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Everything You Always Wanted to Know About Concert Hall Acoustics
(but you couldn't afford the consulting fee)

Meyerson Symphony Center, Dallas

I saw an article by contributing editor Jerry Rosen entitled "Shaping the Sound of Concert Halls" in the August 1986 issue of Mechanical Engineering, the monthly magazine of the American Society of Mechanical Engineers. It was a good primer on the subject of concert hall acoustics and fueled my curiosity about some deeper technical issues that the author didn't cover. I decided to do an acoustics article myself and called up Ron McKay, who at the time was with Bolt, Beranek & Newman, one of the preeminent acoustical consulting firms in the world (he eventually left BBN and started up his own consulting firm, McKay Conant Brook). I visited him at his BBN office and laboratory in Canoga Park, California. After an hour, I came away with a clearer education and fuel enough for a story that ran in the November 16, 1986 issue of the Pasadena Star-News and the following one, which appeared in the August 1996 issue of 20th-Century Music.

Anyone who has read the mass of vague, confusing, sometimes contradictory descriptions of concert hall acoustics knows just how much nonsense has been written on the subject. Look at the hand-waving report cards and hand-wringing eulogies of Davies Hall in San Francisco, Philadelphia's Academy of Music, or Carnegie Hall ~ and you'll soon long for the security and crisp language of home electronics with all its talk about flat response, oversampling and sound-pressure level in dB. Philadelphia Academy of Music

And why not? To most of us, acoustics looks like a black art. We can’t see it. We can’t touch it. Language often fails us when it comes to describing the vagaries of sonic textures. Add to that the fact that some of the greatest sounding halls date from the late 19th century ~ when builders had virtually no practical scientific knowledge of acoustical principles ~ and it would seem that our century has been backsliding from the dark ages.

Fortunately, there does exist a standard of excellence for concert hall acoustics, and experts and lay listeners alike have no trouble coming to an agreement on it. Ask anyone who knows, and the list of favored sites will include Vienna’s Grosser Musikvereinssall (built in 1870), Leipzig’s Gewandhaus (1885), the Concertgebouw in Amsterdam (1895) and Symphony Hall in Boston (1900).

Wiener Musikverein Each of these halls is steeped in a wealth of historical traditions. They each can boast of a great resident orchestra. All are built from the standard construction materials of their day such as wood and masonry and, perhaps more to the point, their interiors share roughly the same aspect and proportions. All were built at around the turn of the last century, a time when engineering and architectural techniques were still in their infancy compared with the last half of the twentieth century. Yet for their acoustics and the sheer quality of the listening experience, each of these structures far surpasses concert halls of more recent vintage. Why is it, then, that these halls sound so wonderful? What secrets did their builders uncover only to be lost again ~ like the secrets of the Stradivarius ~ to later generations of architects. Finding out the answers to these questions has been a decades long journey that may finally have brought us full circle to where we were a hundred years ago.

The search for perfect acoustics was on well before the beginning of this century. One of the earliest pioneers in this field was Harvard University’s Wallace Sabine whose studies of reverberation and attenuation continue to influence acousticians to this day. For the first half of this century, designers concerned themselves primarily with reverberance and the goal of achieving an even distribution of sound throughout the hall in accordance with principles first laid out by Sabine. Then in the early 1960’s, Leo Beranek discovered the importance of time delays in the arrival of successive waves of sound. But it was not until the 1970’s that acousticians fully came to appreciate why listeners preferred the old European halls best.

Manfred Schroeder, a researcher at AT&T Bell Labs in Murray Hill, New Jersey, wanted to know what qualities set the best halls apart from lesser ones. To that end, he studied 20 European concert halls, surveying listener reactions and correlating them to the physical and acoustic properties of the halls. What he found was that listeners preferred the sound of long, narrow halls ~ like the classic halls in Vienna, Leipzig and Amsterdam ~ over the sound of newer, wider halls built for larger audiences.

Carnegie Hall The reasons for the perceived differences between narrow and wide halls are not hard to understand. In either locale, the first sound a listener hears is that of the sound waves coming directly from the stage. The next wave to arrive is the sound reflected from the nearest surface. In a wider hall, this is usually the ceiling overhead which produces a similar signal at both ears. In a narrower hall, though, the first reflections arrive from the left and right walls. These reflected sounds produce slightly different signals at the left and right ears ~ both because of their differing content and because they arrive at the ears at minutely spaced times. Though the differences are subtle, those signals are apparently dissimilar enough for the brain to perceive an enhanced “spatial” quality to the musical signal in a narrower hall, thus giving a listener the impression of being more fully immersed in sound.

The superior acoustical quality of the old European auditoriums didn’t happen by design. It was an accident of history. In the early days of construction, before the advent of reinforced concrete and cold-rolled steel girders, the length of beam spans was limited by the strength and stiffness properties of timber. These antique halls had to be built narrow in order to carry roof loads without resorting to the use of obstructive columns inside the hall. As construction technology improved ~ resulting in both more accurate engineering analysis and better construction materials ~ roof spans got wider. This allowed auditoriums to hold more people and permitted more of those people to sit closer to the stage. On the face of it, better construction technology had struck a blow for democracy, opening up the concert experience to an ever larger audience. However, as the shape of the listening space slowly evolved, there was a cost in terms of lost acoustic quality - and democratically enough, everyone paid the price.

In the last 20 years, the recognized importance of lateral reflections has fueled nostalgia for the proportions of the “classical” European halls. During that time, the virtues of the rectangular “shoebox” design have become lore among architectural acousticians. Many recent designs have incorporated it as their basic shape; e.g., Meyerson Hall in Dallas and the Cerritos (California) Center for the Performing Arts. However, some designer think the case for lateral refections has been overstated. Ron McKay, who designed the acoustics for the Ambassador Auditorium (Pasadena, California) and was consultant on the acoustical renovation of UCLA’s Royce Hall a decade ago, doesn’t discount the role that lateral reflections play but says it doesn’t matter where the first reflections come from as long as they reach the listener’s ears quickly. [At the time this article first appeared, both of these halls were out of commission. Ambassador was closed for inadequate funding by its owner, the World Wide Church of God; the hall remains closed today. Royce Hall was damaged in the 1994 Northridge Earthquake and was not reopened until 1998.]

“Actually, both quick reflections and side reflections are important,” says McKay, “but a quick reflection from overhead also is important and valuable. The time delay between the direct sound reaching you from the stage and the first reflection ~ whether that reflection is from overhead or from the side ~ is very important. If that time delay is short, then you say there’s a great deal of presence to the room, or there is an immediacy to the sound. And all that’s important there is the time delay question, not whether the reflection is from the side or from above.”

To illustrate this from a designer’s standpoint, imagine an auditorium in which there is a piano recitalist on stage and a solitary listener out in the audience. When the pianist strikes a key, the sound of that note radiates out in all directions. One of these sound paths ~ the shortest one ~ leads directly from the piano to the listener’s ear. Another wave ~ the first reflected sound the listener hears ~ bounces from piano to wall to ear like the trajectory of a billiard ball and arrives a fraction of a second later. What Leo Beranek discovered in the 1960’s was that the ideal time delay for that reflected wave is about 0.02 to 0.03 sec. after the direct sound hits. That delay time, multiplied by the speed of sound in air (1100 ft./sec.), says that this detour to the wall should add an extra 22 to 33 feet to the wave’s path. Clearly, this has implications for the ideal sizing of a hall (not to mention the ideal place for a concertgoer to sit). For a listener, the immediacy of sound is enhanced when nearest reflecting surfaces are close by. Too long a delay between the first two waves, say 0.05 to 0.07 sec., and the effect is akin to sitting in a cavernous space like London’s Royal Albert Hall (not a great place for a concert, McKay says, “but it’d be a great place to play basketball!”). Any delay above a tenth of a second would actually be perceived as an echo.

Thereafter, says McKay, it's important for the listener to continue receiving many reflections at very short time intervals ~ and from all directions ~ in order to give the sound more fullness. As an example, he cites his redesign of Royce Hall. The sound that critics reported as having a tremendous presence or immediacy was the result, he says, of successive reflections that get to you quickly ~ "that don't take a long time to go off to distant surfaces and come back."

Boston Symphony Hall As the waves continue bouncing around inside the auditorium "box," they lose energy with each successive reflection. The sound dies down either rapidly or slowly, depending on the absorptive properties of the wall and ceiling materials. Therein lies the second important parameter of acoustic design ~ one which can be measured in seconds rather than milliseconds. Reverberation was a quantity first studied around the turn of the century by Wallace Sabine, the Harvard acoustician who designed Boston Symphony Hall. Technically, reverberation is defined as the time required for a sound to decay by 60 decibels after the source stops. In plainer language, it's simply the time it takes a loud sound to decay away to inaudibility.

The ideal reverberation time depends upon the type of music being played. Liturgical music involving either an organ or a chorus sounds best with a reverberation period of 3 or 4 seconds. Romantic music ~ say, late Beethoven or Brahms ~ sounds best in an acoustic of 2 seconds. Debussy and more contemporary fare that involves complex harmonies is better at 1.6 seconds, an interval that is also well-suited to chamber music. The difference may seem trivial, but according to acousticians, anyone with a good pair of ears can hear the difference between an auditorium with reverberation times of 1.8 and 1.9 seconds. In the theater, where the spoken word makes the highest demands on clarity, reverberation should be 1 second or less. For this reason, it's not uncommon to hear complaints about muddy acoustics and garbled dialogue when a stage play is mounted in a theater whose primary function is musical rather than dramatic.

Bridgewater Hall, Manchester All of this raises an important question about the flexibility of multiple use facilities. One of the classic problems in architectural acoustics is church design, where the extreme reverberation requirements of liturgical music and the spoken word come into play; the standard solution here is to design the space for music and to rely on electronic amplification for sermons. Opera, being part speech and part music, demands a reverberation time somewhere between the 2-second ideal for orchestra and 1 second for speech (possibly 1.5 to 1.7 seconds, leaning a little more to the musical end of the spectrum). In general, though, auditoriums have often been called upon to serve many different functions with as many unique acoustical requirements.

In the 1960s, architects met that challenge by building the American classical, community, multi-purpose auditorium that was expected to do everything. Drama, touring Broadway musicals, local symphony concerts, even the opera ~ all were supposed to be presented under a single roof on a different night of the week. The unfortunate result was that this building was a compromise hall. A little too reverberant for speech and not reverberant enough for music, it didn't serve any function particularly well. For that reason, subsequent designs in the late 1970s and 1980s came to incorporate variable acoustics.

Ambassador Auditorium, Pasadena While there are numerous ways to vary the acoustics, one of the primary tricks is to move very large quantities of draperies in and out of the auditorium. Draperies, being sound absorbing, cut down on reverberation time and allow greater clarity for theater and lectures. At Ambassador Auditorium, for example, the interior has a false ceiling made of bronze bars. Above the bars ~ and invisible to the audience ~ is an enormous open space. For speaking events, a large quantity of black curtains may be pulled out into the space (in speech mode, the reverberation time is about a second). For musical events, the curtains are drawn back into large storage pockets to open up a big, hard, reverberant space for music (which then resonates for a more ideal period of 1.6 to 1.7 seconds).

Rock music, jazz and other popular forms ~ which are typically amplified ~ require a relatively dead, non-reverberant space. It helps if the interior space is large (though the choice of an arena's size for a rock concert has more to do with gate than with acoustic quality). If the sound energy at an amplified pop concert isn't dissipated quickly enough, the environment can get too mushy and overloud. Ideally, the quality of the listening experience at pop concerts has more to do with the audio system than with the building's natural acoustics. Disney Hall, Los Angeles

Given the proportional and volumetric limitations inherent in the requirements for a good hall, there are ideal sizes for halls depending on the type of music being played. Acoustic designers find it more difficult to create good symphony acoustics in auditoriums seating 3,000 or more than, say, halls with capacities of 1,800 to 2,500. Accordingly, the next generation of orchestra halls are being built leaner. The newly opened Bridgewater Hall in Manchester, England; Seattle's Benaroya Hall (opening in 1998); and Los Angeles' proposed Disney Hall all seat about 2,500 people.

The interior shape of an auditorium plays a key role as well, and designers have experimented with almost everything imaginable, including fan-shaped, reverse fan, shoebox, and circular rooms. A fan-shaped hall is undesirable. When a room fans out from the stage, the sound energy dissipates as it travels to the back rows; as the room gets wider and wider, the energy is spread thinner and thinner. In a rectangular shoebox hall, by contrast, the energy doesn't have the opportunity to spread from front to back; consequently, there is greater consistency of listening condition and greater uniformity of loudness.

Reverse fans ~ in which the auditorium is widest at the front and narrows toward the back ~ are poor for both acoustics and sight lines (where a listener sitting in the front row corner has to watch a performance sideways). In the reverse fan, the side walls are set far apart so that it takes the reflections much longer to come back, thus violating the short time delay criterion for good acoustics.

"The goal that everybody's after," says McKay, "is getting those early reflections to everybody in the audience. You always have to balance those things against the requirement for 3,000 seats for box office. And there is always a tendency to go toward some kind of fan shape in order to get good sight lines. What you have to do is be ingenious enough to find ways to do that and still not let the walls get pushed so far out or let the ceiling get pushed up so high that you lose the early reflections.

Maurice Abravanel Hall, Salt Lake City "The classical halls and the newer ones like in Salt Lake City start with a rectangle, admittedly, but it's a very wide rectangle in order to get enough seats," McKay says. "Now how to narrow that down? Well, I start cantilevering out some balconies so that if I look at a section view, I have balcony-main floor-balcony. Now the hall, rather than being 80 feet wide, is effectively 60 feet wide between balcony faces."

Orange County Performing Arts Center (view of lobby) Some designers have played creatively with more complicated geometries in order to experiment with acoustics. A good example is Segerstrom Hall in Costa Mesa, California, whose interior surfaces vaguely suggest the Death Star in "Star Wars." Here, the basic fan shape is broken up by outcroppings of raised seating, creating vertical reflecting surfaces throughout the auditorium. The hall has good presence, but one can sometimes hear the call of a phantom horn coming from somewhere overhead.

Where should a listener sit in order to enjoy the best sound? In a well-designed hall, it shouldn't make much difference whether the seat is on the main floor or in the balcony. In some older halls with high ceilings, though, the balcony is often a better place because of its proximity to the ceiling; here, a listener begins to receive early reflections from overhead that would not reach him otherwise. If a listener wants to hear an orchestra the way the musicians themselves hear, it's best to sit down in front (or in some of the newer halls, beside or behind the orchestra). At these distances, the sound of an orchestra is powerful enough to overwhelm the sound of the hall. As one moves back eight or more rows on the main floor, the hall's acoustics begin to come out. The sound should be fine anywhere on the main floor except under a deep balcony, where the sound is weaker and more remote than in the open hall. Some orchestra halls like the one in downtown Phoenix have a rear balcony that isn't cantilevered from the rear wall but stretches like a beam between the two side walls and is open at the back. This "flying balcony" design allows the sound to penetrate the rear of the main floor by going behind the balcony above; a valid technique, it's one that takes much of the disadvantage out of sitting under a balcony.

Davies Hall, San Francisco In a way, the 20th century could be called the acoustic century. In the last hundred years, this most aesthetic of sciences has experienced a dynamic period of fads, experimentation, intuitive trials and gross errors. Like the changing winds, designers have plied their visions in divergent directions with a single goal in sight ~ to find a sonic ideal for live music. Every concert hall built was like a stepping stone toward that goal ~ a million-dollar experiment leading to the next improvement. Along the way, those designs have been influenced by equal parts physics, politics, art and caprice. There was even a time when some concert hall designers were influenced by the dry, brilliant, unreal sound of stereo recordings with 32-track mixing.

A hundred years ago, an auditorium designer could check the uniformity of sound distribution by building a scale model with mirrored walls, placing a light on the stage and seeing how the light was reflected within the space. Today, designers perform such checks with finite element computer models. In earlier days, the builders of civic auditoriums were artisans engaged in a labor-intensive enterprise. Today, a new orchestra hall is a $100 million proposition. The music of the next century ~ whatever it brings in the way of new sounds, new instruments, new computer-driven technology ~ may yet drive concert hall acoustics in new directions. But for a variety of reasons, the next hundred years of concert hall acoustics will probably not be as dynamic as the last hundred years has been. That high price tag provides some deterrence. But the fact is that in the last hundred years, we've learned much about both the physics and the psychology of acoustics. And if acoustical architects have learned one lesson, it's that our future lies in the past.

Or as Ron McKay puts it: "I think all of the acousticians who are designing concert hall facilities would love to make them sound like the classical halls in Boston and Vienna."

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