having an outer wall sized and configured to seat within the port and an inner wall extending in the elongate direction the inner wall having a circumference which varies along the length of the insert when inserted in a port the insert alters the port resonance so as to vary the acoustic perfoi mance of the loudspeaker application_number US 201514631219 A description fig2 is an isometric cross sectional schematic of an arrangement 2 comprising an acoustic insert 6 seated within an outer tube 4 which is fitted to the enclosure of a loudspeaker not shown the acoustic insert 6 and the outer tube 4 together form an open ended cylindrical tube with a first end 8 opening at the wall of the loudspeaker enclosure and a second end 10 inside the enclosure the insert flares outwardly at the first and second ends with a central portion 14 of smaller radius around the x axis shown than at the two ends the first end of the outer tube 4 has a lip 16 which engages with the wall of the loudspeaker enclosure as is known in the art the insert which is made of low density foam material is also formed with a matching lip 18 which seats on the lip 16 when the insert is fully inserted within the outer tube 4 this prevents the insert from being pushed too far inside the outer tube and instead ensures that the insert 6 is in the correct longitudinal position along the x axis relative to the outer tube 4 also the lip 18 being outside the loudspeaker enclosure is easy to grasp if the insert.