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dc.contributorDepartment of Civil and Environmental Engineeringen_US
dc.contributor.advisorLai, Siu-kai (CEE)en_US
dc.contributor.advisorNi, Yi-qing (CEE)en_US
dc.creatorZhang, Yiting-
dc.publisherHong Kong Polytechnic Universityen_US
dc.rightsAll rights reserveden_US
dc.titleOn investigation of acoustic waves and its applications in noise and air studiesen_US
dcterms.abstractExplosive population growth and increasing migration have led to a boom in mega-cities, posing technological challenges in urban development. To meet long-term sustainability targets and environmental regulations, developing innovative technologies for providing a comfortable, green and safe living environment is of prominent need. The intensive investigation of acoustic waves in both theoretical and experimental studies is largely due to their extensive applications in various engineering fields. In an attempt to minimize the impact of environmental noise and intervene the degradation of indoor air quality, we strive to explore multiple applications of acoustic waves in noise and air studies in this work.en_US
dcterms.abstractThis thesis is mainly divided into two parts. The first part focuses on the investigation of acoustic wave characteristics and noise mitigation strategies. Due to hectic construction activities under a rapid urbanization process in densely populated cities (e.g., Hong Kong), construction noise generated from various powered mechanical equipment (e.g., drillers, hammers and excavators) is a major problem. A long-term exposure in noise can cause an adverse impact on psychological health, life quality and working efficiency. To alleviate this problem, a passive noise control (PNC) approach (e.g., noise barriers) and an active noise control (ANC) method (e.g., generation of anti-noise signals) are commonly conducted to mitigate the unwanted noise. For PNC, the performance of passive-type sound barriers greatly depends on several factors, including material types, design dimensions, surface conditions, and geometric configurations. Nevertheless, the use of PNC approach alone is not practical for the attenuation of low-frequency noise, while ANC method shows advantages in low-frequency range. Motivated by this idea, the combination of both PNC and ANC strategies are investigated.en_US
dcterms.abstractWith the development of digital signal processing, it enables the feasibility of control algorithm for active control. In this work, a learning and forecasting approach is utilized as a pre-treatment process, which combines a Bayesian approach and a dynamic linear model (DLM). Comprising the statistical strategy and descriptive time series, the proposed pre-treatment system is conducive to raw-signal pre-processing and can concurrently generate a predicted signal as a reference signal. The predicted signals based on prior information and Bayesian inference afford an alternative to the normal costs of the secondary path, such as those associated with electro-acoustic signal conversions and computation efforts in the control algorithm. To demonstrate the feasibility of this pre-treatment system, illustrative examples coupling with the existing control algorithms, e.g., the conventional filtered-x least mean square (FxLMS) algorithm and a new convex structure via an FxLMS/F algorithm (C­FxLMS/F), are studied. Making use of the present forecasting technique, stationary signals are acquired for analysis. The updating characteristic "forecast-observation­analysis" loop is advantageous for the implementation of signal processing for an ANC system.en_US
dcterms.abstractTo establish a noise mitigation device, using passive barriers is an effective way for construction sites. With consideration of environmental-friendly barriers, it is preferable to use low-cost, recyclable and lightweight materials. Recycling use of waste wood and rubber materials from construction sites is investigated as their good tensile modulus and mechanical strength. A laminated configuration, which consists of: (i) one-thick-layer recycled composite panel with a mixture of wood pellets and rubber particles, and (ii) a thin-film layer of polyvinylidene fluoride (PVDF) material, is examined. The wood-rubber composite panels are designed for wideband frequency control, while the PVDF thin-film layer as a supplementary part is to eliminate the peak narrowband low-frequency component. Compared to conventional loudspeakers with bulky coils and diaphragms, piezoelectric thin-film materials are flexible and lightweight with strong piezoelectric and ferroelectric properties to couple with passive panels. A scale-down model is fabricated and tested in the in-house semi-anechoic chamber to examine the effectiveness. The barrier shows good performance in noise attenuation (i.e., a reduction of sound pressure levels (SPLs) at average 10dB).en_US
dcterms.abstractTo go beyond the potential application of acoustic waves, the second part focuses on acoustic manipulation technique and thermo-acoustic (TA) wave devices. Acoustic manipulation (e.g., acoustic trapping) is an active but contact-free technique, which utilizes acoustic waves to exert radiation forces on manipulating objects in air. To maintain indoor air quality, heating, ventilation and air-conditioning (HVAC) systems play a crucial role. Through the ventilation duct, viruses and bacteria can spread through the air on dust and microscopic particles within enclosed buildings. Epidemiological evidence reveals that a good ventilation control strategy can reduce the possibility of airborne viral transmission and infection. Generally, particle filters inside mechanical ventilation ductworks having a minimum efficiency rating value (MERV) (ANSI/ASHRAE, 1999) are frequently used, but they are limited of capturing airborne particles at a sub-micron scale (0.3-1.0 μm). Hence, a combination of acoustic-driven pre-filtering techniques and commercial coarse filters is an alternative to enhance filtration efficiency for sub-micron particles.en_US
dcterms.abstractA U-shaped acoustic-driven pre-filtering device is proposed to enhance the working efficiency of coarse filters for capturing sub-micron particles (0.3-1.0 μm). The U-shaped device can optimize spatial homogeneity to improve the removal coefficient of airborne particles under lower sound intensity requirements, which can circumvent the existing problem of using high-intensity sound pressure for acoustic manipulation. Experimental studies are conducted to examine the efficiency of the present pre-filtering device. The results show that an overall filtration efficiency of up to 89% for 1.0-μm airborne particles can be achieved when the acoustic-driven device is coupled together with a low-grade MERV-6 coarse filter. As a standalone device, the acoustic effect works well for the sub-micron particles with a filtration efficiency of up to 61% under a sound pressure level of 116 dB, which is lower than those SPLs as reported in the literature. The design has a high level of flexibility to work with various MERV filters. In the analysis, the influence of relevant parameters (e.g., airflow rates, acoustic frequencies, and sound intensities) on various particle sizes is investigated.en_US
dcterms.abstractTo further develop an acoustic-wave emitter with high sensitivity, the application of thermo-acoustic device is a new direction to go beyond the limitations of vibration-based speakers. Advances in nanomaterials (e.g., carbon nanotube (CNT) and graphene) over the last decade have realized the "thermophone" concept that was discovered a century ago, it basically differs from the working mechanism of conventional acoustic devices to generate sound by mechanical vibration. The operating mechanism of this TA-based technique can be described by providing an alternating current to CNT-or graphene-based thin-film materials, causing the surrounding medium (air) to be heated periodically. An oscillation of the temperature field induced in air can result in thermal expansion and contraction to generate acoustic waves. This next-generation acoustic technique can be scalable to deliver a flat-band frequency range as well as a high-intensity sound pressure level. Test samples of the TA-based emitter are fabricated to characterize its acoustic performance.en_US
dcterms.abstractThe research findings of this work can provide a further understanding of the nature of acoustic waves and its potential applications in various engineering fields. The acoustic-based technologies can contribute better resource efficiency to maintain a healthy, resilient and pleasant urban environment.en_US
dcterms.extentxxxiii, 241 pages : color illustrationsen_US
dcterms.isPartOfPolyU Electronic Thesesen_US
dcterms.educationalLevelAll Doctorateen_US
dcterms.LCSHNoise controlen_US
dcterms.LCSHAcoustical engineeringen_US
dcterms.LCSHAcoustical materialsen_US
dcterms.LCSHHong Kong Polytechnic University -- Dissertationsen_US
dcterms.accessRightsopen accessen_US

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