Master of Science / Department of Architectural Engineering and Construction Science / Julia A. Keen / Throughout recent years, to minimize the cost of construction, a large number of multipurpose spaces have been built using lightweight, less expensive materials without considering or designing for noise control to mitigate any sound that is loud, unpleasant, unexpected, or undesired yet after construction is completed, noise issues are often evident within the space and, if severe enough, may render the intended function of the structure useless. To address this problem, this report is intended to introduce Owners and Facility Managers to some of the common solutions to resolve noise issues in multipurpose rooms. The report focuses on solutions for existing projects primarily, but it is also sensitive to budget constraints and the impact of renovation. Typical multipurpose rooms researched have a volume of 50,000-150,000 cubic feet and are expected to be used for speech activities, small music functions, and some physical sports activities. Therefore, this report will introduce the fundamentals of sound and room acoustics including interior surface materials and construction. Also included are typical noise issues from interior sources, solutions that can be taken within the building to attenuate noise, and the trade-offs associated with each solution.

M.Ing. (Electrical and Electronic Engineering) / Acoustic reverberation is put under the spotlight. A review of the theory was conducted followed by a look into digital artificial reverberation. Measurement methodology is presented including a review of the recently published ISO standard pertaining to reverberation. Experimental testing was conducted for four acoustically different environments with one of them almost completely anechoic. The reverberation characteristic of these four environments were measured and analysed according to the relevant ISO standards. The results were then used in a further study of digital artificial reverberation applied to impulse and vocal sounds. The anechoic sounds were artificially reverberated using Cool Edit Pro software to mimic the sound obtained that had natural reverberation present. The focus was on the RT as well as the EDT of the decay slope. The artificial method of applying reverberation was evaluated using two methodologies, firstly objective methods relying on mathematics; secondly, by subjective personal evaluations using a statistical analysis of a listening test questionnaire. Both the objective and subjective results confirmed that digital artificial reverberation methods could be applied successfully to impulse sounds and vocals. The results provide a basis for the motivation of computerised methods in the studio recording process especially for rooms that are moderately anechoic.

Misturas de poliacetal (POM), polímero de engenharia com boas propriedades mecânicas, com poliuretano termoplástico (TPU), um polímero composto por uma fase rígida e outra flexível visam buscar alternativa de materiais a serem utilizadas em bandas externas de roldanas, visando a diminuição da emissão de ruído das roldanas quando utilizadas em sistema de movimentação de portas de armários, em contato com um trilho metálico. Neste trabalho foram avaliadas as propriedades mecânicas, físicas, térmicas e morfológica de misturas de POM/TPU nas proporções 100/0, 80/20, 70/30, 60/40, 50/50, 40/60 e 0/100 (m/m). As misturas foram preparadas em extrusora dupla rosca corrotante e após os corpos de prova foram injetados em injetora convencional. O aumento dos teores de TPU nas misturas promoveu o aumento da resistência ao impacto, à abrasão e diminuição na dureza do material. Houveram aumentos da temperatura de deflexão térmica das misturas com o aumento do teor de POM. Através da microscopia eletrônica de varredura (MEV) pode ser analisada uma mistura homogênea com partículas dispersas de TPU em formato de gotículas. A avaliação de emissão de ruído mostrou que com a diminuição do módulo de elasticidade, com o aumento da quantidade TPU na mistura, ocorre a redução da emissão de ruído do mesmo, esta redução fica evidente em frequências acima de 1kHz, ocorrendo uma redução da emissão de ruído de 2-5dB. / Bolsa Desenvolvimento de Tecnologia e Inovação / Polyoxymethylene (POM), an engineering polymer with good mechanical properties, blended with thermoplastic polyurethane (TPU), a polymer composed by a flexible and a rigid phase, are an alternative of materials to be used in external bands of pulleys, aiming to reduce the noise emission of pulleys when used in system drive cabinets doors, occurring pulleys contact with a metallic rail. In this work, the mechanical, physical, thermal and morphological properties of POM/TPU blends were evaluated in the proportions of 100/0, 80/20, 70/30, 60/40, 50/50, 40/60 and 0/100 (w/w). The blends were prepared in a twin screw extruder and after the samples were injected in a conventional injector. The increase in the levels of TPU on the blends promoted a rise in the impact resistance, abrasion and a decrease in the material hardness. There was an increase in the deflection heat temperature of the blends with enhanced content of POM. Through scanning electron microscopy (SEM) was observed a blend with the presence of dispersed TPU particles. The noise emission showed that with the decrease in the modulus of elasticity of the polymer occurs a reduction of noise emission. This reduction is evident in frequencies above 1 kHz, causing 2-5dB reduction of noise emission.

Cylindrical shells are common structures that are often used in industry, such as pipes, ducts, aircraft fuselages, rockets, submarine pressure hulls, electric motors and generators. In many applications it is desired to attenuate the sound radiated from the vibrating structure. There are both active and passive methods to achieve this purpose. However, at low frequencies passive methods are less effective and often an excessive amount of material is needed to achieve acceptable results. There have been a number of works regarding active control methods for this type of structure. In most cases a considerable number of error sensors and secondary sources are needed. However, in practice it is much preferred to have the fewest number of error sensors and control forces possible. Most methods presented have shown considerable dependence on the error sensor location. The goal of this dissertation is to develop an active noise control method that is able to attenuate the radiated sound effectively at low frequencies using only a small number of error sensors and secondary sources, and with minimal dependence on error sensor location. The Weighted Sum of Spatial Gradients control metric has been developed both theoretically and experimentally for simply supported cylindrical shells. The method has proven to be robust with respect to error sensor location. In order to quantify the performance of the control method, the radiated sound power has been chosen. In order to calculate the radiated sound power theoretically, the radiation modes have been developed for cylindrical shells. Experimentally, the radiated sound power without and with control has been measured using the ISO 3741 standard. The results show comparable, or in some cases better, performance in comparison with other known methods. Some agreement has been observed between model and experimental results. However, there are some discrepancies due to the fact that the actual cylinder does not appear to behave as an ideal simply supported cylindrical shell.

The prediction of the noise generated by en-route aircraft is gradually gaining in importance as the number of aircraft increases over the last few decades. While the studies of outdoor sound propagation have been focused on near ground propagation, the case when the sound source is high above the ground has not attracted much attention. At the same time there has been a lack of high-quality aircraft acoustic validation data sets that contain detailed acoustic, meteorology, and source-receiver position data. The DISCOVER-AQ data set, which was collected by Volpe in support of the Federal Aviation Administration (FAA), has greatly helped with studying the directivity and the Doppler effect in the comparison between simulation results and measurements. <div><br>To provide a more accurate prediction of en-route aircraft noise, we derived the analytic asymptotic solution of the sound field above a non-locally reacting ground due to a moving point source and a line source using the methods of the steepest descent and a Lorentz transform. The model predicts a much more accurate result for sound field above "soft" grounds, such as a snow-covered ground and sand-covered ground. At the same time, we derived a fast numerical algorithm based on Levin’s collocation for the prediction of the sound field in the presence of a temperature gradient, which can be applied to a wide range of acoustic problems involving integration. The achievements recorded in this thesis can be used to predict the sound field generated by aircraft, trains, and vehicles with a subsonic moving speed. In addition,<br>the model can be used for detection and design of moving sound source. <br></div>

<div><div><div><p>Tonal sounds are a particular problem of concern in building environments, arising from the widely used rotating machinery (e.g., compressors, fans, motors, trans- formers, etc.). In the recent trend of designing and manufacturing high-performance building mechanical systems, higher output power and higher rotation speed are pursued, this inevitably results in a more severe noise problem, since the equipment noise not only becomes louder but also shifts to a higher frequency region (which, in most cases, results in a poorer sound quality due to the shift in spectral balance and tonal components moving into the frequency regions where people are most sensitive to tones). Tonal sounds from rotary machines can be annoying, even at relative low levels.</p><p>Currently, noise criteria guidelines in Chapter 48 of the ASHRAE HVAC Applications Handbook can be used to design the building mechanical system, but this does not apply well for tonal noise. Reducing the limit for noise with perceptible tones is one common strategy in the industry. However, it’s not adequate for some cases, over-design in others. Thus, an adequate understanding of the annoyance threshold of tonal noises associated with building services equipment is valuable technical information not only in the design and manufacture of machines but also in the development of noise regulations related to building services equipment.</p><p>This research aims to develop a sound quality model that cooperates with sound level and tonalness and relates tonal building noises to the perceived annoyance.</p></div></div></div>

The application of the Hershcel-Quincke (HQ) tube as a noise reduction device for one-dimensional plane-wave sound fields has been studied in great detail in previous years. In this thesis, an analytical technique is developed to investigate the potential of the HQ tube concept to control higher-order duct modes. This analytical method involves modeling the tube-duct interfaces as finite piston sources, which couple the acoustic field inside the main duct with the acoustic field within the HQ tube(s). The acoustic field within the HQ tube is modeled as plane-waves and the acoustic field within the main duct is modeled by expanding the sound field in terms of the higher-order modes. This model is then used to investigate the noise reduction mechanisms behind the attenuation of higher-order modes. These mechanisms involve both the reflection of the incident wave as well as the reconstruction and recombination of the modal content of the incident disturbance into other modes. The effects of the modal content of the disturbance along with the HQ tube geometric parameters, such as tube axial position, length, distance between interfaces, and cross-sectional area, are studied with respect to the frequencies of attenuation and the reduction obtained. These results show the potential of the Herschel-Quincke tube concept to reduce higher-order modes in ducts. / Master of Science

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Active noise control (ANC) is a real-time process in which a system measures an external, unwanted sound source and produces a canceling waveform. The cancellation is due to destructive interference by a perfect copy of the received signal phase-shifted by 180 degrees. Existing active noise control systems process the incoming and outgoing audio on a sample-by-sample basis, requiring a high-speed digital signal processor (DSP) and analog-to-digital converters (ADCs) with strict timing requirements on the order of tens of microseconds. These timing requirements determine the maximum sample rate and bit size as well as the maximum attenuation that the system can achieve. In traditional noise cancellation systems, the general assumption is that all unwanted sound is indeterminate. However, there are many instances in which an unwanted sound source is predictable, such as in the case of a song. This thesis presents a method for active acoustic cancellation of a known audio signal using the frequency characteristics of the known audio signal compared to that of a sampled, filtered excerpt of the same known audio signal. In this procedure, we must first correctly locate the sample index for which a measured audio excerpt begins via the cross-correlation function. Next, we obtain the frequency characteristics of both the known source (WAVE file of the song) and the measured unwanted audio by taking the Fast Fourier Transform (FFT) of each signal, and calculate the effective environmental transfer function (degradation function) by taking the ratio of the two complex frequency-domain results. Finally, we attempt to recreate the environmental audio from the known data and produce an inverted, synchronized, and amplitude-matched signal to cancel the audio via destructive interference. Throughout the process, we employ many signal conditioning methods such as FIR filtering, median filtering, windowing, and deconvolution. We illustrate this frequency-domain method in Native Instruments’ LabVIEW running on the Windows operating system, and discuss its reliability, areas for improvement, and potential future applications in mobile technologies. We show that under ideal conditions (unwanted sound is a known white noise source, and microphone, loudspeaker, and environmental filter frequency responses are all perfectly flat), we can achieve a theoretical maximum attenuation of approximately 300 dB. If we replace the white noise source with an actual song and the environmental filter with a low-order linear filter, then we can achieve maximum attenuation in the range of 50-70 dB. However, in a real-world environment, with additional noise and imperfect microphones, speakers, synchronization, and amplitude-matching, we can expect to see attenuation values in the range of 10-20 dB.

Previous work has shown that active noise control (ANC) can be applied to axial cooling fans. Optimization of the control source and error sensor placement is desired to maximize the attenuation using ANC. A genetic algorithm was developed to find the optimal placement of control sources for a given primary source. The optimal configuration of control sources around a single primary source was shown to be a linear arrangement of the sources. This holds true for both two-dimensional as well as three-dimensional configurations. The higher-order radiation of the linear arrangement has also been verified experimentally, but the improvement in the experimental apparatus was not as dramatic as the theoretical model. Multiple flow visualization techniques have been used to find optimal near field error sensor locations. When there is little obstruction to the flow field of the fan, minimal airflow is found along the near field null that is created by minimizing the sound power of the system. Surface mounting of the error sensors can lead to a small increase in the signal-to-noise ratio of the error sensors if vortices exist in the near field of the fan due to obstructions in the main flow. It has also been shown that the introduction of the ANC system does not affect the flow field of the fan.