Application of active noise control to model propeller noise

Abstract

The potential of using “active noise control” to reduce cabin noise on the external fuselage surface of turboprop aircraft is demonstrated by conducting several laboratory experiments. The principle of active noise control is to reduce the noise radiated from a primary source by superimposing a signal from a secondary source, which is made identical in amplitude but opposite in phase to the primary sound signal. A computer-controlled algorithm was developed to implement this concept in a free field environment, in which the noise from the primary source (eventually the propeller) and the noise measured at several locations on a representative surface (eventually the fuselage) were used to create the input for the secondary source. In the first set of experiments, a single-channel system for delay and amplification of secondary source input was used. For single-frequency experiments, an average noise reduction of 8–14 dB was achieved on the surface in the frequency range of 200–1000 Hz. Next, the concept was applied to the propeller problem, where the sound signal contains many discrete tones at harmonics of the blade-passage frequency. For this purpose, a pre-recorded noise-time history of a 1 10 - scale model propeller was used to drive the primary source. An average noise reduction of about 15 dB was observed at the first two blade-passage frequencies, and 12 dB and 5 dB reductions were observed at the 3rd and 4th blade-passage frequencies, respectively. Finally, the active noise control concept was demonstrated by using a 1 10 th scale propeller and fuselage, installed in an anechoïc chamber with flight simulation facility. A substantial amount of noise reduction was achieved on the model fuselage surface. The noise reduction was further improved by using multi-channel system for delay and amplification for secondary source input. Noise reduction of 8–13 dB was achieved for the first four propeller noise tones at some areas on the model fuselage.