An apparatus for range-estimating a noise source including a first passive, vertical hydrophone array of at least two receivers at a first distance from the noise source. The container has multiple compartments for holding different materials. The center wire assembly is located in an axial bore of the inner insulator and delivers a second high voltage ignition pulse to a second spark gap. When the plug portion and socket portion are used together, the magnetic bodies and the elastic flakes contact the metal plate and the contact poles at the same time, respectively. As a result, it is possible to improve an element breakdown voltage as compared with the conventional one. A first grooved portion is formed by removing an excess in thickness in a region between dual raceways on an outer circumferential surface of a hub and an inner race which constitute the inner member of the wheel axle bearing unit, for enlarging a space inside the bearing, thereby increasing an amount of grease that can be retained in the region between the dual raceways.