Survey of human response to control of low-frequency tiltrotor interior noise

Abstract

A preliminary subjective test is conducted in which subjects rank their preference of sounds simulating the interior noise field of a large tiltrotor in forward flight. This noise field is characterized by large-amplitude tonal penetration at the blade passage frequency (<20 Hz) and its first seven overtones. Proposed noise control efforts are simulated by reducing the amplitudes of some or all of these tones. Human response to variations in three noise control parameters is studied: the number of harmonics which are reduced in amplitude, the frequency(ies) of the harmonics which are reduced in amplitude, and the amplitude by which the harmonics are reduced. The results of this test demonstrate that an effective noise control strategy must reduce multiple harmonics, focusing on those which are heard most easily (either the loudest or those with the highest frequency). It is not necessary to control the lowest harmonics, which impose the greatest weight and bulk penalties on a noise control system. It is demonstrated that a harmonic amplitude reduction of only 6 dB may be sufficient to significantly improve passenger response. The results of this test can be used to guide current low-frequency noise control research and development efforts.