In this paper, the computer modelling application based on the modal expansion method is developed to study the influence of a sound source location on a steady-state response of coupled rooms. In the research, an eigenvalue problem is solved numerically for a room system consisting of two rectangular spaces connected to one another. A numerical procedure enables the computation of shape and frequency of eigenmodes, and allows one to predict the potential and kinetic energy densities in a steady-state. In the first stage, a frequency room response for several source positions is investigated, demonstrating large deformations of this response for strong and weak modal excitations. Next, a particular attention is given to studying how the changes in a source position influence the room response when a source frequency is tuned to a resonant frequency of a strongly localized mode.
A simple analytical method is developed to estimate frequencies of longitudinal modes in closed hard-walled ducts with discontinuities in a cross-sectional area. The approach adopted is based on a general expression for the acoustic impedance for a plane wave motion in a duct and conditions of impedance continuity at duct discontinuities. Formulae for mode frequencies in a form of transcendental equations were found for one, two and three discontinuities in a duct cross-section. An accuracy of the method was checked by a comparison of analytic predictions with calculation data obtained by use of numerical implementation based on the forced oscillator method with a finite difference algorithm.
Elżbieta M. Walerian, Ph.D., D.Sc., a retired employee of the Institute of Fundamental Technological Research of the Polish Academy of Sciences (IPPT PAN), passed away after a serious illness, on the 26th December 2013. She was one of the scientific leaders in the Section of Environmental Acoustics of IPPT PAN and her career, educational and organizational activities were inseparably linked with the acoustics. Elżbieta Walerian was born on August 9th 1950 in Poznań. She graduated from the Faculty of Mathematics, Physics and Chemistry of the Adam Mickiewicz University in Poznań, receiving her Master of Science degree in the environmental acoustics in 1973. Five years later, under the supervision of Professor Ignacy Malecki, she obtained her PhD title, in the physical acoustics, in IPPT PAN in Warsaw. In 1979 she began working at the Section of Environmental Acoustics of IPPT PAN, where she dealt with the diffraction of acoustic waves and a description of the sound field produced by vehicles moving in an urban area.
Reverberant responses are widely used to characterize acoustic properties of rooms, such as the early decay time (EDT) and the reverberation times T20 and T30. However, in real conditions a sound decay is often deformed by background noise, thus a precise evaluation of decay times from noisy room responses is the main problem. In this paper this issue is examined by means of numerical method where the decay times are estimated from the decay function that has been determined by nonlinear polynomial regression from a pressure envelope obtained via the discrete Hilbert transform. In numerical experiment the room responses were obtained from simulations of a sound decay for two-room coupled system. Calculation results have shown that background noise slightly affects the evaluation of reverberation times T20 and T30 as long as the signal-to-noise ratio (SNR) is not smaller than about 25 and 35 dB, respectively. However, when the SNR is close to about 20 and 30 dB, high overestimation of these times may occur as a result of bending up of the decay curve during the late decay.