Musical Timbre Combfilter-coloration from Reflections

Coloration is defined as changes in Timbre/ ”Klangfarbe”. Adding a reflection will automatically change the frequency response of a signal, giving some kind of coloration. This might be looked upon as distortion. However, reflections has been a natural part of sound distribution since the Greek amphi-theatres, indicating that some coloration must be acceptable, or even “wanted” , depending of the type of signal/musical material. The question is: “Which reflections give disturbing/unwanted coloration”? Part 1 gives a general overview of combfilter-effect for different time-delays. Part 2+3 gives the main results of a large practical investigation of coloration on orchestra platforms in concert halls. In Part 4+5, these results are compared with psycho-acoustical studies on coloration. 1. COMB FILTERS, GENERAL OVERVIEW A distinct reflection arriving after the direct sound will always give a Comb-filter. For the further investigation of Comb-filters we need to define the Comb-Between-Teeth-Bandwidth as the distance in Hz between two successive dip’s of a comb-filter FF. Comb-Between-Teeth-Bandwith For a single reflection with a time delay of ∆t msec, this “Comb-Between -Teeth-Bandwidth is 1/∆t. A “short reflection” will give a very broad combfilter, and reflection arriving “late” will give only small “ripples” in the FFT, (and probably echoes in the time domain). 1.1) EXAMPLE, SOUND CONTROL ROOM The Impulse response measured from one of the loudspeakers in a well treated , but not perfect soundcontrol-room to the mixing position might look something like this: This Impulse Response does not show any strong/dangerous reflections. The FFT of the whole measurement-time (2 sec) does not show any visual signs of coloration: Psycho -acoustic studies (see part 4 +5) indicate that coloration is formed in the first part of the sound. For this sound controlroom, it is therefore interesting to investigate the FFT of the direct sound + the first reflection (localised to arrive from the mixing console). Therefore, we will “zoom in” on the part of the Imp. Resp. Before the cursor in the figure above, and perform a FFT for this time window: Proceedings of the 2nd COST G-6 Workshop on Digital Audio Effects (DAFx99), Tor Halmrast NTNU, Trondheim, December 9-11, 1999 W99-2 The Comb-Between-Teeth-Bandwidth from this “mixing console-reflection” is some 800 Hz. Examples of other combfilters are given in the Appendix. (Multiple Reflections etc.) Are these combfilter “dangerous” concerning coloration? That of course depends on the type of source, its frequency content etc. First we will investigate the situation on an orchestra platform, then compare our results with psychoacoustic studies with broadband-noise, and at last consider Loudspeakers. 2. INVESTIGATIONS IN CONCERT HALLS Our work on orchestra-podium acoustics started with an investigations for the Oslo Philh./Mariss Jansons in Oslo Concert Hall. Investigations for improving the musicians ability to hear each other were carried out in a full-scale test with the orchestra playing short musical examples, and in sequence introducing reflecting surfaces closer to the orchestra. The subjective impressions of these changes were reported in questionnaires by both orchestra and public seats[1]. The following figure shows that the questionnaire parameter “Klangfarbe” (Timbre) was reduced for the audience when adding the reflectors over the orchestra. (Arbitrary Score scale.) Remarks were given that when introducing the suspended reflectors, “it sounds like if the orchestra is placed inside a bucket or a small box”. The term “Box-Klangfarbe” will be used for this type of coloration. 3. TOR –MEASUREMENTS (Through Orchestra impulse Response) TOR-measurements are MLS Impulse Response Measurements used to investigate the acoustic conditions between different members of the orchestra, taken diagonally across the with the orchestra on the platform.[1] 3.1) TYPICAL TOR-IMPULSE-RESPONSE A typical Impulse Response from TOR-measurements on an occupied stage with suspended reflectors or a low ceiling over the orchestra is : (Oslo Concert Hall)