may have a tubular portion 18 a ring or flange portion 19 and a disc portion 20 while the prototypes for this assembly have been fabricated as illustrated in fig4 with the disc portion 20 being fillet welded to the tubular portion 18 and the ring or flange portion affixed by means of an interference or press fit it is contemplated that the cap may be manufactured as an integral piece by forging or casting it will be seen that rotation of the cap 17 with respect to the sleeve 16 will advance the cap into or out of resonator tube 15 depending upon the direction of rotation as the cap is screwed out of resonator tube 15 the air column is lengthened and the resonant pitch of the air column is lowered conversely if the cap 17 is rotated in the opposite direction so that it advances into the resonator tube the length of the air column is shortened and the resonate resonant pitch thereof is raised it is axiomatic that loose or poor fitting threaded couplings are less expensive than smooth tight fitting threaded couplings loose threaded couplings however cannot be used for tuning adjustment means because the vibrating air column will cause any loose element to vibrate and produce a very undesirable buzzing sound thus it is essential that the threads be smooth running and have no 34 shake 34 so that this undesirable buzzing sound will be avoided it is also necessary that the threads provide a reasonable pneumatic seal between the cap 17 and the resonator tube 15 this necessity arises from the need to have highly efficient acoustical resonators as any.