Sound absorption with variable acoustic resistance means

DETAILED DESCRIPTION It is an object of this invention to maximize the attenuation of sound by varying the acoustic resistivity of nonlinear duct acoustical linings to optimize lining resistance throughout the range of changing conditions encountered thereby.
The present invention contemplates a system for adjusting the noise absorption quality of an acoustical lining by changing its acoustic properties so that its resistance will be optimum for the sound level and grazing flow conditions encountered by providing means for imposing a selected steady flow of air through the material.
In accordance with the present invention, a system for varying the acoustic resistance of an acoustical lining disposed in an air propulsor duct comprises a nonlinear sound suppression liner having a porous facing sheet and a substantially close-ended, side and bottom enclosure member defining a cavity and having an open top surface, the facing sheet extending over said top surface and a plurality of sidewalls extending between the facing sheet and the enclosure member in the cavity to divide the cavity into a plurality of cells, header means communicating with each of the cells, and air pumping means for forcing a predetermined steady flow of air sequentially through the header means, the cells and the facing sheet to vary the acoustic resistance of the facing sheet to make it linear for a selected sound level and grazing airflow condition in the duct.
The duct lining may be constructed from a porous sheet overlying either channel or cellular sound absorption cavities dead-ended by an impervious hard back wall with means provided for conducting air through apertures formed in the hard back wall and/or adjacent channels and/or slots in the cell walls.
Some of the advantages of the present invention are: 1.
Changes in liner acoustic resistance because of contamination of the duct lining due to dirt, oil, etc.
, may be compensated for by reducing the amount of airflow.
2.
For applications for which the optimum acoustic properties are not too well known, they may be empirically determined after installation by varying the throughflow until a maximum attenuation is achieved