Nb. Roozen et al., VORTEX SOUND IN BASS-REFLEX PORTS OF LOUDSPEAKERS - PART II - A METHOD TO ESTIMATE THE POINT OF SEPARATION, The Journal of the Acoustical Society of America, 104(4), 1998, pp. 1919-1924
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 fr
equency of the bass-reflex port, air is pumped in and Out at rather hi
gh velocities, vortex shedding occurs at the end of the port, and blow
ing 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 separa
tion from the wall at which vortices are formed. This paper presents a
method for estimating the point of separation for an unsteady flow li
ke 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 oc
curs, it was found that the point of separation can be estimated on th
e basis of measurement of the sound pressure inside the loudspeaker bo
x 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 separ
ation is determined by the particle displacement rather than by the pa
rticle velocity. It was also found that a good indicator of the onset
of severe vortex shedding is the Strouhal number based on the radius o
f curvature of the port edges. (C) 1998 Acoustical Society of America.
[S0001-4966(98)01009-1]