This report is my preview of the eagerly awaited new music hall at the Isabella Stewart Gardner Museum. The new Calderwood Hall is replacing the Tapestry Gallery as the site for concerts. My observations are based partly on a guided tour a few weeks ago during which we heard no music, and partly on my observations from today’s official press opening which included a rehearsal by A Far Cry chamber orchestra.

Calderwood Hall, designed by Renzo Piano and Yasuhisa Toyota, is built into a cube 44 feet on a side. Two rows of audience surround the musicians on the floor. The rest of the seating is in three tiers of four-sided balconies – each only one row deep. Seating capacity is approximately 300, similar to the 330 seats of the Shalin Liu Hall in Rockport, but the designs of these two halls, and their sounds, could not be more different.

Current fashion for concert venues of all types tends toward a shoebox shape – long, narrow, relatively high ceilinged, and with the performers in some kind of a stage house. The goal in such spaces is usually a reverberant sound, often at the expense of clarity. But at the Gardner we are being treated to an entirely different concept. The musicians are on the floor, and the audience surrounds them, as close as is practically possible.

The design of the Calderwood is unusual for our time, but it is not historically unprecedented. Most chamber music was written for performance in small spaces – holding at most a few hundred people, and richly supplied with sound absorbing furniture and fabric. The idea that it should be performed in a reverberant space would have been very foreign. Even churches, particularly Protestant churches with their emphasis on vernacular text, were hung with tapestries and banners that reduced reverberation to low levels. The Thomaskirche in Leipzig, where Bach performed his Cantatas and Passions, is estimated to have had a reverberation time of only 1.6 seconds, a very low value for such a large space. It would have been much easier to hear all the words in that space than in the recent passion performance in Boston Symphony Hall.

There was a need for such spaces – and their existence drove the evolution of music. In the Baroque period music became more highly ornamented, both in chamber settings and in opera. Opera houses were typically U shaped, with audiences in tiers. According to Marshall Long in his excellent book Architectural Acoustics:

‘Baroque music was performed in salons, drawing rooms, and ballrooms, as well as in churches. In general the former were not specifically constructed for music, and tended to be small. The orchestras were also on the smallish side, around twenty-five musicians, much like chamber orchestras today. … The problem of distributing sound evenly to the listener was soon recognized, but there were few useful guidelines. In England Thomas Mace published (1676) suggestions for the designer in his Musick’s Monument or a Rememberancer of the Best Practical Musick..He recommended a square room with galleries on all sides surrounding the musicians, much like a theater in the round.’

The Calderwood Hall takes Mace’s suggestion literally. The advantages are clear, the disadvantages equally clear. Ensembles typically arrange themselves facing a particular direction – often with a conductor. Their instruments are directional, with much of the sound directed forward or upward. Audiences behind them certainly get a different sonic and visual picture. The violins will be softer, the violas louder (sometimes not a bad thing…) Those audience members will see the face of the conductor, the backs of the musicians. With the top off a piano radiates mostly upward – a great benefit to the audience in the balconies, a slight detriment to those on the floor.

I was fortunate today to have several conversations with Yasuhisa Toyota, the acoustician from Nagata Acoustics in Los Angeles who was responsible for the acoustics, and Scott Nickrenz, the artistic director of the music at the Gardner. Nickrenz, an accomplished violist, had always been dissatisfied with the sound of concerts in the Tapestry Gallery. Although I am not personally familiar with concerts there, I have heard many reports of very dry sound, poor sight lines, and a great difference in sound between the front of the hall and the rear.

I asked Toyota and Nickrenz how the idea of the new space arose. Why put musicians on the floor, with audience in three balconies? Nickrenz, perhaps anticipating the question, pulled from a folder a wonderful picture of an early Italian opera house. “This is what I said I wanted”, he said. I was in complete agreement, as I have felt for some time that putting the audience in tiers around the musicians is the best way to give a large number of people the kind of clarity that chamber music demands. At least two halls in this area give credence to the idea: Jordan Hall and Sanders Theater. “But why”, I asked “did you decide to put the audience on four sides of the musicians, and not just three?” Nickrenz explained (with Toyota nodding) that this idea arose at the first meeting between Toyota and Piano at Piano’s home in Paris. Piano had always been fascinated with cubic forms. Nickrenz said some good ideas take a long time to develop. This good idea took only 15 minutes. “Good,” they both decided, “let’s go for it”.

In the Calderwood the goal was to make the sound for each audience member as uniform as possible, giving each both a sonic and visual unobstructed view of the performance. When BMInt visited in December the reverberation time was quite low – about half a second. It is not obvious why the room was so absorptive. The visual walls are made of decorative plywood cut with linear slits. Eighteen inches behind the slits there is a structural wall. There were absorptive curtains in the space between the visual and the structural wall. These were mostly withdrawn into the corners of the structure during our visit, exposing the walls behind. During our December visit the sound in the balconies from people speaking on the floor was clear, highly intelligible, and not reverberant.

In contrast, Shalin Liu is a shoebox, with the musicians occupying a stage area in front of a large glass window. The shoebox has become a standard shape for a music performance space, possibly because of the success of our own Boston Symphony Hall. But Boston Symphony was designed for orchestral music, and has a capacity of 2700 seats. It is not clear that a shoebox is the ideal shape for a chamber music hall with a much smaller capacity. In a shoebox some the sound up close is louder and clearer than the sound in the rear. A shoebox hall also typically has large surface areas that are sound reflecting. These surfaces reflect sound between each other and eventually into the audience, increasing the loudness and adding – in large halls – a warm enveloping reverberation. In small halls these reflections are a mixed blessing, as they come sooner and stronger than they do in a large hall. When there are too many early reflections the sound is muddled, and notes are difficult to separate one from another.

In the front of a large shoebox orchestral hall the sound in is dominated by the direct sound – sound that travels directly from the instruments to the listener. There are typically a great many prompt reflections – both from surfaces of the stage house, and from the ceiling and walls of the hall. These reflections increase the loudness of the sound somewhat as you move back – but they do so at the expense of clarity. The direct sound decreases in pressure by 6dB for every doubling of the distance, so in the rear of the hall the direct sound is weak. Early reflections – those that bounce off only one or two surfaces – are similarly weakened as they travel to the rear. But there are a great many later reflections that have bounced off many surfaces before they reach the listener, and these combine to make the loudness more uniform. For listeners in the middle and the front of the hall it is this late reverberation that gives orchestral sound its warmth and envelopment. But in a small hall we do not need the extra loudness if the audience is close to the instruments. The direct sound is strong enough.

So why do we need reflections in a chamber music hall? Were we happy with the dry sound in the Tapestry Gallery? The answer depends in part on what experience an individual audience member would like. When direct sound is un-muddled by early reflections it is perceived as close to the listener. It demands our attention. Clarity is very high, and with careful listening one can hear every note, and identify which instrument played it. Listeners intent on hearing every note will be delighted – those wanting a less demanding “well blended” sound may be disappointed. Assuming no-one in our distinguished audience is in the latter category, what other purposes do reflections serve, and does the Calderwood deliver them?

Musicians do not like playing in a reflection-free space. They rely on sound from the hall to judge the balance between their own instrument and those of their colleagues. A solo string player needs sound from the hall to judge the effects of bow speed and position on the timbre and tone of their playing. It is also much easier to hear one’s intonation when the sound in the hall can be heard. But reflections that arrive earlier than 50 milliseconds are masked by the sound of their own instruments. All the beneficial reflections arrive later than that – ideally more than 100ms later. Like the perception of warmth and envelopment to an audience member, musicians depend on late reverberation. How can you have late reverberation in a small, absorptive hall?

Turns out – you can. Reports from several musicians that have played in the new hall are quite favorable. A glance at the geometry of the hall explains why. The underside of each balcony makes a right-angle with the wall behind it, forming a corner reflector. Sound striking the underside of the balcony will bounce against the wall behind, and return back to where it came from. If we imagine a musician near the center of the floor, his sound will strike the underside of three balconies in four different directions. Reflected energy will return to him at twelve different times from twelve different directions. If the curtains are removed the corners of the side walls the balcony undersides add an additional twelve reflections. The dimensions of the space are such that the first set of reflections arrives before 50ms, but the later ones arrive well after. There is sufficient strength and time delay in the combination of reflections to give the musicians a satisfactory sense of the hall. But there is some danger: If there are too many of these reflections, clarity will suffer. The audience also benefits, although to a lesser degree. Reflections directed to the floor are not absorbed – at least if the floor is not packed with musicians. The sound will bounce off the floor and back up to the balcony, creating at least some late reverberation.

Audience members also appreciate reflected energy. In a reflection-free space each member of an ensemble is perceived as a soloist, and the group never seems to blend together into an ensemble. This problem is well-known to recording engineers. Closely-miked instruments never sound together unless some early reflections are added. My initial impression in the Calderwood during the first visit was that that there would be sufficient reflections in the time range of 80ms to 100ms to provide a sense of ensemble. This would certainly be true on the floor, where the parallel wall surfaces will provide them. But once again, if there are too many reflections, clarity will suffer.

So – how does the new hall sound? Short answer: it sounds fantastic. Nickrenz is in heaven. I listened to the rehearsal of A Far Cry, the resident chamber orchestra of the Gardner, in many of the balcony seats. (We were shooed away from seats on the floor.) The sound in every seat, according to both Toyota and Nickrenz, was “clear and warm.” (I think they had rehearsed their speech…). I am delighted to concur. One can quibble, and maybe this was my charge in being assigned to this article by BMInt. So here goes.

When we visited in December the curtains behind the perforated walls were mostly withdrawn. The reverberation time was on the order of half a second – although I was not able to measure it. Yesterday the reverberation time was immediately perceived as longer – my guess is about 0.8 seconds, and quite uniform with frequency. (This means the bass response is rich.) The change in reverberation time between the visits is significant. I inquired as to what had changed. The answer from Toyota was that the curtains had been completely withdrawn into enclosed spaces. I worried a bit about this increase, as I had looked forward to unusual clarity in the hall – much like an early opera house. (The Staatsoper unter den Linden in Berlin (currently under renovation) had 1500 seats and a reverberation time of 0.9 seconds). Small spaces lose clarity quickly as the reverberation time increases. But I was relieved when I heard A Far Cry play. Yes, the hall is more reverberant than before, and yes, the clarity has gone down a bit. But it is still good in every seat, and very good in most.

It is worth explaining what I mean by clarity, as I use the term somewhat differently than current acoustic texts. I have found that humans can instantly detect when a sound is close to them or far away. The detection is usually subconscious, but it can be very important. We pay attention immediately to sounds that we perceive as close. Those far away can be attended to more leisurely.

The quality of sound that allows us to perceive “close” is the same quality that allows the brain to separate simultaneous sounds from several instruments, determine from what direction each came from, and attend to one of several conversations at the same time. To me, sound is “clear” when all these perceptions can be made. These perceptions are mostly subconscious, but when you listen carefully you can hear them. A Far Cry was rehearsing a Haydn’s C Major cello concerto and Britten’s Variations on a theme of Frank Bridge. They were arranged conventionally in a semicircle, with the violins on the left, the violas on the right, and the cello in the middle. When I was sitting in the balconies in front of the group the clarity was very high. Even in the third tier I could clearly localize the cello with my eyes closed. The violins were equally clear, the violas mostly inaudible, as is normal. When I moved to the rear of the orchestra in the third tier the balance was still very good, the music terrific, but with my eyes closed the cello was not sharply localized, just somewhere in front. (With eyes open you always hear it just where you see it.) The violas were suddenly audible and localizable. The sound was good – but not as gripping as in front. The cello was more localizable in the rear part of the second tier, and the cello localized sharply in the rear of the first tier. The localization and gripping clarity was good at the sides of the orchestra in all the tiers – but you have to look over the rail. If you settle back in your seat behind the glass balcony front the sound is “well blended”. It is good but not gripping.

In short – no problems, and an amazing success story. Nickrenz and others have plenty of time to play with the curtains in the walls. Just a bit more absorption might bring the clarity up a bit, at the expense of what is currently audible reverberation.

Calderwood represents a bold break with current fashion in chamber music hall design. It portends an acoustic much closer to the kind expected by the great composers of the Baroque and Classical periods – strong, balanced, and exceptionally clear. I look forward to hearing many different types of concert there.

Griesinger’s first ISGM concert review, “Acoustics vs. Performance: Claremont Trio at ISGM,” is here.

David Griesinger is a Harvard-trained physicist who is eminent in the field of sound and music. His website is here.