Are visual intimacy and sonic-envelopment mutually exclusive?
Christopher Blair – full time acoustician, part-time conductor, 2nd time blogger. Following up on our previous discussion of acoustic conditions for orchestral players, comes a discussion of the two most popular ways of arraying the audience for concerts, along with the acoustical and visual advantages and disadvantages typically encountered in each. The question for today’s exploration is how many of these positive and negative attributes are hard-wired into these configuration options, or might there be a ”middle-way” that can achieve the best of both worlds?
For those readers that may be unfamiliar with insider jargon, traditional “shoebox”-shaped concert halls have their design roots in the 18th and 19th Century court ballroom form in which the classical orchestral concert was born. The prototypical shoebox room is tall, narrow, with parallel side walls, and most of the audience is formally arrayed in front of the orchestra. The main floor seating is on the flat or shallowly raked. Overhanging balconies and side tiers are often employed in an attempt to bring listeners closer to the performers.
Positive acoustical attributes most often associated with shoebox halls are an even distribution of sound to all seats, blending of the various orchestral sections into a unified whole, and the sense of being immersed in reverberant energy everywhere in the room. On the negative side of the roster for some of these rooms are distance of some seats from the stage, less than optimal sightlines (some side tier seats in Vienna’s esteemed Musikvereinssaal have no view of the stage), and less clarity than might be optimal for some intricately scored repertoire.
“Vineyard” rooms are, for the most part, a 20th Century development in concert hall design, following an arena concept that wraps the audience around the performers. The result is that the audience is seated closer to the performers, in a less hierarchal arrangement than found in the shoebox model. These rooms are typically wide, with audiences organized in terraces around the orchestra, and whose wall surfaces provide critical early reflections of acoustic energy. Audience seating is more steeply raked than is common in shoebox halls for both visual and acoustical reasons. There is a large seating area located behind the orchestra.
In general, vineyard halls are viewed most positively for the intimate physical relationship between audience and performer, and the strong sonic impact which accompanies that condition. Clarity is typically high, which is a real positive for modern works employing large and enthusiastic percussion sections, but may detract from sectional blending, flattering to earlier styles of music. On the downside, the acoustic experience often varies widely from seating area to seating area, particularly in the seats behind the orchestra when there is a soloist, and reverberation may only be noticeable at the end of stopped chords.
What are the acoustic conditions that create the discrepancies between these perceptions? If you paid attention to Chapter 1 of this series and think “forward masking”, you are on the right track.
A common design feature of shoebox hall is the zone at the top half of the room known as the “hard cap”. The parallel side walls in this zone and their minimal acoustic absorption give rise to a localized extended reverberation time. Once this resonant reservoir of acoustic energy becomes excited by a transitioning soundwave, it can take a long time for this energy to decay and propagate back downward to the seats. The result is a very audible and enveloping reverberant response, which also serves to even out spatial variations in the room and promote sectional blending.
In addition, because the direct propagation distance from the orchestra to the listener are usually longer, and the lower side walls not shaped to direct early reflections in preferential ways, the early energy arrival at your seat in a shoebox hall is likely to be somewhat less powerful than may be found in vineyard halls. This, in turn, renders the later energy easier to hear.
The early-to-late energy conflict and associated auditory masking found in many vineyard halls may be traced back to a single basic acoustic design philosophy: that early energy is good (true) and that more is better (not so true, as it turns out). Vineyard halls today are designed with their wall and ceiling surfaces carefully oriented to direct strong first order reflections into the various seating areas. This design technique serves to “ground” early energy in the audience, thereby increasing loudness and clarity. The louder the early energy, the less audible the consequently weaker late energy will be. There is no free lunch when it comes to apportioning energy over time.
Moreover, musical instruments are directional radiators, and if the discrete reflecting surfaces are not aligned in a particular way, spatial variations in the strength and tonal color of various orchestra sections are observed.
One possible method for combining the best physical and acoustic attributes of both types of rooms might be to redirect some of the very early reflected energy characteristic of a vineyard-style room design upward, away from the audience, and into a tall hard cap zone similar to those found in shoebox rooms. This redirected energy then can become part of the late sound field. Removing even a little early energy in this way would rescue a great deal of pleasing late energy usually consigned to inaudibility.