Digital evaluation of the acoustics of small models: The midas package

Abstract

Compared with more fashionable computer modeling, acoustical modeling is far from being superseded. In fact, there is often no alternative to acoustical modeling when designing an auditorium. Careful analysis of the laws of similarity shows that there remains some possibility of improving the acoustical testing of small models. This improvement does not solely concern transducers but the processing of impulse responses as well. Digital processing provides both accuracy of computation and compensation for air attenuation. Based on these principles, a computer-aided testing plant was designed and a computer package was written to drive it. A modular structure was selected as giving the maximum flexibility. Thus a digital evaluation of objective room-acoustical parameters can be achieved within a few minutes. So far, computer-aided acoustical modeling has proved well adapted for research and designing purposes.

Numerical Compensation of Air Absorption of Sound in Scale Model Measurements

“…Application of a scale model implies that certain rules defined by the laws of acoustical similarity have to be fulfilled (Xiang, Blauert, 1993;Picaut, Simon, 2001;Polack et al, 1989). In this way, it is possible to have the sound field in a scale model identical or similar to the field in the full scale enclosure (space).…”

“…This excess of air absorption can be scaled or compensated for by applying different procedures. The usual procedures are related to filling the model by nitrogen (Gade, 2007;Xiang, Blauert, 1993) or drying the air to about 2% of relative humidity (Gade, 2007;Polack et al, 1989). While some authors prefer these procedures, the others point out the procedures' limitations (they require special equipment and can be time-consuming; in addition, the accuracy of the air absorption compensation is not sufficient) (Picaut, Simon, 2001;Polack et al, 1989).…”

“…As an attempt to overcome the limitations of the previously mentioned procedures, an alternative procedure called computer compensation was proposed more than 20 years ago (Polack et al, 1989). This computer (numerical) compensation is based on the signal processing, where the energy response is compensated by the fast Fourier transform (FFT) (Polack et al, 1989).…”

“…The rules are related to the lengths, time, and frequency, but also to the absorption of sound in the fluid (air) filling the model and full scale. Thus, the air absorption in the model, α m , should be S times greater (S is the scale factor) than the absorption in the full scale, α, at the corresponding frequencies, f , (Xiang, Blauert, 1993;Polack et al, 1989) α m (Sf ) = Sα(f ).…”

“…Apart from the important advantages, this compensation shows certain drawbacks and limitations. Some of them are caused by the increase of background noise (Polack et al, 1989;Akil et al, 1994). In order to eliminate this artifact of the compensation, a modified approach to the numerical compensation is proposed and analyzed in this paper.…”