No abstract

Performance of scale model fan noise experiments in a unique free-field heavy gas test facility required that a method be developed for determining microphone response in the high molecular weight gas. Using an electrostatic actuator, calibration experiments in air, argon, helium, and freon-12 were conducted at several gas pressures. Results indicated that microphone pressure response is essentially independent of the test gas composition. At diaphragm resonance frequency, where the greatest sensitivity variation was observed, changes due to gas composition were only about 1 dB. Apparently diaphragm damping is governed by pressure rather than gas density. This result obviates the need for microphone calibration in gases other than air.

No abstract

Spectral broadening of discrete-frequency compressor noise was observed during a series of noise measurements for small compressors. The phenomenon appeared to be similar to “haystacking,” which results from turbulence in the inlet flow. Measurements made in a long flow-measuring inlet duct indicated that near the compressor the blade-pass noise was discrete frequency in character, while at some distance upstream it was spectrally broadened, having a bandwidth (at −6 dB) up to 15% of the tone frequency. Thus, the broadening occurs in propagation rather than generation. The origin of the spectral broadening was found to be random frequency modulation of the compressor tone resulting from propagation through orifice-generated turbulence. Analysis and discussion of the broadening process is presented.

Definitive experimental knowledge of discrete-frequency noise reduction, which can be obtained in an aircraft turbofan engine through increased axial spacing between the fan rotor and outlet guide vanes, has been lacking. An experimental scale-model research program has been undertaken to fill this void. Using a unique heavy-gas scale-model anechoic test facility, rearward radiated discrete frequency and broad-band noise was investigated for a scale model fan. Data has been obtained for fan tip Mach numbers of 0.7–1.4, and rotor to OGV spacings of 0.13, 0.5, 1.0, and 2.0 rotor chords. Discrete frequency noise reductions up to 17 dB were obtained. At close spacing, discrete frequency reduction was at a rate greater than 6 dB per double spacing. Relative harmonic content was not influenced by spacing and reduction was independent of rotor speed. No significant change in broad-band noise was observed.