Generally, it is well known that auxiliary power units of the turbine engine type produce a large amount of noise which is a direct result of the high velocities at which air or exhaust gases are moving through the various stages thereof and the interaction of these components with inlet air and exhaust gases. For instance, the noise emissions from gas turbine auxiliary power units are particularly troublesome in aircraft or other installations where personnel are required to be in close proximity while the auxiliary power unit is in operation. At such times, noise levels can exceed values consistent with International Civil Aviation Organization (ICAO) Guidelines for civil aircraft, Occupational Safety and Health Administration (OSHA) Regulations for human safety, and military specifications.
For these reasons, it is most desirable to be able to maintain safe noise levels in and around auxiliary power units to avoid hearing damage where continued human exposure is required. Unfortunately, there have been no truly practical means of modifying auxiliary power unit turbine design without penalties to performance, particularly in cases where efforts have been made to effect a reduction of noise transmission through the air inlet duct of the turbine. In other words, there has remained a need to reduce such noise, including high frequency noise above 4,000 Hz, of the type which is generated by passage of air through the blades in the compressor stage.
Among the efforts to provide noise reduction is that disclosed in Smith et al. U.S. Pat. No. 3,715,009, issued Feb. 6, 1973. Smith et al. is specifically directed to a noise suppression system for testing jet engines which includes both an intake silencer and an exhaust noise suppressor, but the intake silencer is simply a portable acoustical absorptive chamber whose walls are formed to have acoustic characteristics capable of absorbing objectionable high frequencies in intake-generated sound during jet engine testing operations. Clearly, Smith et al. would not be suitable for use during normal operation for attenuating high frequency noise of compressor blades at an inlet duct.
An attempt to eliminate vortex whistle by utilization of a pair of tabs in an intake assembly of a load compressor is disclosed in Linder U.S. Pat. No. 4,531,356, issued Jul. 30, 1985. These vortex disturbing tabs or baffle members are arranged relative to a series of adjustable inlet guide vanes associated with a circular radially outwardly facing inlet opening. However, Linder fails to disclose or suggest a mechanically simplified modular sound attenuation assembly for reducing high frequency noise caused by air passage through compressor blades.
Among other attempts to provide a sound attenuation assembly, or to provide noise suppression of one form or another, are those disclosed in U.S. Pat. Nos. 3,739,872; 3,620,329; and 3,011,584.
The present invention is directed to overcoming the above stated problems and accomplishing the resulting objects.