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Math, Magic, and Music: An Interview with Carl Rosenberg, Acoustician

Oct 29, 2015 9:57:00 AM
By BLO Staff

We sat down recently with Carl Rosenberg, an acoustician with Acentech, Inc. who consults with BLO on Opera Annex productions, including the upcoming In the Penal Colony at the Cyclorama at Boston Center for the Arts. A fascinating mix of math, music, and magic, read on for his insights about what makes a site-specific work sound glorious!

BLO: How did you get into this field?
Carl Rosenberg: Well, I studied to be an architect but found that I didn’t have a design flair. But I really resonated with the course I took in architecture school on architectural acoustics, which combine my interest in buildings with some facility in music and a more engineering bent on how buildings are put together.

Now I’m primarily a consultant. I work for colleges and universities, for classrooms, for atriums and public spaces, lots of small auditoriums at schools, office acoustics, a variety of things. 

BLO: Are you a musician yourself?
CR: I studied music the way kids do—taking piano lessons! But I like learning more about how spaces sound and what one does in the design of spaces to influence that acoustical quality, which in turn affects how we hear, how you can understand speech, the excitement of a dramatic production if you can understand the words, and so on.

BLO: How did you begin working with BLO?
CR: I think I was referred to Esther [Nelson, the General & Artistic Director of BLO] by a mutual friend, who suggested that if there were concerns or questions about the acoustics for these more challenging or non-traditional spaces, I might be able to help. And indeed the consultant work I do as my career deals with a wide variety of acoustical problems in spaces. So it started with learning more about the Annex productions, and my association with BLO developed through that.

BLO: What are some of the hallmarks of good acoustics? What do you listen for in a space?
CR: Good acoustics obviously depends dramatically on what the goal is of the space—it would be different for an office, for a restaurant, for a classroom, for musical productions, different for different types of musical productions. Where there would be amplified sound, you would want a certain type of acoustical quality, but for live music (especially the voice) you want a different type of acoustical quality. It also depends on the space and what the relationship is between the audience and the performers.

With opera, you want to allow the voice to resonate, to be heard, and you also want make sure there’s a degree of articulation and clarity. And those are the challenges that we’re faced with in any operatic venue.

BLO: When you walk into a space that we’re considering for Opera Annex, what are the things that you’re listening for or evaluating in that space?
CR: The relationship between the audience and the performers, because that’s the most important way to gauge the audience for the performance. [I also evaluate] the degree to which the space will support the propagation of the voice and the music that goes with it without sounding too muddy, or without overpowering the voice with extraneous sounds. One of the variables that helps us evaluate that balance is reverberation, which is the multiple echoes that you hear in a space. So you would rather the environment be more supportive of the voice than, for instance, singing into a closet.

BLO: What contributes to that? Is it the hardness of the materials that comprise the space?
CR: Exactly. The reverberation, which is one measure of the quality of the sound, is directly proportional to the volume and inversely proportional to the amount of absorptive materials in a space. So with those two variables, we can control that reverberation, which is measured in seconds. The audience is always a major component of the absorption in a hall—in so many spaces, the reverberation changes dramatically from an empty space to a full one. But if we make allowances for that and look at the other materials as well as the volume of the space, we have some sense of what that reverberation will be like.

Set model for In the Penal Colony. Design by Julia Noulin-Mérat.

BLO: So in the Cyclorama, we had to add a lot of absorptive materials to the set. Can you talk a little bit about how you came that conclusion?
CR: Anyone who goes into the Cyclorama, especially when it’s empty, will be overwhelmed and impressed in a positive manner by the huge volume, and the fact that all the surfaces that are on the finished materials now—floor, ceiling, oculus—are hard, sound-reflecting materials. There’s nothing soft or porous or fuzzy about any of them. Again, that will change with the addition of an audience, because all of us introduce absorptive materials, but with some rough calculations my colleagues and I figured that this space would be too echoy [for the opera]. So we worked with the design team to incorporate additional absorptive materials. The problem with the Cyclorama is exacerbated by the fact that its circular shape will focus sound in certain ways, just like a lens…or anything that has a concave surface. In the Cyclorama, both the ceiling above, which is a concave dome, as well as the circular floor plan [contribute to this]. So the location of the treatments is our attempt to control that focusing that will occur.

BLO: Are sounds generally more reverberant when they are in a space that is circular?
CR: [In a circular space,] the sound isn’t reflected or bounced around in an even manner…the extreme case of this focusing is evident at the Mapparium at the Christian Science Center, where there’s an entire sphere and if you go inside you can be overwhelmed by the echoes and the reverberation, the focusing, that occurs. The problem is not that severe at the Cyclorama, of course, but [the Cyclorama] was obviously not designed for live performances, it was to look at a painting of the battle [of Gettysburg].

BLO: What has been the most challenging venue that you’ve worked on for the Opera Annex series?
CR: Well, I think the Cyclorama will be one of the most challenging. Although, the Castle was an even larger volume and potentially even more reverberant. So that was a challenge too. The balance of these Annex productions is getting the absorption to be part of the stage set, the design; it’s not a permanent installation, [and we’re] limited by budget. In the Cyclorama what we could do was also limited by the structure itself and its historical significance…

BLO loads in to the Temple Ohabei Shalom for
The Love Potion, November 2014.

Another challenging space was the Temple Ohabei Shalom, where we did the production of The Love Potion last Season. An interesting space and  a huge volume, but as it turned out, that entire interior surface had already been treated with a sound-absorbing finish. Which made it quite non-reverberant, or dead, which is not what you would expect given the visual impact. You walk in and you expect to hear a long reverberation like a cathedral, but it’s not that at all. So our challenge in that case was to help bring some of the sound back to the audience that would otherwise be lost.

BLO: Anything you want to add?
CR: I would want to say that it’s been a profound honor to work with the production staff and the music staff; obviously we all share the same goals, but they have a love of opera which is inspiring and contagious and that’s been a wonderful opportunity for me.

Topics: 2015/16 Season, Acoustics, In the Penal Colony, Opera Annex

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