This paper analyses the changes in transfer characteristics of the vocal tract when closed by a mask, i.e. a chamber. The analysis was performed in two ways: by analytical estimation and by measurements in the vocal tract physical model for the case of mask with inner volume V = 430 cm3, corresponding to the oxygen masks used in combat airplanes. It was shown that closing the vocal tract with a mask cavity increases the first formant frequency by about 10% in front and high vowels (/e/, /i/, and /u/) and the frequencies of the first two formants by about 5% in the remaining two vowels (/a/ and /o/). It was also revealed that longitudinal and transversal resonances in the mask chamber can lead to errors in the recognition of the vowel formant frequencies. The results point to the need for additional knowledge about resonances in mask application.
Problems associated with designing silencers are presented. Results of direct tests of silencers for cooperation with systems of axial fans, as well as results of numerical tests of a two stage acoustic silencer, are given. The numerical tests enabled determining the distribution of acoustic field inside the silencer and in the surrounding area. In those tests A sound insertion losses for different variants of installation inside the silencer, as well as for two different types of absorbing material used to fill the silencer walls, were determined. Impact of design features of silencers on effectiveness of noise reduction is described. Also, a technical sketch of a universal silencer with significant noise reduction (DipS = 39:1 dB) which can be successfully used in many ventilation systems is presented
The present paper is comparing the results of research studies carried out for three road acoustic screens of different design and different number of damping layers. For the tests, we selected timber or steel screens with a traditional multilayer structure and also one innovative type of simplified design. With respect to particular panels, their sound absorption properties were investigated in the reverberation chamber after they had been subjected to simulated weathering. In the process, two screens were subjected to the aging tests of 50–500 cycles in a special climatic chamber, and the innovative screens were subjected to 1000 cycles. The procedure was repeated every 50 or 100 cycles in order to obtain the changes of acoustic characteristics. The changes taking place in the absorbing material were also investigated with the use of scanning electron microscopy method (SEM). Basing on the obtained results and on the statistical analysis, the capability to maintain acoustic properties by the panels during their service life in natural conditions was estimated. For that purpose, linear statistical models were worked out, which were then applied to estimate the value of the single number sound absorption coefficient after successive aging cycles as well as the predicted time periods of acoustic class changes.