Extraction of helicopter-radiated noise by frequency domain processing

Abstract

Real data observations of farfield radiated noise from an approaching UH-1 helicopter revealed that the received time-domain radiated noise contained impulses. The cited literature attributes these observed impulses to blade slap which is caused by, among other mechanisms, blade vortex interactions. This paper is concerned exclusively with extracting these measured impulses by a frequency-domain processing method. As is well known, the total radiated noise of a helicopter is quite complex. No attempt is made in the paper to discuss the full complexity of the radiated noise field. However, references are cited in the text which support our findings and give a more general treatment to helicopter-radiated noise. The method for extracting impulses generated by an approaching helicopter utilizes a nonlinearity implemented in the frequency domain. It is shown with real helicopter-radiated noise data that autocorrelation estimates of a signal, interfered with by additive helicopter-radiated noise, can be improved by extracting some of the interfering components. The extracted components are also available as an enhanced time-domain representation.