Levels Of Spectral Components Research Articles

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

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.

A TERAHERTZ PHASE FREQUENCY CHANGER WITH THE CRYSTAL QUARTZ PHASE SECTIONS

A phase frequency changer (PFC) for quasi-optical transmission line with phase sections made of crystal quartz and polarizers executed on the basis of small-period wire gratings has been considered. The PFC is intended for operation in terahertz (THz) frequency region. It has been examined at the frequency f o= 0.89THz. The influence of the differential phase shift deviation in the sections on the output signal spectrum and the influence of the sections mismatch on the reflection signal spectrum have been considered. In the frequency region ± 10% f o the levels of the spurious spectral components of output signal are less than −40 dB with regard to the level of the useful signal of the shifted frequency. The levels of all spectral components of the reflection signal are less than −60 dB with regard to the level of the useful component of the PFC output signal.

Measurement of interference-pattern parameters of the near acoustic field of a deep-sea noise source

We present the results of a study of the space-frequency energy distribution of the near acoustic field of noise radiation from a moving source. Dynamic spectrograms of the received signal of noise radiation in the frequency range of 0–2.0 kHz at distances of up to 1300 m are given. It is shown that the variation in the levels of spectral components of the signal due to interference modulation in the near field can attain 12 dB. The interference pattern is shown to occur over the entire range of considered distances of 100–1300m. Within the framework of ray theory, using hydrological data for C(z) in the region of measurements, we calculate the near field for a lumped noise source. The calculation results are in good agreement with the results of field experiments. It is shown that the sound velocity profile C(z) in the region of measurements has a pronounced influence on the interference pattern of the near field. Allowance for C(z) in the calculations of the near acoustic field within the framework of the existing theory allows one to predict the interference pattern and choose optimal distances for measurements under near-field conditions.