ACOUSTIC RETROREFLECTORS FOR MUSIC PERFORMANCE MONITORING Heikki T Tuominen, Jussi Rämö, and Vesa Välimäki Department of Signal Processing and Acoustics Aalto University, Espoo, Finland htuo@iki.fi ABSTRACT This paper is concerned with acoustic retroreflectors, which reflect sound back towards any sound source. They are constructed here of two reflecting panels connected with hinges and placed on a hard reflecting floor. Acous- tic retroreflectors can replace electroacoustic monitoring in music performance when sufficiently large panels are placed at an appropriate distance from performers. A good distance is between about 3 and 8 m from a player, corresponding to propagation delays of between approx- imately 20 ms and 50 ms from a player to the retroreflector and back. We have conducted acoustic measurements in an anechoic chamber using various retroreflector structures, including symmetric V-shaped and asymmetric L-shaped reflectors of two different heights with various opening angles and incident angles. Our data show that the 90° opening angle produces the strongest reflection. Surprisingly, increasing the opening angle to 100° or more decreases the magnitude of reflec- tion by more than 10 dB, while a smaller angle, such as 80°, mainly weakens the reflection at high frequencies. User tests with musicians indicate that acoustic retroreflectors can provide the desired feedback in per- formance spaces in which natural reflections to the stage are missing, such as in large halls far away from the walls or outdoors. 1. INTRODUCTION For a performer, some places are easy to play in while others are difficult. This ease or difficulty is related to the reflections of the performance space sending the player’s own sound back to her or his ears [1]. Without such audi- tory feedback provided by the walls or other reflective structures, the performer does not hear her or his playing well, which feels uncomfortable. This influences espe- cially amateurs. Professionals notice this as well, but they are more competent to make adjustments. Through expe- rience they look for a better position to stand or sit and their muscle memory helps them to maintain good ergo- nomics even when the space does not support the sounds they are making. Reflections that are too early or too late do not help (see Fig. 1). This paper considers a light-weight acoustic arrangement corresponding to ‘stage monitor speakers’ used in ampl i- fied music performances. The function of monitoring is for players to hear their own performance and also a mix of other performers. How much monitoring feedback is needed, and how can this be implemented acoustically without building heavy structures on the stage? Can a portable set of good reflectors be made which can be carried by performers themselves to schools, museums, outdoor places, and other environments not allowing a loudspeaker-based monitoring system? Figure 1. Rough division of usefulness of retroreflections in music and speech performances. This paper suggests acoustic retroreflectors, which are constructed of three orthogonal acoustically reflective boards: two boards connected together at a right angle placed on a hard floor, which acts as the third board. Such a retroreflector echoes sound back towards a sound source placed at any angle in its vicinity [1, 2]. The best location for a retroreflector is inevitably in front of the performers, i.e., between the players and the audience. This implies that the construction must not be visually intrusive. The main emphasis in this work is to find low, ramp-like constructions, that can be hidden among chairs and music stands. Furthermore, knowing how far on the side of an ensemble a retroreflector can be installed while still working efficiently, is of interest. This is, in practice, connected to finding out how wide a horizontal range of angles can be covered with one retroreflector, i.e., how large a group of performers can benefit from each reflec- tor. This paper does not consider what the audience hears, although good stage acoustics may greatly improve a music performance. The rest of this paper is organized as follows. Sections 2 and 3 discuss the basics of reinforcing sound reflections and acoustic reflectors. Section 4 explains our arrange- Proceedings of the Sound and Music Computing Conference 2013, SMC 2013, Stockholm, Sweden 443