Most models for jet noise assume that the turbulence producing the noi
se is far enough removed from any solid surface that the presence of s
urfaces (such as the nozzle itself) has negligible effect on the sound
field. This paper addresses the validity of this assumption. Experime
nts were performed on a low Mach number jet in which the dominant soun
d source, the pairing of vortex rings, was well documented by previous
work [1]. The vortex rings were stabilized spatially and temporally b
y artificial excitation at St(Dj) = 1.14 and became coplanar (one insi
de the other) at x/D-j congruent to 2.5 with a frequency of occurrence
St(Dj) = 0.285. In the current study, the directivity of this source
was measured for various external nozzle geometries. The external nozz
le shape was changed from a conventional conic shape to a flat plate w
hose diameter was then changed by a factor of three to determine how e
xternal nozzle shape and size affected the sound of the vortex pairing
in the jet. To explain the variations in directivity observed with th
e different nozzle geometries, a simple model of the vortex ring pairi
ng was created using Biot-Savart vortex simulations. Vortex sound theo
ry, including surface dipole terms, was applied to this estimate of th
e vorticity field to calculate the resulting dipole and quadrupole sou
nd sources. The dipole sound was of the same order as the freestream q
uadrupole sound. When the phase-average sound field measured in the ex
periments was decomposed into multipole components, the relative stren
gths of the low frequency dipole and quadrupole components were in goo
d agreement with those of the simulation, supporting the general concl
usion that the dipole produced by the presence of the nozzle is not ne
gligible for vortex motions within the first few diameters of the jet,
and supporting the validity of the vortex sound theory itself. The de
composition also unveiled a weaker monopole source, which is seen as e
vidence of subharmonic feedback from the pairing to the jet nozzle, he
lping stabilize successive pairings even though no excitation was prov
ided as these subharmonic frequencies. (C) 1995 Academic Press Limited