Kyungjun Song

Emission enhancement of sound emitters using an acoustic metamaterial cavity.

The emission enhancement of sound without electronic components has wide applications in a variety of remote systems, especially when highly miniaturized (smaller than wavelength) structures can be used. The recent advent of acoustic metamaterials has made it possible to realize this. In this study, we propose, design, and demonstrate a new class of acoustic cavity using a double-walled metamaterial structure operating at an extremely low frequency. Periodic zigzag elements which exhibit Fabry-Perot resonant behavior below the phononic band-gap are used to yield strong sound localization within the subwavelength gap, thus providing highly effective emission enhancement. We show, both theoretically and experimentally, 10 dB sound emission enhancement near 1060 Hz that corresponds to a wavelength approximately 30 times that of the periodicity. We also provide a general guideline for the independent tuning of the quality factor and effective volume of acoustic metamaterials. This approach shows the flexibility of our design in the efficient control of the enhancement rate.

Directional Reflective Surface Formed via Gradient-Impeding Acoustic Meta-Surfaces

Artificially designed acoustic meta-surfaces have the ability to manipulate sound energy to an extraordinary extent. Here, we report on a new type of directional reflective surface consisting of an array of sub-wavelength Helmholtz resonators with varying internal coiled path lengths, which induce a reflection phase gradient along a planar acoustic meta-surface. The acoustically reshaped reflective surface created by the gradient-impeding meta-surface yields a distinct focal line similar to a parabolic cylinder antenna, and is used for directive sound beamforming. Focused beam steering can be also obtained by repositioning the source (or receiver) off axis, i.e., displaced from the focal line. Besides flat reflective surfaces, complex surfaces such as convex or conformal shapes may be used for sound beamforming, thus facilitating easy application in sound reinforcement systems. Therefore, directional reflective surfaces have promising applications in fields such as acoustic imaging, sonic weaponry, and underwater communication.

Sound Pressure Level Gain in an Acoustic Metamaterial Cavity

The inherent attenuation of a homogeneous viscous medium limits radiation propagation, thereby restricting the use of many high-frequency acoustic devices to only short-range applications. Here, we design and experimentally demonstrate an acoustic metamaterial localization cavity which is used for sound pressure level (SPL) gain using double coiled up space like structures thereby increasing the range of detection. This unique behavior occurs within a subwavelength cavity that is 1/10th of the wavelength of the incident acoustic wave, which provides up to a 13 dB SPL gain. We show that the amplification results from the Fabry-Perot resonance of the cavity, which has a simultaneously high effective refractive index and effective impedance. We also experimentally verify the SPL amplification in an underwater environment at higher frequencies using a sample with an identical unit cell size. The versatile scalability of the design shows promising applications in many areas, especially in acoustic imaging and underwater communication.

Concentric artificial impedance surface for directional sound beamforming

Utilizing acoustic metasurfaces consisting of subwavelength resonant textures, we design an artificial impedance surface by creating a new boundary condition. We demonstrate a circular artificial impedance surface with surface impedance modulation for directional sound beamforming in three-dimensional space. This artificial impedance surface is implemented by revolving two-dimensional Helmholtz resonators with varying internal coiled path. Physically, the textured surface has inductive surface impedance on its inner circular patterns and capacitive surface impedance on its outer circular patterns. Directional receive beamforming can be achieved using an omnidirectional microphone located at the focal point formed by the gradient-impeding surface. In addition, the uniaxial surface impedance patterning inside the circular aperture can be used for steering the direction of the main lobe of the radiation pattern.

Enhanced water collection through a periodic array of tiny holes in dropwise condensation

This paper introduces a simple method of water collection by increasing the coalescence effects in dropwise condensation with the use of microscale holes. The tiny holes modified the surface free energy states of the droplets on the plate, yielding a surface free energy barrier between the flat solid surface and the holes. The spatial difference in the surface free energy of the droplets enabled the droplets to move toward the adjacent droplets, thus increasing the possibility of coalescence. The water collection experiments were performed using a Peltier-based cooling system at 2 °C inside a chamber at 30 °C and 70% humidity. The results demonstrated that the perforated plates without any additional treatment provided the water collection rate of up to 22.64 L/m2 day, which shows an increase of 30% compared to that demonstrated by the bare plate. By comparing the experimental results for the surface of filmwise condensation, it was proved that the dominant water collecting improvement results from the in...

Fabrication of Si3N4-Based Artificial Basilar Membrane with ZnO Nanopillar Using MEMS Process

This paper presents the fabrication of Si3N4-based artificial basilar membrane (ABM) with ZnO nanopillar array. Structure of ABMs is composed of the logarithmically varying membrane fabricated by MEMS process and piezonanopillar array grown on the Si3N4-based membrane by hydrothermal method. We fabricate the bottom substrate containing Si3N4-based membrane for inducing the resonant motions from the sound wave and the top substrates of electrodes for acquiring electric signals. In addition, the bonding process of the top and bottom substrate is performed to build ABM device. Depending on sound wave input of the specific frequency, specific location of the ABM produces a resonant behavior. Then a local deformation of the piezonanopillar array produces an electric signal between top and bottom electrode. As experimental results of the fabricated ABM, the measured resonant frequencies are 2.34 kHz, 3.97 kHz, and 8.80 kHz and the produced electrical voltages on each resonant frequency are 794 nV, 398 nV, and 89 nV. Thus, this fabricated ABM device shows the possibility of being a biomimetic acoustic device.

Fabrication and Vibration Characterization of a Partially Etched-type Artificial Basilar Membrane

The structure of the human ear is divided into the outer ear, the middle ear, and the inner ear. The inner ear includes the cochlea that plays a very important role in hearing. Recently, the development of an artificial cochlear device for the hearing impaired with cochlear damage has been actively researched. Research has been carried out on the biomimetic piezoelectric thin film ABM (Artificial Basilar Membrane) in particular. In an effort to improve the frequency separation performance of the existing piezoelectric thin film ABM, this paper presents the design, fabrication, and characterization of the production and performance of a partially etched-type ABM material. O2 plasma etching equipment was used to partially etch a piezoelectric thin film ABM to make it more flexible. The mechanical-behavior characterization of the manufactured partially etched-type ABM showed that the overall separation frequency range shifted to a lower frequency range more suitable for audible frequency bandwidths and it displayed an improved frequency separation performance. In addition, the maximum magnitude of the vibration displacement at the first local resonant frequency was enhanced by three times from 38 nm to 112 nm. It is expected that the newly designed, partially etched-type ABM will improve the issue of cross-talk between nearby electrodes and that the manufactured partially etched-type ABM will be utilized for next-generation ABM research.

Improving Efficiency of Dehumidifiers via Nature-Inspired Technology

Even though global warming and humid climate have resulted in an increase of use of dehumidifiers, they are not becoming more common because of high energy consumption. Furthermore, conventional dehumidifier technology finally reaches the limit to increase energy efficiency of water collection. As an alternative, nature-inspired technology may lead to a major breakthrough in the dehumidification performance. In order to improve the efficiency of dehumidifiers, we first analyze the energy consumption of commercial dehumidifiers and then study bioinspired water collection methods adopted by Namib beetles and grass. † Corresponding Author, helim@kimm.re.kr C 2013 The Korean Society of Mechanical Engineers 윤성진 · 송경준 · 박병길 · 김완두 · 강상현 · 이선용 · 임현의 212 에너지 효율 등급 R 값을 표시한다. 이 제도의 목적은 소비자에게 고효율 제품을 구매하도록 유도하는 데 있는데, 미국에서도 제습기의 에너지 운전 효율에 따라 ‘Energy Star’ 라벨은 부착할 수 있도록 프로그 램을 운영하고 있다. 이 프로그램은 국내와 달리 등급이 없고, 에너지 효율이 기준을 충족하는지 여부만 판단한다. 또한, 국내 시민단체에서도 국내 제품의 경쟁력 강화 및 에너지 소비율을 낮추기 위해 제습기의 에너 지 효율을 좀 더 높일 것을 주장하고 있다. 이런 상황에서 제습기의 성능을 증대시키고 에너지 소비량 을 줄일 수 있는 방법 중의 하나가 자연모사 기술을 적용하는 것이다. 자연모사기술(Nature-inspired Technology)이란 자연의 생태계와 자연 현상 그리고 살아있는 생명체의 기본 구조, 원리 및 메커니즘에 서 영감을 얻어 공학적으로 응용하는 기술을 말한다. 최근 지속가능한 발전을 추구하면서 고효율·친환 경 기술개발을 위하여 자연모사공학이 많은 관심을 받고 있다. 왜냐하면, 자연은 기술적으로 생산된 공 정이나 제품보다 현저히 적은 물질과 소량의 에너지를 소비하면서도 고효율과 고성능의 최적시스템을 항상 유지하고 있기에 지속가능한 기술개발에 최적의 해답을 제공할 수 있기 때문이다. 에너지를 사용하지 않고 공기 중의 수분을 포집하는 기술은 지구상에서 가장 건조지역으로 알려진 대 륙의 나미브 사막의 동·식물로부터 배울 수 있다. 이곳에 사는 생물들은 긴 시간 동안 건조지역에서 생존하기 위하여 공기중의 수분을 수자원으로 활용하는 지혜를 가지고 있으며 그들의 구조와 행동양식 을 진화·발달시켜 왔다. 나미브 사막에 사는 딱정벌레와 잔디가 가장 대표적인 예인데 Fig. 1(a)에서 보여 주는 딱정벌레는 등껍질을 이용해 아침 이슬을 수집하여 식수로 활용하고 있으며, Fig. 1(b)의 자생식물인 나미브 사구 부시맨 잔디(Stipagrostis sabulicola)는 잎 표면을 통해 안개로부터 물을 수집하여 생존한 다. 이들이 가지는 이러한 지혜를 제습기에 도입한다면 에너지를 덜 소모하면서도 효율적으로 공기 중의 수분을 포집하여 제습기의 개발이 가능할 것이다. 본 논문에서는 현재 사용 중인 10 개 상용제습기의 제습 성능을 소비전력, 풍량에 따라 분석하고, 사 용 조건에 따라 그 성능이 어떻게 변하는지 살펴 보았다. 그리고 이들의 제습성능을 높이고 소비전략을 낮추기 위하여 자연모사기술이 어떻게 응용될 수 있는지 기술개발 되고 있는 연구현황을 알아보고, 앞 으로의 기술 발전방향을 전망해보고자 한다.