Noise Control for NACA Supersonic Wind Tunnel

To reduce an intense noise with frequency components extending from 5 to 10 000 cps due to the burning of a jet engine in the test section of a supersonic wind tunnel, a special muffler was built having an open cross-sectional area of 600 square feet. For frequencies below 11 cps, large Helmholtz type resonators were used. Between 11 and 20-cps intermediate-sized Helmholtz resonators were employed. Between 20 and 800 cps a tuned set of six Fiberglas lined square ducts, were constructed. At higher frequencies two lined bends were employed to reduce the noise. Performance data measured in models and in the full scale wind-tunnel are presented. The result is a wind tunnel that is so quiet that nearby listeners are unaware of its operation.