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Loudspeaker Placement for Enhanced Monitor Sound Field and Increased Performer Source Positioning

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When handling the electro-acoustics in a church, reverberation time is often long enough to make the reverberant and delayed sound field irritating and confusing for the performer, typically a singer or talker. This is often compensated by using monitor loudspeakers placed on stage, facing the performer. The sound field from this monitoring system will then be reflected in the wall behind the performer and will decrease intelligibility for the audience due to these reflections, especially in the area close to the stage. If the wall behind the performer is soft or absorbent, this is not a problem, but in many churches and auditoriums, the podium floor is hard and the wall behind the performer is hard. By mounting loudspeakers on the wall, facing the audience, the monitoring aspect can be resolved, and other advantages can be achieved at the same time, like increased localization. Since the performer's sound field is not very loud, the position is often given by the loudspeaker system, thus negatively affecting the localization of the performer. This will result in a lower intelligibility, especially for persons with decreased spatial hearing. By using loudspeakers behind the performer, a well defined wave front is accomplished, creating a clear localization. The next set of loudspeakers are then adjusted to stay withing the 8-10 dB sound level that are stipulated by the so called 'Haas Effect.' Haas found that the second wave front will not contribute to the direction, given that the sound field stays within the 8-10 dB and is delayed about 25 ms, measured at the listeners position relative to the first wave front. This effect can be achieved given that sound level and time delay stays within these psychoacoustic boundaries, at most positions in the auditorium. This idea has resulted in a patent, and an implementation in several churches in Sweden, one of them being a church in Bankeryd, Sweden. A large series of measurement have been performed, consisting of several hundred measurements, quantifying the effect this innovative placement of the loudspeakers, analyzing the subjective effects but also the RASTI performance. This paper analyzes the background to the sound challenges normally found in many churches and auditoriums, and how some of these can be handled given a different loudspeaker placement. The paper also describes the results accomplished, and what other possible side effects could occur, like decreased RASTI values (RApid Speech Transmission Index), despite a better subjective sound and increased intelligibility.

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