was described, followed by characterization frameworks to model the stacks subject to the edge-constraint effect as well as varying excitation levels. These frameworks were validated by comparing the absorption spectra predicted by using the inferred material properties with impedance tube measurements of GAC stacks with varying depths, diameters, and exposure levels. In the end, a novel sound absorption treatment was presented (a GAC stack supported by a soft, porous layer), which was subsequently optimized to develop broadband absorbers.</p>

[EN] The behaviour of different types of textile fibres, obtained from extraction and separation methods in the recycling processes of ELT (End of life tyres) (from now on "fluff"), as acoustic absorbent material has been studied in this research work. This work is a starting point for the study of the use of this type of materials in architectural acoustics applications. Experimental design was used to study the relevance of the forming conditions: pressure, time and temperature variables. The adjustment of the manufacturing variables: different temperature range, curing time, fluff mass variation, and compaction factor were studied in order to investigate the relation of these parameters to the acoustic intrinsic properties: porosity, tortuosity and air flow permeability. The objective was to achieve the best conditions in the production of panels with the best sound absorbent qualities within the widest frequencies rank. The production process was carried out by using an octahedral mould, made up of the enclosed body covered by a piston-like lid with downward-upward trajectory. Inside the mould a specific mass of fibres was homogeneously displayed, and then it was placed in a press with heated plates, using the piston to compress the fibres until the established level of pressure. Panels were obtained by applying the precise heating energy in order to reach the required temperature for each particular sample and remaining in these conditions during the fixed time. / [ES] En este trabajo estudiamos el comportamiento como absorbentes acústicos de diferentes clases de fibras textiles, provenientes de métodos de extracción y separación en el reciclado de neumáticos fuera de uso, en adelante (fluff). El trabajo es un punto de partida en el estudio de este tipo de materiales para su uso en aplicaciones de acústica arquitectónica. Mediante diseño de experimentos, se estudia la influencia de las condiciones de conformación: presión, tiempo, temperatura, masa de fluff. Aplicando unas variables en los parámetros de conformado, variación de temperatura, variación de tiempo de exposición, variación de la masa de fluff, y variación del factor de conformación a fin de estudiar la relación de estos parámetros y las propiedades intrínsecas acústicas: porosidad, tortuosidad, permeabilidad al flujo de aire. Consiguiendo las condiciones más optimas, para la posterior conformación de un panel que obtenga las mejores cualidades absorbentes del sonido, en el mayor rango de frecuencias. El proceso de conformado, se ha llevado a cabo mediante un molde ortoédrico, compuesto por el seno, cubierto por una tapadera tipo pistón descendente en el cual, se han dispuesto homogéneamente una determinada masa de fibras, el molde ha sido situado en una prensa con platos calefactores. Comprimiendo las fibras mediante el descenso del pistón hasta el nivel establecido, aplicándoles energía calorífica, hasta obtener la temperatura indicada en cada muestra, permaneciendo en estas condiciones durante un tiempo determinado. / [CA] En aquest treball estudiem el comportament com a absorbents acústics de diferents classes de fibres tèxtils provinents de mètodes d'extracció i separació en el reciclat de pneumàtics fora d'ús, d'ara endavant (fluff). El treball és un punt de partida en l'estudi d'aquest tipus de materials per al seu ús en aplicacions d'acústica arquitectònica. Mitjançant disseny d'experiments s'estudia la influència de les condicions de conformació: pressió, temps, temperatura, aplicant unes variables en els paràmetres de conformat, variació de temperatura, variació de temps d'exposició, variació de la massa de fluff, i variació del factor de conformació a fi d'estudiar la relació d'aquests paràmetres i les propietats intrínseques acústiques porositat, tortuositat, permeabilitat al flux d'aire, aconseguint les condicions més optimes per a la posterior conformació d'un panell que obtinga les millors qualitats absorbents del so, en el major rang de freqüències. El procés de conformat s'ha dut a terme mitjançant un motle ortoedre, compost pel si cobert per una tapadora tipus pistó descendent en el qual s'han disposat homogèniament una determinada massa de fibres, el motle ha sigut situat en una premsa amb plats calefactors, comprimint les fibres mitjançant el descens del pistó fins al nivell establert, aplicant-los energia calorífica fins a obtenir la temperatura indicada en cada mostra romanent en aquestes condicions durant un temps determinat. / Zamorano Cantó, M. (2016). INVESTIGACIÓN CON FIBRA RECICLADA DE NEUMÁTICO (F.R.N.) PARA APLICACIONES ACÚSTICAS [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/61972

La zona en estudio, ubicada entre los kilómetros 31 y 35 de la carretera Panamericana Norte, posee un nivel de presión sonora promedio de 76.48dB y 74.76dB en el horario diurno y nocturno respectivamente, y tomando como base Los Estándares de Calidad de Ruido Ambiental (ECA) se puede determinar que la zona en estudio se produce contaminación acústica. Por ello, las personas que frecuentan y/o circulan por esta zona se encuentran propensas a contraer distintas enfermedades y molestias producto de la contaminación sonora, entre ellas: Estrés, pérdida de audición con el paso del tiempo, problemas cardiológicos, entre otros. Ante lo mencionado, la presente tesis busca controlar el ruido generado por el parque automotor mediante la instalación de Barreras Acústicas en la sección vial de la Panamericana Norte, utilizando como materiales principales dos tipos de concreto, el concreto convencional y el concreto con agregados ligeros de aserrín y viruta. Para ello, el concreto con agregados ligeros de aserrín y viruta, material absorbente del sistema de Barrera Acústica, se lleva al laboratorio acústico para ensayar mediante el método del Tubo de impedancia, para obtener el coeficiente de nivel de presión sonora. Siendo este valor promedio de absorción sonora máxima de las muestras (α) 0.23. En conclusión, el sistema de barrera acústica diseñado alcanza a absorber hasta 23% del nivel de presión sonora percibido, y con ello la zona en estudio se encuentra dentro de los Estándares de Calidad de Ruido Ambiental. / The area under study, located between kilometer 31 and kilometer 35 of the Northern Pan-American Highway, has an average sound pressure of 76.48dB during daytime and 74.76dB during nighttime. Taking the Environmental Noise Quality Standards (ECA, in Spanish) into consideration, it can be determined that the noise pollution occurs in the area under study. Therefore, the people who frequent or pass by this area are prone to contract various diseases or malaise as a result, such as stress, gradual hearing loss, heart issues, among others. Given the situation, this thesis aims at controlling the noise produced by motorized vehicles by installing sound barriers on said road section of the Northern Pan-American Highway, using two types of concrete as main materials: conventional concrete and concrete with light sawdust and woodchip aggregates. The concrete with light sawdust and swarf aggregates, absorbent material of the sound barrier system, is taken to the sound laboratory to perform tests through the impedance tube method to obtain the sound pressure level. This is the average value of the maximum absorption level of the samples (α) 0.23. In conclusion, the designed sound barrier system manages to absorb up to 23% of the perceived sound pressure level. With that, the area under study falls within the Environmental Noise Quality Standards. / Tesis

This thesis describes methods of high frequency noise and vibrations computation of cabin part of EV–55M (aircraft developed by Evektor Kunovice). There is a brief summary of methods used for determining high frequency noise and vibrations in the first part of the thesis. Detailed explanation is given for Statistical Energy Analysis (SEA) which is nowadays the most dominant method in this area. The energy balance equation is derived in this chapter and SEA parameters such as modal density, damping loss factor, coupling loss factor and power input are introduced here. Next part deals with main noise sources of propeller driven and jet aircraft and passive and active noise controls are discussed. Practical part of this thesis deals with modeling aircraft EV–55M fuselage using VA One SEA module. Two models were created. First of them is only an outside fuselage with aircraft flooring and the second one is extended by interior trim panels and is applicable for simulation of noise control treatments. Computational modeling is accompanied by experimental measurement of passive noise control material characteristics. Postprocessing of information obtained from impedance tube measurement was performed in FOAM – X. Determined characteristics of porous material were used as inputs to VA One and reduction of sound pressure level in fuselage cavities by using noise control treatment was found. In conclusion there is a summary of noise transmission paths from sources to interior cavity and some treatments of them are simulated