Acoustic liner

Claim 1

In combination with an engine having rotor blades generating an airflow and acoustic field from the tips thereof, a slant cell cavity perforated acoustic liner positioned about the periphery and adjacent to the plane of rotation of said blades, said liner comprising a backing plate, a perforated face plate, and a layer of partitioned cellular material between said plates, said layer having cellular cavities extending between said plates with the axes of said cavities extending through said plates at angles other than normal thereto, said cavities having walls of a length greater than the spacing between said plates, said walls being inclined in azimuth and pitch relative to the axis of rotation of said rotor blades and in a direction approximately parallel to the expected direction of propagation of sound waves from said acoustic field over said perforated face plate, said liner being tuned to absorb high-intensity sound and sawtooth shock waves in said acoustic field to thereby suppress low frequency noise emanating therefrom.

Claim 3

The method of suppressing with an acoustic liner around the fan blades of a jet engine noise over a relatively narrow frequency range, said fan having a first operating condition of one fan speed and a second operating condition of another fan speed, wherein said fan generates a noise of a first frequency range under said first operating condition and a second frequency under said second operating condition, said method comprising: a. determining the environmental parameters in which said acoustic liner is to operate, including the angular direction of propagation of wAves from said fan blades on said liner, b. providing said liner with a sufficient open area to obtain an absorption coefficient at resonance approaching unity, c. providing attenuation cavities in said liner having liner resonance frequencies substantially equal to said first frequency range and to said second frequency range, and d. positioning the angular direction of walls of said cavities in azimuth and pitch substantially parallel to the angular direction of propagation of said waves from said blades on said liner.

Claim 4

The method of suppressing with an acoustic liner around the fan blades of a jet engine noise over a relatively narrow frequency range, said fan having a first operating condition of one fan speed and a second operating condition of another fan speed, wherein said fan generates a noise of a first frequency range under said first operating condition and a second frequency under said second operating condition, said method comprising: a. determining the environmental parameters in which said acoustic liner is to operate, including the angular direction of propagation of wAves from said fan blades on said liner, b. providing said liner with a sufficient open area to obtain an absorption coefficient at resonance approaching unity, c. providing attenuation cavities in said liner having liner resonance frequencies substantially equal to said first frequency range and to said second frequency range, and d. positioning the angular direction of walls of said cavities in azimuth and pitch substantially parallel to the angular direction of propagation of said waves from said blades on said liner.