referring to fig1 a rivet setting tool generally designated 10 comprises a pneumatic cylinder 12 for receiving in its annular bottom chamber 13 pressurized air from a source not shown , an upper draw cylinder 14 housing hydraulic rivet pulling means not shown but , for instance , of the general type fully described in the mentioned u . s . pat . no . 3 , 254 , 522 , and a hollow handle portion 16 interconnecting the cylinder 14 to an upper end closure 18 of the cylinder 12 . it may be assumed for present purposes that the tool 10 is essentially as disclosed in the referenced patent except for novel exhaust muffler means hereinafter to be explained , it being understood that the muffler is not limited in use to riveting devices nor to use in the exhaust system of any particular pneumatic mechanism whether manually operable or otherwise . by way of background information , the illustrative tool 10 is adapted to set a tubular rivet ( not shown ) by exerting tension on a mandrel stem until the latter is broken . the considerable force normally required is , in the illustrative tool , derived from a hydraulic supply cylinder 20 ( fig1 ) arranged in the handle 16 , and pneumatic booster means 22 to be described for forcing fluid from the fluid supply cylinder 20 to the draw cylinder 14 to effect a working stroke . though not shown in fig1 the rivet mandrel pulling means may be assumed to be a draw rod having gripping means on one end and an axially reciprocable piston on the other end , the piston being subjected to recoil upon mandrel breakage . as shown in fig1 an air piston seal assembly 24 operable in the cylinder 12 is connected by a rod 26 carrying at its upper end a fluid seal 28 slidable in the hydraulic supply cylinder 20 . air in the cylinder 12 above the piston assembly 24 is vented to the interior of the handle 16 via an aperture 30 . it has been found that , especially when a restrictor 32 provides a small channel for flow in the working stroke and relatively large channels for flow in a rapid return stroke , there is often a need for a noise reducing means , as will next be described , to be associated with the exhaust system of the intensifier chamber 13 beneath the air piston seal assembly 24 . a control trigger 34 ( fig1 ) and associated toggle link 36 are actuated to open a valve for pressurizing the intensifier chamber 13 when a mandrel is to be pulled for rivet setting . the lower end of the link 36 is coupled , in the illustrative arrangement , to the upper end of an exhaust muffler assembly 38 ( fig1 - 5 ) by means of a pin 40 transversely extending through a tube 42 and one end of an internal tubular spring guide 44 ( fig2 - 5 ). the muffler assembly 38 is disposed for communicating with an exhaust passageway 46 ( fig1 ) leading from the pressure chamber 13 . in its relatively open condition shown in fig2 the assembly 38 has one end of a compression spring 48 engaging the pin 40 , and its other end yieldingly engages an axially shiftable flow restrictor 50 preferably having a central orifice 52 , the restrictor then abutting an inner face 54 of a fixed stop or concavo - convex retainer disc 56 secured in an inlet end of the assembly . the arrangement is such that in the relatively open position of the restrictor the orifice 52 is blocked by the face 54 but exhaust air exits from the passageway 46 , flows through laterally opposed openings defined by the inner wall of the tube 42 on the one hand and concave walls 58 ( fig4 ) of the disc 56 and the circular periphery of the restrictor 50 on the other hand , and then through the other end of the muffler , to an outlet 60 formed in the handle 16 . fig5 shows how the restrictor 50 is shifted to a relatively closed or first stage operating position , against resistance of the spring 48 , in response to a rapid increase in air pressure in the intensifier chamber 13 , as for example when the mandrel is broken by tension exerted by the tool 10 . thus the sudden spurt of exhaust air under pressure causes the restrictor to be axially displaced until it abuts a shoulder or end 62 ( fig2 ) of the spring guide 44 , and the air flow is then restricted to the orifice 52 which is of smaller cross - section than afforded by the effective total opening when the muffler was in its relatively open position as above described . this closed position of the restrictor is maintained during exhausting of the high pressure air , and with a reduced sound level , until the compressed spring 48 can overcome the substantially reduced exhaust pressure of the gas flowing from the passageway 46 . then the restrictor is automatically repositioned to the second stage condition shown in fig2 to allow substantially unrestricted flow of the exhaust gas . from the foregoing it will be apparent that this invention not only attenuates sound levels by providing for exhaust in two stages ; it also enables the pneumatically actuated device 10 to function more quietly without increasing to any significant extent the operating time of its cycle . briefly reviewing this cycle , an operator of the tool had initially depressed the trigger 34 to pressurize the chamber 13 and thereby force the piston assembly 24 to the upper end of the cylinder 12 . pressure in the chamber 13 having caused a buildup of hydraulic pressure in the cylinder 14 , the hydraulic pressure and recoil influence at mandrel breaking causes the rod 26 and hence the piston assembly 24 to move toward the bottom of the chamber 13 . resultant high exhaust pressure entering the tube 42 at its lower end displaced the restrictor 50 from relatively open to relatively closed position , i . e . from fig2 condition to that shown in fig5 . compressed air is expelled through the orifice 52 to the expansion chamber provided by the handle 16 to attenuate the sound level of impulse and exhaust noise and escapes to atmosphere via the opening 60 . the operator will have released the trigger when the mandrel was broken . when pressure in the chamber 13 has dropped to some intermediate value , dependent on the strength selected for the spring 48 , the restrictor 50 will be re - seated against the retainer disc 56 , i . e . returned to relatively open position so that air from the chamber 13 can be dumped to atmosphere without incurring objectionable sound .