Sound-attenuating inlet duct

A sound-attenuating inlet duct for reducing acoustic wave propagation, said inlet duct comprising:

• a casing having an inner surface defining, in part, a motive fluid flow path and open at opposing ends to accommodate the passage of the motive fluid therethrough, the casing defining, in serial flow relationship, a minimum area throat of predetermined axial extent through which the motive fluid is adapted to be accelerated to a high subsonic Mach number thereby providing a first quantum of acoustic attenuation, a relatively rapid diffuser, and a cylindrical section;
• and
• sound-absorption material disposed upon the casing inner surface defining the diffuser and the cylindrical section;
• the diffuser and the throat being substantially defined by the relationships:
• L.sub.d /D = 2.6 M - 1.12,
• and
• L.sub.t /D = 1.28 - 1.6 M,
• wherein:
• D = diameter of the cylindrical section;
• L.sub.d = axial extent of the diffuser;
• L.sub.t = axial extent of the throat;
• and
• M = average throat Mach number substantially between 0.7 and 0.8;
• whereby the motive fluid is accelerated to a substantially higher axial velocity proximate the sound-absorption material than the axial velocity at the duct longitudinal axis to thereby turn the acoustic waves toward the sound-absorption material resulting in a normal impact of the waves upon the material and the additive second quantum of acoustic attenuation.

In a method of attenuating noise propagating along an inlet duct in a direction generally opposite to the fluid flow, the duct having an inner wall partially defining a fluid flow path, the steps of:

• accelerating the fluid at a throat to a high average subsonic Mach number;
• rapidly diffusing the fluid along the inner wall which is contoured in such a manner that flow separation from the inner wall is avoided, the axial velocity of the fluid proximate the inner wall exceeding that at the inlet centerline;
• and
• refracting the acoustic waves with the fluid toward sound-absorption material disposed along the inner wall downstream of the throat.