Page 34 - Spring2019

Art of Concert Hall Acoustics
dynamics, the spectrum also changes; at higher levels, more overtones are present. These nonlinear radiation characteris- tics are not represented by an omnidirectional sound source, and the directivity of the sound source has a significant im- pact on perception of room acoustics (Hochgraf et al., 2017). This problem becomes more pronounced, complicated, and perceptually important when considering the radiation of an entire orchestra.
In the audience, the listener hears the orchestra spatially, not monaurally. Binaural and directional parameters for spatial impression are helpful, but measurement of these param- eters can be unreliable due to the impacts of microphone calibration and orientation on the parametric calculation. In addition to directivity mismatches between real sources and receivers and those used to measure impulse responses, the frequency ranges of human hearing and radiation from musical instruments are both significantly larger than the ca- pabilities of most loudspeakers used for room acoustics mea- surements. Beyond the inherent limitations of the standard parameters, impulse responses are often measured in unoc- cupied rooms and averaged across multiple seats. Occupied rooms, however, are significantly different acoustically from unoccupied rooms, and acoustical conditions vary signifi- cantly between different seats.
As the significance of these limitations has become clearer, acousticians have developed new ways of analyzing impulse responses to obtain useful information. More emphasis is being placed on visual and aural inspection of the impulse response instead of on parameters that can obscure impor- tant acoustical details. Acousticians also modify standard pa- rameters in a variety of ways, including changing temporal increments, separating calculation for early and late parts of the impulse response, and averaging over different frequency ranges. Although these adaptations may help an individual acoustician make sense of data, it makes it difficult to com- pare halls with each other—one of the primary reasons for documenting parameters in the first place.
New parameters are also emerging, including Griesinger’s (2011) localization metric (LOC) for predicting the ability of a listener to detect the position of a sound source and Bassu- et’s (2011) ratios between low/high (LH) and front/rear (FR) lateral energy arriving at the sides of a listener’s head. The potential utility of objective metrics that correlate well with perception is undeniable, but as more parameters emerge and their application among acousticians diverges, we are continually faced by the question of whether parameters fun- damentally support or suppress excellent design.
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Figure 3. Boston Symphony Hall. Photo by Peter Vanderwarker.
Figure 4. Philharmonie de Paris. Photo by Jonah Sacks.
How Do Perceptual Goals and Parametric Criteria Inform Design?
The form of a concert hall is determined by a variety of factors, including, but not limited to, acoustics. One of the greatest challenges and responsibilities of an acoustician is to educate the design team of the implications of room shape and size, which fundamentally determine the range of pos- sible acoustical outcomes, so that design decisions are well- informed and consistent with the perceptual goals.
Case Studies of Two Concert Halls:
The Influence of Shape, Size, and Parametric Criteria Boston Symphony Hall (Figure 3) and the Philharmonie de Paris (Figure 4) were built over a century apart, and although they are used for the same purpose, they are fundamentally different acoustically.
Constructed in 1900, the shoebox shape of Boston Sym- phony Hall is the result of architectural evolution from royal courts, ballrooms, and the successful, similarly shaped Ge- wandhaus Hall in Leipzig, Germany (Beranek, 2004). It was the first hall built with any quantitative acoustical design in- put, courtesy of Wallace Clement Sabine and his timely dis-