Journal of Sound and Vibration 2000(1) 352 ORCHESTRAL TIMBRE COMBFILTER-COLORATION FROM REFLECTIONS TOR HALMRAST Statsbygg, Acoustics, p.b 8106 Dep N-0032 Oslo, Norway tor.halmrast@statsbygg.no Coloration is defined as changes in Timbre/”Klangfarbe”. Coloration might be the reason why an orchestra-/opera-hall sounds bad, even if all the common room acoustic parameter show good results. In this paper we will take a closer look at the reflections within the time-intervals commonly used for the room acoustic parameters, investigate their contribution on coloration, and compare our results with psycho-acoustic studies. Part 1 shows that placing reflecting surfaces closer to the orchestra will improve the ensemble- conditions, but might give undesirable coloration, like if the orchestra was “placed inside a small box”. Such a coloration effect is called “Box-Klangfarbe”. In Part 2 it is shown that Coloration can be detected by analysing the Frequency Response of the first part of the Impulse Response between members of the orchestra. Such Through O rchestra Impulse Response (TOR)-measurements must be done with an orchestra on the platform to give information about coloration and ensemble conditions. We will call the difference between the successive dip´s in a comb-filter the “Comb-Between-Teeth-Bandwidth”. It is shown that Box- Klangfarbe is likely to appear only when reflections give a “Comb-Between-Teeth-Bandwidth” that is comparable to Critical Bandwidth. A strong/discrete reflection with a time-delay of about 5-20 msec (Comb-Between-Teeth-Bandwidth of 200-50 Hz) will give Box-Klangfarbe. This time-delay region is called the “Box-Klang-Zone”. It is shown that adding more reflections into this zone will reduce the Box-Klangfarbe. Part 3 shows that the results are valid also for the audience area. Part 4 shows good agreement with psycho-acoustical studies. In earlier studies, however, there seems to be an underlying assumption that all coloration must be “bad”. Part 5 gives some practical/musical comments on “good” and “bad” coloration for orchestra platforms. 1) INVESTIGATIONS IN OSLO CONCERT HALL Our work on orchestra-podium acoustics started with an investigations for the Oslo Philharmonic/Mariss Jansons in Oslo Concert Hall. This hall has triangular shape, a very large ceiling height over the orchestra, sidewalls placed far away from the orchestra, see Jordan [1] or Halmrast [2] for drawings and general information about the 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; (see [3] and App.A): - 7 Flexible Suspended Reflectors, 2,8x2,8m, height 6-7m, over the orchestra, - Flexible Walls at all sides, and - “Tilted Top´s” of the sidewalls. The subjective impressions of these changes were reported in questionnaires by both orchestra and public seats. Sidewalls closer to the orchestra and “Tilted Top´s” on these were appreciated both by the orchestra and the audience. Suspended reflectors over the orchestra were appreciated by the orchestra, but not by the audience. (See App.A and [4] ). Fig. 1 shows that the questionnaire parameter “Klangfarbe”(Timbre) was improved for both the audience and the orchestra when introducing sidewalls closer to the orchestra and the tilted top´s of sidewalls, but reduced for the audience when adding the reflectors over the orchestra. Similar results, indicating that coloration is most likely remarked from the audience, is also given by Ando [5]. Zeon PDF Driver Trial www.zeon.com.tw