Page 36 - Spring2019

Art of Concert Hall Acoustics
Where do we go from here? It can be tempting to conclude that all new concert halls should be shoebox shaped and that the acoustics in more complex geometries remain an unsolv- able mystery. But as long as architectural and public demand for creative room shapes continues to grow, we must keep pursuing answers.
Two Emerging Design Trends from
Recent Research and Experience
The importance of lateral reflections for spatial impression is well-understood (Barron, 1971; Barron and Marshall, 1981; Lokki et al., 2011), but more recent research has shown that these reflections are also critical to the perception of dynam- ic responsiveness. Pätynen et al. (2014) have recently shown that lateral reflections increase the perceived dynamic range by emphasizing high-frequency sounds as the result of two important factors: musical instruments radiate more high- frequency harmonics as they are played louder and the hu- man binaural hearing system is directional and more sensi- tive to high-frequency sound that arrives from the sides. If lateral reflections are present, they will emphasize high-fre- quency sound radiated from instruments as they crescendo, and our ears, in turn, will emphasize these same frequencies. If they are not present, then the perceived dynamic range will be more limited. Increased perception of dynamic range has also been shown to correlate with increased emotional re- sponse (Pätynen and Lokki, 2016).
Building on these developments, Green and Kahle (2018) have recently shown that the perception threshold for lateral reflections decreases with increasing sound level, meaning that more lateral reflections will be perceived by the listener as the music crescendos, further heightening the sense of dynamic responsiveness. From an acoustical design per- spective, it is easier to provide strong lateral reflections for a larger audience area in a shoebox hall by simply leaving large areas of the lower side wall surfaces hard, massive, and flat. In a vineyard hall, the design process is more difficult because individual balcony fronts and side wall surfaces are smaller and less evenly impact the audience area.
The role of diffusion has been hotly debated in architectural acoustics for a long time. A diffuse reflection is weaker than a specular reflection and scatters sound in all directions. Diffusion is helpful for avoiding problematic reflection pat- terns (such as echoes or focusing effects) without adding un- wanted sound absorption. It can also be helpful for creating a more uniform late sound field (such as in the upper volume of Boston Symphony Hall). Haan and Fricke (1997) studied
the correlation between estimated surface diffusivity and overall acoustical quality perceived by musicians playing in 53 different halls. As a result of the high correlation that they found, as well as design preferences of many acousticians and architects at the time, many halls built in the last two decades have a high degree of surface diffusivity. Not all of these halls have been regarded as acoustically successful, particularly when the articulation has all been at the same physical scale (meaning that surfaces diffuse sound in a narrow range of frequencies) and when the diffusion has weakened lateral reflections that we now better understand to be critical to multiple perceptual factors.
The title of a recent presentation is particularly illustrative of the growing opinion among acousticians who caution against the use of too much diffusion: “Halls without qualities – or the effect of acoustic diffusion” (Kahle, 2018). Although the tide seems to be shifting away from high surface diffusiv- ity and there is more evidence to substantiate the need for strong lateral reflections, there is still limited evidence from research to explain exactly how diffusion impacts the listen- ing experience.
How Will Concert Hall Acoustical
Design Change in the Future?
In parallel with applying lessons learned from existing halls, the future of concert hall acoustical design will be trans- formed by the power of auralization. An auralization is an aural rendering of a simulated or measured space, created by convolving impulse responses with anechoic audio record- ings, played over loudspeakers or headphones for spatially realistic listening. Auralizations have been used in research and limited design capacities for several years, but recent technological advancements associated with measurement, simulation, and spatial audio have the potential to leverage auralization for more meaningful and widespread use in the future, potentially answering previously unresolved ques- tions about concert hall acoustical perception and design. Rather than averaging and reducing impulse responses to single number parameters, auralizations strive to preserve all the perceptually important complexities and allow acousti- cians to make side-by-side comparisons with their best tools: their ears.
Auralizing the design of an unbuilt space requires simu- lating its impulse response. Commercially available room acoustics software currently relies on geometric simulation methods that model sound waves as rays, which is a valid ap-
34 | Acoustics Today | Spring 2019