Abstract: A method of and a device for attenuating the noise radiated by a free-expion gas jet throughout the audible-frequency range as well as in the ultrasonic and infrasonic frequency ranges. Concomitantly with the complete and shock-free expansion of the gas into the atmosphere, the acoustic field of the jet is organized by the substantial diminution of the low-frequency tones by causing them to pass from the annular section of the jet to the narrow annular section of the nozzle slot. This is followed by sound-wave diffraction on emergence from the slot in the presence of a baffle, and by a wave reflection towards the sound-absorption lining of the device housing. The final noise results from an intensive ejection of ambient air by the primary gas which, leving the nozzle slot, entrains the air by means of turbulence, the air entering through the annular channels of the case, reaching the outer orifices of the nozzle, and dividing into two flows, namely inside and outside the nozzle.

Abstract: A sound attenuating laminate exhibiting a relatively low change in acoustic resistance with respect to particle velocity is disclosed. The laminate includes a gas permeable mat of randomly arranged, small diameter filaments interleaved between two layers of resin impregnated, open weave web or cloth. A predetermined pattern of adhesive applied to the filamentous mat penetrates the mat to bond the laminate into an integral structure. The laminate can be easily fabricated to form sound attenuating panels having a variety of contours by curing the composite within a mold.

Abstract: Sound-suppressing structure comprising stacked acoustic panels wherein the inner high frequency panel is mounted for thermal expansion with respect to the outer low frequency panel. Slip joints eliminate the potential for thermal stresses, and a thermal expansion gap between the panels provides for additional relative thermal growth while reducing heat convection into the low frequency panel.

Abstract: An acoustic duct such as a jet engine fan duct is lined with a plurality of circumferentially spaced strips of sound-absorbing materials which extend helically about the duct axis and which are effective to scatter spinning mode acoustic pressure fields to higher order attenuating modes for improved noise suppression. To maximize noise reduction, the pitch angle of the helix may be aligned to the direction of the wave front generated by the first stage rotor blades of a fan or compressor disposed in the duct when operating at a supersonic velocity relative to the incoming airstream. Alternate strips may be tuned to different frequencies and/or be separated by untreated strips.

Abstract: A combination fan jet engine nozzle plug and muffler. The combination comprises a nozzle plug structure formed from a plurality of inwardly slanted spaced apart elements. The elements form outwardly open and inwardly closed cavities. The cavities' openings are covered by porous skin material attached to the elements.