Vortex sound in bass-reflex ports of loudspeakers : part II. a method to estimate the point of separation

In part I of this paper, the vortex shedding that may occur in a bass-reflex port of a loudspeaker system was discussed. At the Helmholtz frequency of the bass-reflex port, air is pumped in and out at rather high velocities, vortex shedding occurs at the end of the port, and blowing sounds are generated. It was explained that the key in the design of a port with a minimum of blowing sounds is the point of flow separation from the wall at which vortices are formed. This paper presents a method for estimating the point of separation for an unsteady flow like the flow through a bass-reflex port. Assuming that the flow can be described by a potential flow up to the point where flow separation occurs, it was found that the point of separation can be estimated on the basis of measurement of the sound pressure inside the loudspeaker box and measurement of the sound pressure at a distance of 1 m from the port exit. Application of the proposed technique to a cylindrical port with rounded edges at both port ends revealed that the point of separation is determined by the particle displacement rather than by the particle velocity. It was also found that a good indicator of the onset of severe vortex shedding is the Strouhal number based on the radius of curvature of the port edges. © 1998 Acoustical Society of America.