19th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 COLORATION DUE TO REFLECTIONS, FURTHER INVESTIGATIONS

Close Reflections might give audible Coloration, both on Orchestra Platforms and in Recording Studios/Sound Control Rooms. This paper gives an overview of Coloration from such close reflections. To investigate the stage acoustics of an Orchestra Hall, one needs to do measurements with the orchestra on the platform, Trough Orchestra impulse Responses (TOR). The question is: “When will discrete reflections give clear, audible comb-filtering? ” (“Box-Klangfarbe”)? This paper presents new measurements of TOR-impulse responses, and a practical case study, showing how the TOR-impulse responses have changed due to improved design of an over-orchestra reflector in a concert hall. INTRODUCTION ORCHESTRA PLATFORM ACOUSTICS My first interest in podium acoustics came from playing on the stage of the Oslo Concert Hall. Most reference data and measurements regarding podium acoustics are given for empty stages. The orchestra is often, for practical/economical reasons, not in included when measuring stage acoustic parameters like ST. My measurements for the Oslo Phil./Mariss Jansons in several high profiled concert houses showed that the orchestra has a mayor influence on the acoustics on stage. Stage Acoustics should be measured with the musicians present on stage. COLORATION DUE TO REFLECTIONS A distinct/discrete reflection arriving after the direct sound will always give a Comb-filter. For investigations of Comb-filters Comb-Between-Teeth-Bandwidth is defined as the distance in Hz between two successive dip’s of a comb-filter FFT. Comb-Between-Teeth-Bandwith For a single reflection with a time delay of Δt msec, this “Comb-Between -Teeth-Bandwidth is 1/Δt. A “short reflection” will give a very broad combfilter, and reflections arriving “late” will give only small “ripples” in the FFT, (but probably echoes in the time domain). [1,12] Coloration due to reflections are important for the correct positioning of loudspeakers, both in halls an in sound control rooms. [12] This paper concentrates on Orchestra Platforms. Measurements for details of stage acoustics should be done with a loudspeaker giving a more “directional” sound than for overall room acoustic measurements. For our earlier measurements, a Fostex SPA-11 was used. For the new measurements in Oslo Concert Hall, a smaller Monitor-speaker was used. For all measurements MLS was used as signal (MLSSA and WINMLS). Measurements of podium acoustics with the orchestra on stage were named TORmeasurements: Trough Orchestra impulse Response. (for me to remember), [1,2,12]. TORmeasurements should be performed between many as many positions on the stage as possible. Understanding that orchestra rehearsal time is expensive, it has been shown, that, for a typical Symphony Orchestra, measurements between the left-most violin1 and the rear-most bassoon might give an overall idea of typical podium acoustics. [1,2,3] TYPICAL IMPULSE RESPONSES with/ORCHESTRA A typical Impulse Response from TOR-measurements on an occupied stage with suspended reflectors or a low ceiling over the orchestra is: [1] 19 INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRID 2