Abstract
A narrow canal is sometimes used in order to shield the platform on which a delicate instrument is placed from the disturbing vibrations caused by ground noises. The shielding effect of the canal was examined in a model experiment using an ultrasonic technique. Whenthe canal is as deep as 3/4 of the wave-length, the maximum amplitude of an elastic pulse decreases to 1/3 of that in the case without a canal. Several other types of canals were also examined. Even when the canal width is enlarged up to half of the wave-length, the shielding effect is not so. much improved in comparison with the case of a narrow canal. The increase in the number of narrow canals is also found to be not effective. When two narrow canals are dug at an interval of half of the wave-length, the maximum amplitude is about 70% of that in the case of a single canal. In actual cases, therefore, canals of any type scarcely serve the purpose of shielding.
