Author: Zhang, Yiting
Title: On investigation of acoustic waves and its applications in noise and air studies
Advisors: Lai, Siu-kai (CEE)
Ni, Yi-qing (CEE)
Degree: Ph.D.
Year: 2022
Subject: Noise control
Acoustical engineering
Acoustical materials
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxxiii, 241 pages : color illustrations
Language: English
Abstract: Explosive 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.
This 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.
With 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.
To 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).
To 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.
A 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.
To 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.
The 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.
Rights: All rights reserved
Access: open access

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