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Fig. 4. Rates of nerve firings for the direct sound and build-up of reflections in Boston Symphony Hall, front of first balcony, row A, seat 23.
Fig. 3. Nerve firing rates for the direct sound and the build-up of reflections in unoc- cupied Boston Symphony Hall, row DD, seat 11.
The direct sound is weaker here—but there are no strong early reflections. The ratio of areas is +2.2dB, and localization is better than in row DD on the floor. (See Fig. 3) The localiz- ability predicted by the ratio of areas is poorer than my sub- jective impression in the fully occupied hall—but the differ- ence in the way the reflections build up is easy to see.
Mitigation
Halls need not sound either muddy or too dry. Some of the old shoebox halls, and almost all of the new ones, lack the coffers and niches that make BSH work. But it is possible to add elements that perform the same job. Plastic saucers or cloud elements of variable size over the orchestra can be arranged to reflect frequencies above 1000 Hz down into the orchestra and the front rows of the audience, while letting lower frequencies excite the upper volume of the hall. The high frequencies will be absorbed, increasing the high fre- quency D/R in the rear of the hall without changing the reverberation time. Clarity in the rear will improve. The direct sound is strong in the front, and the prompt early reflections will be appreciated. Beams and columns added to the side walls perform the same function—namely they reflect the high frequency portion of the lateral reflections back to the front, giving people in the rear more time to detect the direct sound.
But one needs to be careful. It is possible to reduce the early reflections too much, or make a hall too wide. There needs to be enough energy between the direct sound and the bulk of the reverberation to prevent the brain stem from detecting the reverberation as a separate foreground event— an echo. The marvelous Concertgebouw in Amsterdam, with substantially greater width than BSH, is at the limit. Orchestral music is gorgeous. The sound is both clearer than BSH and more reverberant. But in some seats during a piano performance the reverberation is heard disconnected from the notes.
In smaller halls all the reflections come sooner, and the reverberation builds up more quickly. In small halls the
sound is loud and muddy almost everywhere, especially with student orchestras playing modern instruments. Such halls are also perceived as too dry, as the volume is not large enough to allow a strong late reverberation. The owners of such halls are very reluctant to add absorption, as this will make lower the reverberation time. But adding absorption to the stage can be surprisingly effective. Vaudeville stages
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Clarity, Cocktails, and Concerts 21
