Noise suppression by an acoustically treated three-ring inlet on a TF-34 engine

Abstract

Acoustic performance tests were conducted with a three-ring inlet noise suppressor designed for a TF-34 engine. For all tests the aft noise sources were highly suppressed. The measured inlet suppression was large, reaching levels greater than 30 dB at the peak. Comparisons between the data and performance predictions were in reasonably good agreement. The frequency of peak attenuation was well predicted, while the magnitude and spectral shape were reasonably well predicted. This good agreement occurred when the distribution of sound energy across the inlet was taken into account in the performance predictions. Tests in which the length of treatment was varied showed an orderly progression of attenuation with length. The performance predictions for the different lengths also showed an orderly progression with length. At the highest speed of the engine, multiple pure tones were present throughout the spectrum in the source noise signature. These tones were very effectively suppressed by the inlet liner, even at low frequencies, while the liner was designed to work best at blade-passing frequency.