In his review here in the Intelligencer of the recent recital by violinist Thomas Zehetmair at the American Academy of Arts and Sciences Christoph Wolff mentioned that the acoustics in the Stephen D. Bechtel Auditorium, designed primarily for symposia and lectures, “…was remarkably good in every respect. ” Wolff’s comment deserves some expansion on the whole question of acoustics.

The Bechtel auditorium is very well designed for its purpose. The audience sits in semicircles around the podium, forming a wide fan. There is ample space behind the performer, and the back of the stage is filled by a moderately sound-absorbing projection screen. The seats are upholstered with sound-absorbing fabric, and there are carpets in the aisles. The high ceiling gives an unusually large internal volume for a speech auditorium, and the extra volume increases the reverberation time sufficiently that there is a noticeable, although quiet, reverberation — under one second. Reverberation is audible, but at a low enough sound pressure that it does not obscure the music in any way. The music, even eight rows back, is as clear as if one were standing next to the performer. The net result is an exciting, highly engaging, concert experience, increasingly unusual in concert venues.

There are two major perceptions that define the quality of acoustic spaces: clarity and reverberation. Of the two, reverberation is the easier to notice. We hear it when music stops, and we can learn to hear it as the music is playing. Reverberation, if it is not excessive,  adds beauty to a musical performance, as it transforms arpeggios into harmony and adds to a sense of unity between the audience, the performers, and the hall.Reverberation blends the individual parts of a piece into a whole that some believe is more beautiful than the sum of the parts. The perception of the clarity we heard at the at the Zehetmair recital is mostly subconscious and difficult to define scientifically. Acousticians usually define clarity as the ability to understand speech – but there is more to clarity than that. If most of the syllables of speech are recognized, and many the notes in a musical performance can be heard, we tend not to notice the ones that are missing. Our ear to adapts to acoustics. After five or 10 minutes our consciousness of the clarity of the space – good or bad – disappears. The actual clarity of sound is difficult to remember. But sufficient clarity enables sound to grab and hold our attention – and makes it possible to hear every note.

Although clarity and reverberation are both desirable, they are usually mutually exclusive. The physics of spaces decrees that efforts to increase reverberation will usually reduce clarity. In the past, where un-amplified speech was the rule, spaces were designed, often by trial and error, to convey drama effectively to the listener. Old halls and theaters provided excellent clarity in a majority of seats. Churches were filled with banners and tapestries, as were recital salons and halls. Traditional opera houses were loaded with velvet – both on stage and in the hall. But they had very little late reverberation. (Many of these historic spaces are currently much more reverberant, as the fabric has been removed long ago.) Given the choice between a sub-conscious perception and a perception that is easily perceived, modern conductors (who perform close to the musicians) and acousticians have pushed for more reverberation.

In most modern halls reverberation reigns supreme. I have been told by prominent acousticians that with the advent of opera titles clarity is obsolete, and that”no one needs to hear the words, which are in any case probably in Italian.” Design specifications for new halls insist on a reverberation time of two seconds, regardless of the volume of the hall or the number of seats. Clarity is usually specified by “C80” a measure that assumes all reflections that arrive within 80 milliseconds of the direct sound are beneficial. It is popular to believe that an abundance of early reflections – particularly lateral reflections – are vital to the quality of a hall.

But the Zehetmair concert amply demonstrates that this is not the case. Zehetmair remarked that he has never before played to an audience that listened so intently, completely silent and absorbed by the music. Why did this occur? Surely the performance was brilliant, but Zehetmair is always brilliant. Could the unusual clarity in the auditorium contribute?

It is well known in psychology that sounds that are perceived as close to you demand attention. Distant sounds can be ignored. This fact was taken for granted in old theaters – designed for Shakespeare or Vaudeville. The importance of dramatic clarity is known to every drama and film director, who demand that theaters be dry, and that cinemas utilize highly directional loudspeakers for the reproduction of dialog. In the quest for reverberation, many modern concert halls and operas have a beautiful resonance, but it is difficult to hear inner voices over most of the seats. The audience will enjoy the performance, and there are always a few good seats if you can manage to get a ticket for them. But in the long run excess reverberation may be dangerous. Classical music recordings provide good clarity; people will not buy them if they sound too distant. I believe that if current concert hall designs delivered real clarity along with sufficient reverberation, more people would be attracted to the concert experience. Halls that achieve this balance exist, but they are rare.

There is a scientific basis for the kind of clarity that demands our attention.   At a recent concert I was sitting about 15 feet from the cellist in a string trio as he tuned his instrument.  To my surprise, I could hear the change in pitch as he adjusted the fine tuner on his D string one-quarter turn.   A few rows further back and I would not have been able to do it. Our wonderful ears use at least two methods to detect pitch and timbre, but our extraordinary abilities to hear fine details of pitch, timbre, direction, and distance of several instruments at the same time depends on the distinctness to which we can hear the direct sound from the instruments. The most sensitive mechanism for these perceptions relies on the phase coherence of tone harmonics, particularly those in the speech intelligibility bands from 1000Hz to 6000Hz. These phases get scrambled by reflections. Early reflections (which tend to be the strongest) are particularly harmful. As the harmonics lose their coherence our perception of the sound goes from “near” to “far,” our ability to hear details of intonation is lost, and inner voices become inaudible.   The sound has achieved “blend” but our attention can wander.

The physics of the phase coherence detection mechanism offers solutions to the problem of providing both clarity and reverberation at the same time. The richness and blend that reverberation provides lie chiefly in frequencies below 1000Hz, where the fundamentals of nearly all musical instruments reside. If it is possible to design a hall where the strength (and not necessarily the reverberation time) of the reverberation is reduced in the intelligibility bands, we can achieve clarity without losing richness. In addition, the neural mechanism that detects harmonic coherence is time-dependent. Our ear integrates sound energy over a time period of about 100 milliseconds. We can still hear the direct sound with clarity if there is sufficient time before reverberation overwhelms it. In a large hall, the time delay is larger than it is in a small hall. But if the delay is too great reverberation will be perceived as a disturbing echo. So hall size and shape is very important.
In the Bechtel auditorium the reflections from the wall behind the performer are softened by the projection screen, the walls and ceiling are far away, and the side walls are angled to not reflect sound directly into the audience. There are very few strong early reflections, and the clarity is magnificent. The reflections we do hear have taken their time to get to us and do not affect clarity very much. But the sound is less than ideal. The reverberation time is below one second, and the reverberant level is low.

A few days after the concert Thomas Zehetmair played in my studio as we used electronics to gradually increase the audibility of late reverberation. The clarity in the studio was very high, but with no added reverberation it was uncomfortable to play. Just a slight increase in the late energy improved the ease of playing (and the sound of the instrument). Zehetmair preferred more late reverberation than exists in the Bechtel auditorium. The small audience in my studio also appreciated more. The most successful reverberation for us was close to the natural reverberation in Jordan Hall at New England Conservatory – 1.4 seconds of reverberation time, at a relatively low level.

There are several reasons that Jordan Hall has a good reputation for chamber music. In my opinion, the near ideal balance between clarity and reverberation tops the list. Like the Bechtel auditorium, Jordan Hall can be seen as a wide fan. The audience sits relatively close to the performers, providing good clarity. But the internal volume, enhanced by the high ceiling, is much larger than the Bechtel. The reverberation time is longer, providing more late reverberation, and the level of the reverberation is low enough that clarity is not lost. Jordan includes a large enclosed stage house that can reduce clarity if the performers are not in front of the proscenium, but most small groups know this, and perform in front. Sanders Theatre at Harvard University is similar to Jordan, and lacks the deep stage house. Chamber music is wonderful in Sanders. Such halls are rare in other cities. Boston is blessed to have two.

Our Symphony Hall in Boston also provides a good balance between clarity and reverberation. Seats on the floor in front of the cross aisle are almost uniformly very good. As you move from row N to the cross aisle the only substantial change in the sound is the angle subtended by the orchestra. It is easier to separate parts from one another in the front seats, as the azimuth difference between the sections is larger than for seats further back. But the sound is definitely engaging. Behind the cross aisle instruments increasingly blur together, and fine details are lost. This happens because there are no niches in the lower walls to provide frequency dependence, and there is less time delay between the direct sound and reflections in the rear seats. The side reflections are attenuated for seats in the front of the first balcony, restoring clarity.
I believe the reasons that Boston Symphony Hall succeeds in providing both clarity and reverberation over an unusually wide range of seats lie primarily in the size of the hall, the frequency dependence provided by the rectangular niches in the walls and ceiling, and the unusually shallow and open stage house.  Smaller halls of similar shape do not work as well. They need to look more like Jordan.

In conclusion, performances in halls where fine details of pitch and azimuth of each note can be heard (in the absence of vision) grab and hold the attention of listeners.   We should strive for more halls and opera houses that can provide this form of clarity – along with adequate late reverberation. Such halls exist, and it is possible to understand why they work.

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