New Method for the Directional Representation of Musical Instruments in Auralizations Felipe Otondo, Jens Holger Rindel Ørsted DTU, Acoustic Technology, Technical University of Denmark email: {fo,jhr}@oersted.dtu.dk Abstract The issue of the representation of sound sources that vary their directional pattern in time in auralizations is introduced. Musical instruments are used as a reference for the discussion of the traditional representations with assumed fixed directional characteristics. A new method for the representation of the spatial sound contributions in time is proposed using multiple-channel recordings and various virtual sources in room auralizations. Possible developments of the proposed recording/reproduction method are described. 1 Introduction The term “auralization” has been coined as an analogous term to visualization – it therefore names the process of rendering audible (imaginary) sound fields. Room auralizations have as main objective a simulation as accurate as possible of the binaural listening experience in a certain location within a modeled space (Kleiner, Dalenback, Svensson 1993), (Odeon 2002). An important factor to be taken into consideration in an auralization is the directional characteristics of the sound source. Musical instruments have a complex directivity pattern, which generates a particular acoustic behavior in a room. The aim of this investigation is to take a closer look at their directivity in the case of a real performance and to provide a better representation of this behavior in room auralizations. 2 Directional characteristics of musical instruments The sound produced by musical instruments involves many different acoustic features that are related to intrinsic characteristics of the instrument. One of these features is the directional characteristic, or directivity, which is the way in which the sound of the instrument is radiated in different directions at different frequencies. The directivity of a musical instrument is affected by the different notes played on the instrument (Meyer 1978), the different performing intensities (Rossing 1990) and the different playing techniques. These changes are different for the different families of musical instruments, due to the complexities of the musical instrument as a multi-resonating system (Fletcher, Rossing 1998). An example of the measured directional characteristics of four isolated notes played on a Spanish guitar at the 500 Hz octave can be seen in Figure 1. In this case the notes were played within two octaves by the performer trying to maintain the same intensity. 3 Musical instruments as sound sources in auralizations When musical instruments are used as sound sources for auralizations, it is important to take into consideration their radiation characteristics in order to have a representation of the sound source in the room model. As already mentioned, musical instruments are sound sources that have a complex directivity which cannot be easily described in a real performance situation. If a fixed directivity pattern per octave were to be considered, such as the case of a loudspeaker, the result would be rather poor and inaccurate. The directivity changes in time would be ignored and the consequence would be a wrong directional pattern with the level at certain frequencies of the particular spectra either emphasized or diminished. Perceptually this will deteriorate the listening experience due to the added colourations. A more accurate representation that will contain the source directivity changes in time is therefore necessary. 4 Improvement of the spatial representation of sound One could offer a better representation of the spatial sonic characteristics of a musical instrument in a room auralization, or of any source that changes its directivity in time, by taking into account the various samples of the sound field created by the source; they are to be used afterwards in the reproduction process. One method of achieving this is through simultaneous anechoic recordings of the musical instruments with microphones surrounding the source in order to capture the sound radiated in different directions. An example of an anechoic 4-track recording of a