Page:Elementary Text-book of Physics (Anthony, 1897).djvu/184

 CHAPTER III.

VIBRATIONS OF SOUNDING BODIES.

143. General Considerations.—The principles developed in § 133 apply directly in the study of the vibrations of sounding bodies. When any part of a body which is capable of acting as a sounding body is set in vibration, a wave is propagated through it to its boundaries, and is there reflected. The reflected wave, travelling away from the boundary, in conjunction with the direct wave going toward it, produces a stationary wave. These stationary waves are characteristic of the motion of all sounding bodies. Fixed points of a body often determine the position of nodes, and in all cases the length of the wave must have some relation to the dimensions of the body.

144. Organ Pipes.—A column of air, enclosed in a tube of suitable dimensions, may be made to vibrate and become a sounding body. Let us suppose a tube closed at one end and open at the other. If the air particles at the open end be suddenly moved inward, a pulse travels to the closed end, and is there reflected with change of sign (§ 133). It returns to the open end and is again reflected, this time without change of sign, because there is greater freedom of motion without than within the tube. As it starts again toward the closed end, the air particles that compose it move outward instead of inward. If they now receive an independent impulse outward, the two effects are added and a greater disturbance results. So, by properly timing small impulses at the open end of the tube, the air in it may be made to vibrate strongly.

If a continuous vibration be maintained at the open end of the tube, waves follow each other up the tube, are reflected with