REPOST: pardon my reverb

Fools!Jen-Luc Note from your errant blogger(s): Life is currently kicking our collective asses, both professionally and (for some of us) personally, hence the eerie quiet of late at the cocktail party. So much cool stuff, so little time to blog about it! For my part, I've just finished copy-editing The Calculus Diaries manuscript, and am heading off to the San Diego Science Festival, where I will be on a panel on science and science fiction (with David Brin and Scott Sigler, no less!). But I've been blogging for enough years now, that some of you may have missed cool posts from yesteryear. So to tide you over until I get back to blogging this weekend, here's one of my favorite early posts, on concert hall acoustics, inspired by a Jordi Savall performance. And hey, if you hunger for fresher fare, I'm still blogging over at Discovery News.

Back in March, I visited my beloved New York City, having snagged tickets to see a concert performance by the incomparable Jordi Savall, world-renowned master of the viola da gamba and two-time Grammy nominee, whose soundtrack for the French film Tous Les Matins du Monde has sold more than a million copies worldwide. For this event, Savall performed with the Hesperion XXI ensemble and La Capella Reial de Catalunya, both of which he founded. Jen-Luc Piquant is as big a fan of Jordi Savall's music as I am, and we were thrilled at the opportunity not only to hear him perform live, but also by the featured pieces: early Renaissance and Baroque compositions from the Iberian Peninsula region. These are unusual, little-known compositions that Savall has taken upon himself to revive and introduce to classical audiences around the world.

A bit of history: the first Jesuit missionaries in Brazil and Paraguay adapted traditional Amerindian melodies of that region to Christian doctrinal texts translated into the local languages. They even recruited local musicians to take part in the ceremonies, clad in their native garb and playing their own culture's instruments. Inevitably, the two musical traditions merged, combining (according to the program notes) the "European model system, formal structure and contrapuntal techniques… with their own melodic and rhythmic patterns," particularly those associated with native dances. This resulted in some odd, yet affecting hybrids, such as Juan Garcia de Cespedes' Ay qye me abraso — literally, "I am burning." It features a rhythm based on a popular Mexican dance. The secular aspect is offset by the lyrics: the characters are "panting and sighing because of the excessive heat generated by their emotions at the sight of the newborn Christ." Our own more sonorous, politely composed Christmas carols seem downright emotionally tepid in comparison.

If only the organizers of the event had paid more attention to the venue, the evening would have been an unqualified success. There's no shortage of performance spaces in Manhattan, after all, with decent enough acoustics for this type of music — the gloriously Gothic St. John the Divine would have been ideal. Alas, the concert was held at St. Paul the Apostle on the Upper West Wide, a small Romanesque-style house of worship whose architectural details added up to a veritable acoustical "perfect storm." Unless one happened to be sitting in the first few rows, the sound was muted, muddy, and each separate part of the complex compositions seemed to reach the audience at different times, so that the percussion always seemed a few beats behind the singers, and/or the instruments. 

Needless to say, the evening was an unmitigated disaster. Several audience members left in disgust, and those who remained to the bitter end were forced to wander forlornly about the sanctuary in hopes of finding one of the "live" spots where the acoustics weren't quite so ruinous. Since it was impossible to fully appreciate the performance, our little group instead found ourselves discussing the scientific aspects of what might be happening to cause the horrific effects.

Composers have always recognized the importance of the acoustics of a given performance space, and many tailored the music they composed to fit those spaces. For instance, Gregorian chants fare well in medieval cathedrals, known for long-reverberation times; ditto for organ music, such as Bach's "Toccata in D Minor." In contrast, Mozart and Haydn composed music to be played in highly furnished chambers, for smaller, intimate audiences. Such pieces lose their clarity when played in highly reverberant spaces. But no one really thought about how to design a concert hall or opera house to achieve optimal acoustics; it was done through trial and error, with little acoustical theory to provide much of a framework to centuries of experiment.

The father of modern architectural acoustics is an American physicist named Wallace Clement Sabine. In 1895, he was a lowly faculty member of Harvard's physics department, who was handed the knotty problem of improving the infamously bad acoustics of the university's Fogg Lecture Hall, part of the recently constructed Fogg Art Museum. Sabine didn't have any particular expertise with sound — he didn't even hold a PhD (the horror!) — but he doggedly tackled the challenge  as he would any other physics experiment. He spent several years studying the acoustical qualities both the museum's lecture hall, and the Sanders Theater, widely considered to have excellent acoustics, in order to determine  what might be causing the difference in sound quality.  Specifically, he was attempting to find some objective formula or standard by which to measure and assess the acoustics of performance space designs.Wcsabine

It wasn't an easy task because so many variables had to be taken into consideration. He and his assistants tested each space repeatedly under varying conditions, moving materials back and forth between the two halls — such as hundreds of seat cushions from the Sanders Theater — and making careful measurements armed only with an organ pipe and a stop watch. He timed how long it took for different frequencies of sounds to decay to inaudibility under those varying conditions: with and without Oriental rugs, various numbers of people occupying the seats, and so forth.

Ultimately, he was able to determine that there was a definitive relationship between the quality of a room's acoustics, the size of the chamber, and the amount of absorption surfaces that were present. And he came up with the formula for calculating reverberation time, still the critical factor for gauging a space's acoustical quality.

Sabine concluded that the Fogg Lecture Hall's reverberation time was too long — a spoken word would remain audible for 5.5 seconds, as opposed to the optimal reverberation time of 2.25 seconds — so there was too much resonance and echo. He solved the problem by outfitting the space with sound-absorbent materials to reduce the "echo effect." His success cemented his reputation and he went on to serve as acoustical consultant for the design of Boston's Symphony Hall, still considered one of the best halls in the world in terms of sound quality. And oh, yes: the unit of sound absorption, the sabin, was named after him.

The field of concert hall acoustics has advanced far beyond Sabine's rudimentary first measurements, although there are still purists who believe that there will always be a subjective element that eludes attempts at strict mathematical description. Nonetheless, using just those sorts of quantifiable tools, Leo Beranek, one of the most eminent acoustic engineers, has identified three basic aspects to achieving a sufficiently good sound in a concert hall: (1) Listeners should be as close to the orchestra as possible; (2) Listeners should have a line of sight to the orchestra so the sound can travel unobstructed; and (3) the interior surface of the hall should be made of a hard material so that sound energy is not absorbed or lost. So an acoustical consultant needs to balance strength, reverberation and clarity requirements when designing a performance space.

Computer modeling has become one of the modern acoustician's most important tools. It turns out that the sound diffusing through a performance space can be modeled as particles of light bouncing around that space, much like a billiard ball bounces around a table in response to being hit by the cue. Acousticians can use those models to calculate the impulse response of the sound. Odeon
Once the sound field of a modeled space has been determined, it's possible to simulate the sound as it would appear within the real space — a technique called auralization, the acoustic equivalent of visualization techniques. Auralization software can help acoustical consultants predict, and perfect, how a proposed peprformance space design will sound, based on a wide range of parameters: room size and shape, surfaces, and materials, among other design features.

Auralization is still in its infancy when it comes to concert hall acoustics, but a team of Korean scientists has developed a way to paint "sound pictures" in space: specifically, a technique for controlling the sound in a selected region of listening space. They compare it to painting a picture on a canvas using "acoustical paintbrushes." They mix and overlap sound strokes by playing multiple loudspeakers simultaneously to create interference effects. The end result is a "photocopy" of, for example, a symphony hall's sound picture. The technique is called sound field reproduction.

Getting back to that infamous Savall concert: there were clearly many different factors at work on the night of the botched acoustics. For instance, different musical styles require different reverberation times. If reverberation time is too long, the notes blend together too much,
so that it becomes difficult to distinguish individual notes in fast,
complex passages. I had been especially looking forward to hearing my first villancico: a
religious song performed in the vernacular, incorporating percussive
African dance rhythms and a highly theatrical call-and-response effect
between soloists and the chorus (tutti). All the pieces that night featured quick tempo changes, unusual rhythms and complex vocal combinations. Throw in the odd percussionist and a woman with castanets and high heels stomping her way through some of the pieces to add even more of a staccato beat, and the result was a hopeless muddle of reflected sound — because of an overly long reverberation time.

The shape of the performance space plays a vitally important role in all of this, because it affects how sound is distributed in that space. Fan-shaped halls, favored for theaters and early cinemas, lack sufficient side reflections to achieve good sound quality (specifically, the pleasurable sensation of being "enveloped" in the music), while the circular and elliptical shapes favored by many architects tend to focus sound on certain "hot spots," leaving much of the remaining space acoustically "dead." St. Paul the Apostle's main sanctuary features a long gallery topped by a high curved roof — I believe the technical term is a "barrel vault" — lined on both sides by decorative archways. I'm not sure if that qualifies as a "shoe box" shape, but the poor acoustics would indicate that it is not. Making matters worse, the floor is mostly wood, with marble tile borders, so too much sound is absorbed. Add in poor line of sight, and you've got that muted, muddied quality to the music in question.

Frankly, the performance by Savall's ensemble du jour would have been better served if it had been given in the subway station. There's a reason so many buskers favor specific spots in New York City's vast subterranean transit network. Alex Case, director of Fermata Audio and Acoustics in Portsmouth, New Hampshire, has conducted extensive studies of local musicians who play regularly in subway stations. I interviewed Case — who wears a second hat as a professor at Berklee College of
Music in Boston — back in 2004 during a meeting of the Acoustical Society of America about his NYC subway work. Basically, he recorded numerous subway "performances" with
portable digital recorders and then analyzed the data he'd collected.

The walls of those stations are made of rigid heavy materials like tile, stone, steel and concrete, all of which are better at reflecting sound waves. This allows sound pressure (volume) to build up naturally, with no need for extra amplification — that all-important reverberation. The same sound, heard up close, has much less reverberation. (This is also the same reason why so many people enjoy singing in the shower.) In fact, Case likes to mention that there's a very famous tile-and-concrete stairwell outside a professional recording studio in Manhattan that serves as an excellent "reverberation chamber" and has featured on several classic rock recordings.

Not surprisingly, he found that those stations with acoustical qualities in keeping with the established numerical criteria embraced by professional sound consultants, also proved most popular with street musicians. For instance, buskers instinctively seek out locations near hard walls and under low ceilings to amplify their music above the routine din. They also tend to avoid stations with lots of announcements, or major transfer hubs, since there are too many trains running through the station to provide enough time for an extended performance period.

Of course, the very qualities that make subway stations so good for the street musicians can also be blamed for the notorious incomprehensibility of the routine service announcements over the MTA's public address system. (Jen-Luc Piquant firmly believes the audibility of such pronouncements is inversely proportional to the importance of any given announcement.) Amplifying speech requires far less reverberation than music. What sustains musical notes by building up sound reflections causes speech to become mushy and unintelligible. The reflections all mix together so that individual words can't be deciphered. Add in the electronic amplification, and it's little wonder the announcer sounds like he/she is speaking with a mouthful of marbles.

We love subway buskers, and we love Alex Case for choosing to study the acoustics of those performances rather than (or in addition to) the more hoity-toity, elitist performance spaces — conventional opera houses and symphony halls, for example, which are quite nice in their own right, don't get me wrong. They just exclude a large percentage of the population. Case loves the broad accessibility of the subway, pointing out that some 7 million people ride the NYC subway system every day. Even if only 1 in 10 passengers pay attention to the music, that still adds up to about 700,000 listeners per day, from every conceivable social demographic. In comparison, Carnegie Hall and Lincoln Center, combined, would have to sell out 54 shows every day, on all six main stages, just to compete. So the subway is the concert hall of the huddled masses.

Savall himself has something of Case's "common touch." The Spanish and Portuguese composers featured in the program that night combined high and "low" art, seeking to embrace the "pop culture" of their time. This might explain why there were so many young people in the audience. Classical musical aficionados are constantly bewailing the absence of youth in their audiences. Perhaps it's because the mainstream programs are so predictable: great music, yes, but always the same pieces by the same composers. This isn't an approach likely to appeal to younger, hipper audiences, who are far more likely to gravitate towards fresh alternatives, such as lost or rediscovered historical gems from, say, the 10th to the 18th centuries. That's Savall's specialty. His bio in the program notes offers his success as "proof that early music does not have to be elitist or of interest only to a minority, and that it can and indeed does appeal to an increasingly large and young audience." Savall is making classical music cool.

On the way out after the performance, I picked up a church bulletin, hoping to glean a little information about the building's history and design. Mostly it contained the usual church announcements, but on page 2 there was an item about the new sound system being installed in the sanctuary to address the chronic acoustical problems. It made mention of the need to eliminate certain "dead spots" in the sanctuary, not to mention increasing clarity in the spoken word in the nave. (The church leaders declined, however, to carpet the marble floors and stone pillars to better absorb sound to eliminate the echo effect completely, purely for aesthetic reasons.) Given the fact that Savall is such a terrific ambassador for early
music, in addition to being a world-class musician, it's doubly
appalling that the organizers would deliberately book his ensemble in such a well-known acoustically challenged
space. Yes, it was malice aforethought. Savall et al deserved better. And so did his audience.

11 thoughts on “REPOST: pardon my reverb”

  1. >Physics-minded types know this as Snell’s Law: the angle of reflection equals the angle of incidence, whether we’re talking >about light or sound waves traveling in rays that reflect off of surfaces.
    I know that as “momentum conservation”; Snell’s Law is the closely related law that describes how varying indices of refraction affect propagation through an interface.
    >Savall is making classical music cool.
    Maybe but I vote for Ethel, http://ethelcentral.dreamhost.com.
    The 2.25 second number is fascinating. Wonder what that says about the acoustic processing part of the brain?

  2. “I know that as “momentum conservation”; Snell’s Law is the closely related law that describes how varying indices of refraction affect propagation through an interface.”
    I’m glad Alison said that, because I thought for a moment that I’d slipped sideways into a parallel universe. Again.

  3. That’ll teach me to be more carefully discriminating in my sources. 🙂 It’s also the great thing about blogging — these errors come to light so much more quckly. Things have been duly corrected. Thanks!!

  4. All too true. I’ve seen quite a few live performances where the audience was not even aware how horrible the sound was (esp. when it matters more to be seen *out*).
    Re: classical music. I don’t think it’s the lack of ‘freshness’ that turns the younger audiences away. You almost have to grow up listening (almost exclusively) to classical music to appreciate it in your youth to the extent the older afficionados think possible. I grew up listening to a lot of pop & hard rock, and even though I am possibly the person most into classical music among my friends, I sometimes have to *try* to like it. The format is too different. It requires a good setup to hear the low passages and quite a bit of dedication to sit through a symphony. And it’s always hit and miss: there are some famous composers that I just can’t stand.
    The number of songs (and genres) that are out there increases every day and so does the ipod capacity. It’s going to be more and more difficult for classical music to compete for that space.
    PS: There’s also a typo in the sentence that starts with auralization software.

  5. Nobody will be listening to Best Said Fred 200 years from now. Baroque, classical, and romantic style compositions will last an eternity.

  6. Thank you for helping to bring the value of good sound forward into the spotlight. For some reason, sound usually takes the back seat in most any performance. I don’t mean the making of sound takes a backseat. We have lots of talented sound sources. I do mean the handling of sound once it is made, as you have noted.
    1) A word I didn’t see in your work is “intelligibility” It speakes for itself in meaning. It is one of the most obvious acoustical requirements to be met in performance spaces and yet, it is one of the least discussed.
    2) There is a huge difference between sound and light. Wavelength. For sound it’s between 1″ and 50 feet. For light, well, it’s pretty small. Light (very small wavelengths goes pretty much in a straight line. Sound (relatively huge wavelengths) expands and bends.
    Try this. You and Sean go out at night. Find a well lit corner of a building. A glass corner would be even more fun. Then you two stand against the wall, a couple feet back, either side of the corner. You no longer are in a direct line of sight of each other. Still, you can talk clearly with each other, around the corner.
    Sound goes around objects, light does not. Computer modeling of sound only does “ray tracing” which does not include the very important mechanism of diffusion, the spreading of sound.
    Arthur Noxon, Acoustical Engineer

  7. Today–March 24–is the 2nd annual Ada Lovelace “Blog About Women in Science & Technology” Day!
    (via http://findingada.com )
    So I just blogged a little tribute to two entities:
    1. the woman professor whose “physics for idiots” class I took in college
    2. this happiness-inducing blog (and that’s even without drinking the cocktails on the sidebar!)
    I’m sorry to hear life is being personally hard for some of you wonderful CPP bloggers,
    but I want to express gratitude to you all.
    You are a collective antidote to all those [otherwise wonderful] 1950s sci-fi movies I watched at midnight throughout my childhood in which an alien appears and ALWAYS the wobbly-shoed woman turned to the tweed-jacketed man and asks,
    “What is it?”
    I always wanted to scream, How the f*** should he know! Figure it out yourself!
    And so you have.
    Thanks you.

  8. Partially Deflected

    There are ways to improve acoustics in a setting like that without carpeting the walls. Hanging tapestries works quite well, and can work with the aesthetics.

  9. The number of songs (and genres) that are out there increases every day and so does the ipod capacity. It’s going to be more and more difficult for classical music to compete for that space.

  10. Hi,
    As a recently retired architectural acoustician I found your blog on acoustics quite accurate and right on. As my degree was in architecture, I’ve always understood what impediments are put in architect’s minds regarding achieving acoustics. Everything is about visual form and design. Despite the admonition by Vitruvius, during the time of Rome, that architecture should provide “firmness, commodity and delight” (in today’s terms, structural strength and safety, usefulness for the occupants and users, and esthetic pleasure), most architects are taught to focus on the last one, and then primarily in visual terms. They are taught to visualize their designs in their mind’s eye, but they never listen to their designs in their mind’s ear.
    My epiphany in our little half-semester introduction to this formidable subject in architecture school was that acoustical criteria and correct functioning could be used as “form-givers” which determine the shaping and furnishing and proportions of a space every bit as much as Euclidean geometry, the Golden Rectangle, Palladian motifs,understanding of historical structures and all the other stuff. Not only that, in terms of the usefulness of a space, a lot more people will complain or abandon work and living spaces that are too noisy, too reverberant, afford insufficient privacy or prevent concentration on tasks than will ever leave because they aren’t handsome or beautiful in some esthetic way. It is a bitter pill for architects and interior architects to swallow, that people might consider the impact noise of someone walking around upstairs to be more important than the orchestrated flow of space and framing of vistas in their own apartment.
    Music and its acoustics were not systematically studied in detail until the mid-20th century, allowing architects to move away from the traditional temple and gothic forms. Unfortunately, most did so with no understanding of acoustical consequences, and this remains a problem today. Religious forms constitute a sort of design ‘baggage’ which can be difficult to overcome for the acoustician, as the architect and the minister need the house of worship to “look like” something that they and the congregation will recognize as such. Once the architect encloses space, she commits acoustics. -Whether she has any consciousness of that fact or not!
    Another difficulty is reconciling the requirements of pleasurable listening to music with clarity and intelligibility of human speech, in any flexible, mixed-use venue. What improves the one, up to a point, usually degrades the other. This is a tough compromise to make. If an instrument like an organ is to be reconciled with clear speech communication, one is lucky not to make everybody unhappy.
    There are both architectural and electronic signal-processing approaches to this conundrum, of course.
    Thanks for bringing this absorbing subject up and giving it a wider ear!
    J Johnson
    Olympia WA

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