The Radiation Force Acting on a Sphere in a Spherical Sound Field

Abstract

Using a method due to King an analysis has been made of the time average forces acting on a rigid spherical obstacle placed in a progressive spherical sound field. The theoretical results are presented. The theory has been verified using a series of hollow glass spheres (0.22- to 3.03-cm radius) suspended by a fine glass fiber in the sound field in air; the deflection of each sphere was measured at known frequencies (300 to 7000 cps) and amplitudes (0 to 1400 dynes/cm2) of the sound field. Care was taken to protect the sphere from any gas streaming to or from the source and to insure that the field was accurately spherical. It is found that, as the source of the field is approached, the radiation force of repulsion decreases to zero and then becomes a force of attraction. The distance to which this region of attraction extends from the source is determined both by the frequency of the field and by the size of the detecting sphere—lower frequencies and smaller spheres both extend the region. Even when allowance is made for an inverse square law the attractions may by many times greater than the repulsions under similar conditions of frequency and intensity.