Feasibility Study of Phononic Crystal Structure Applied as Underwater Absorptive Material.

• Main Authors: Yi-Hsien Lin, 林宜賢
• Other Authors: Shiuh-Kuang Yang
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/66485928263186368842

碩士 === 國立中山大學 === 機械與機電工程學系研究所 === 93 === “Phononic crystal,” a binary-composite medium composed of a square array of parallel circular brass cylinders in a water matrix is reported. Phononic crystal exists total band-gaps phenomenon which is caused by destructive interference of Bragg reflection in their acoustic transmission spectrum. This Bragg reflection theorem is also a basis for searching the total band-gaps in this thesis. Because of the band-gaps of the phononic crystal, it is very appropriate for applying phononic crystal in underwater absorptive materials. This research presents the Bragg theorem prediction of brass/water acoustic forbidden bands structure with three kinds of different filling fractions, 5 %, 10 %, and 20 %, and three kinds of transducers. Their central frequency are 300 kHz, 500 kHz, and 1 MHz, respectively, and their bandwidths are 210 kHz~390 kHz, 350 kHz~650 kHz, and 700 kHz~1300 kHz, respectively. Furthermore, in order to find total band-gaps, [100] and [110] directions are measured in this research. The band-gaps of phononic crystal in this research are designed by the couple probes of lowest frequencies 300 kHz in our laboratory. Although the devices of underwater acoustics usually operate in 15~200 kHz, it is also proved indirectly that to design and to apply phononic crystal in underwater absorptive materials are workable. In addition, the measurement results of band-gaps of single frequency are the same as broad-band frequencies using ultrasonic analyzer in this thesis. Therefore, it is a good way to survey the band-gaps with broad-band frequencies method first, and then to use single frequency method measuring deeply drop of the band-gaps. This research uses Bragg reflection theorem, to calculate approximate position of band-gaps, and predicts n=1~3 total band-gaps successfully in experiments. It is also proved that using this kind of underwater absorptive materials of phononic crystal has the effect of camouflaging submarine purpose with specific frequencies. This is an easiest theorem to survey band-gaps of phononic crystal, and must be a most useful tool to design all kinds of absorptive materials of phononic crystal.