Reverberation Condition Evaluation for Rectangular Rooms with Non-Uniformly Distributed Sound Absorption

The most common cause of reduced diffusion of the acoustical field is extensive sound absorbing applied to the ceiling and the floor, and contrarily walls reflect sound well and they are vertical and poorly subdivided. In such rooms, sound waves propagating in directions close to horizontal attenuate slowly form a horizontal reverberant field. The reverberation time appears to exceed the value calculated using Sabine’s equation. Since this is a common situation in real rooms, one of the authors proposed to calculate RT as a sum of two reverberation processes decaying independently along horizontal and vertical main axes of the hall (R. Neubauer, 1999). The model divides the sound field into two parts considering each as a corrected Eyring value and results in a socalled New Formula or Fitzroy-Kuttruff equation (R. Neubauer, 1999), (R. Neubauer, B. Kostek, 2001). Introduction here are numerous parameters that were developed in order to asess quality of room acoustics (Sabine, 1964), (Beranek, 1992, 1996). t may be said that two main approaches dealing with acoustic quality f a room are based on either objective or subjective evaluation or oth. One of the most relevant sensations of the sound field in rooms s the cognition of reverberation (Vorländer, 1998). In addition room coustical parameters derived from the room impulse response such s clarity, definition or lateral energy fraction are also adequate for ssessing a room intended for speech and music (Beranek, 1996), Vorländer, 1998). On the other hand, it is highly desirable that all alculated parameters are assessed subjectively (Kostek 1999). herefore one of the most important issues in acoustics is to find reations between objectively measured parameters of rooms and their ubjective quality assessed by listeners (preferably experts) (Voränder, 1998). Moreover, it is most important for the acoustical deign of a room to have a tool, which enables prediction of the releant acoustical properties (Neubauer, 1997). 3 Subjective Tests The impulse response of the auditory room of TUG was measured using both directional and omnidirectional characteristics of microphones. The next step was to convolve these signals with anechoic recordings. For this purpose a number of sound excerpts recorded in an anechoic chamber were used. Experts judged resulted sounds. Their task was to rate all signals from 1 to 5 and to choose the signal that was perceived as the best one. In Fig. 1 a view of the auditory hall is shown with positioning of microphones and place in which a source was placed. Correspondingly in Fig. 2 RT measured in this room is shown. In addition Figures 3 and 4 present impulse responses recorded both by omnidirectional and cardioid microphones. In the last attached Figure (Fig. 5) a convolution of impulse response from Fig. 3 and anechoic recording is shown. This recording was judge by experts as the best one. This means that experts’ subjective preference as to modelled reverberation conditions is such as that in case when RT is directional. his paper aim is two-fold. First, a short review on some acoustical rinciples related to the room acoustic assessment is presented. Then roblems related to the acoustical design of rooms in cases most enountered in practice namely rectangular halls where the sound field s not diffuse are discussed. The RT formulae known so far, cannot redict the RT characteristics accurately in halls with non-uniformly istributed absorption. Moreover, the formulae needed in practice ught to be matched to conditions in occupied rather than in empty alls, those being as a rule highly absorptive, i.e. filled with direcional sound field, due mainly to audience presence. RT prediction or such cases is highly desirable for acoustic consultants and archiectural project engineers. Secondly, it is also important to check in hich way and if sourceand microphone-directional characteristics nfluence subjective perception of reverberation. It will be shown that uch an influence exists and experts’ judgement resulted from subjecive tests will be cited. The paper shows selected impulse responses f an auditory room, enabling to assess reverberation subjectively.