Abstract-Parametric modeling of room acoustics aims at representing room transfer functions (RTFs) by means of digital filters and finds application in many acoustic signal enhancement algorithms. In previous work by other authors, the use of orthonormal basis functions (OBFs) for modeling room acoustics has been proposed. Some advantages of OBF models over all-zero and pole-zero models have been illustrated, mainly focusing on the fact that OBF models typically require less model parameters to provide the same model accuracy. In this paper, it is shown that the orthogonality of the OBF model brings several additional advantages, which can be exploited if a suitable algorithm for identifying the OBF model parameters is applied. Specifically, the orthogonality of OBF models does not only lead to improved model efficiency (as pointed out in previous work), but also leads to improved model scalability and model stability. Its appealing scalability property derives from a previously unexplored interpretation of the OBF model as an approximation to a solution of the inhomogeneous acoustic wave equation. Following this interpretation, a novel identification algorithm is proposed that takes advantage of the OBF model orthogonality to deliver efficient, scalable and stable OBF model estimates, which is not necessarily the case for nonlinear estimation techniques that are normally applied.

This report introduces a new database of room impulse responses (RIRs) measured in an empty rectangular room using subwoofers as sound sources. The purpose of this database, publicly available for download, is to provide acoustic measurements within the frequency region of modal resonances. Performing acoustic measurements at low frequencies presents many difficulties, mainly related to ambient noise and to unavoidable nonlinearities of the subwoofer. In this report, it is shown that these issues can be addressed and partially solved by means of the exponential sine-sweep technique and a careful calibration of the measurement equipment. A procedure for estimating the reverberation time at very low frequencies is proposed, which uses a cosine-modulated filterbank and an approximation of the RIRs using parametric models in order to reduce problems related to low signal-to-noise ratio and to the length of typical band-pass filter responses. [7] contain binaural and head-related RIRs, and are useful in hearing-aids applications. Other databases present specific configurations of the microphones, usually arranged into arrays. What is common to all these databases is that they use fullrange loudspeakers, whose frequency response typi-* To whom correspondence should be addressed. Tel: +32-16-321817; e-mail: giacomo.vairetti@esat.kuleuven.be cally has a lower bound of 50-100 Hz. While these databases cover a frequency range sufficient for the development and evaluation of speech enhancement algorithms, information about a significant portion of the modal response of the room is missing. INTRODUCTIONNowadays, home audio systems generally include a subwoofer, which is intended for the reproduction of low-frequency content typically in the region between 20 Hz and 150 Hz. In this frequency range, small-sized typical rooms operate within the modal frequency region [8]. In small-sized rooms, most of the acoustical problems are actually due to poor acoustics at very low frequencies (LFs). The modal resonances are usually well separated, energetic, and detectable by the human ear [9], thus degrading the perceived sound quality. A subwoofer with small enough lower cut-off frequency can even partially excite the so-called cavity mode (i.e. the modal resonance centered at 0 Hz). Therefore, algorithms for home audio system applications, such as room compensation algorithms, should be validated also on

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