an exhaust passageway is provided with a sound muffler comprising an orifice , and means operable against a spring in response to sudden increase of fluid exhaust pressure for relatively restricting the orifice thus to abate the sound of exhausting high pressure gas flow . as the gas pressure to be relieved diminishes in a first stage , the spring automatically enlarges the orifice , in effect , whereby the second stage exhausting is rapidly completed at unobjectionable noise level to permit recycling of a pneumatic machine or tool .