Ambient Background Noise under Acoustic Tests of Aircrafts at the Local Aerodrome

Abstract

To obtain the qualitative experimental data about acoustical characteristics of light propeller aircrafts is a relevant problem when conducting the full-scale tests at the local aerodrome. For such tests [1 - 3], it was very important that a desired signal level was significantly higher than the ambient background noise. In this regard, it is relevant to form a databank both of the spectral structure of aerosphere acoustical radiation, corresponding to the ambient background noise for various climatic zones and natural landscapes, and of the basic regularities to form an ambient background noise in a bottom layer of aerosphere. The article describes the authors’ developed procedure of data acquisition about a spectral structure and the procedure to form a bank of statistical data about spectral and integrated characteristics of an ambient background noise. The article provides measurements of spectral characteristics of an ambient background noise taken in conditions of local aerodrome. It shows the normalized graphical matrixes of 1/3- oktave spectra of the sound pressure levels (SPL) of an ambient background noise. The paper also estimates confidence intervals to assess mathematical expectation of the spectral sound pressure levels of a background to have a reliability level of 90 % for various values of the meteorological parameters characterizing aerosphere condition. In the course of tests, the aerosphere parameters were changing within the following ranges: wind speed - 0-6 m/s; temperature - 8- 30оС; relative humidity - 30-59%; air cover condition - from «clear sky» to « cumulus clouds». The obtained data allowed us to find that the acoustical radiation corresponding to the ambient background noise is broadband, within the range of frequencies of 16-10000Hz. In the background noise spectra, it is possible to distinguish, at least, four frequency ranges, which have the notable characteristic frequency-changing levels of spectral components. It may be possible that these frequency ranges match to various sources (or mechanism of sound generation) of a natural origin. It is shown that even in conditions of flat ground, spectral levels of the background ambient noise can be rather unstable even in a time interval, equal to one minute, with no or low (up to 2 m/s) wind speed. And, on the contrary, a change of the spectral levels of sound pressure can be poor in a time even with increasing (up to 6 m/s) wind speed.