Sound & Vision
After Carnegie Hall was renovated in 1986, critics and musicians complained that the hall’s legendary acoustics had been compromised somehow. The restoration firm begged to differ—nothing had changed, they said. But eventually, it was discovered that the addition of a concrete slab under the stage floor was the culprit. The slab was removed. No more complaints.
Acoustics is both an art and a science. A science because acousticians now have an arsenal of precision tools to measure the components that comprise the auditory experience. An art because arriving at the right combination of components is, to some extent, a matter of intuition and imagination.
One of those components—diffusion of acoustic energy—is the research focus of a Vassar physics professor, David T. Bradley. In architectural spaces like rooms, acoustic energy is either absorbed (by carpets and curtains, for example) or reflected (bounced back by hard surfaces, like walls). The reflected energy can behave in one of two ways, specularly or diffusely. A specular reflection is similar to when a billiard ball hits a side rail on a pool table, in which the arriving angle and the reflected angle are equal. A diffuse reflection is one in which there are many possible reflected angles; basically the sound spreads out in all directions. Diffuse reflections help create a widely scattered sound field, where the sound energy is evenly distributed throughout the room.
“High diffusion is often the goal in an acoustic environment like a concert hall or, for that matter, a classroom,” says Bradley. “Diffusion is what allows all the listeners to have a similar and rich auditory experience.” To achieve this effect, architects often employ acoustic diffusers—those hard panels with unusual surface geometries that you often see affixed to walls and ceilings in performance spaces.
The concept of diffusion (also called “scattering”) in architectural acoustics is not new. Bradley points out that the highly praised acoustics of the great 19th-century concert halls of Europe are in part the result of unintended diffusers—the statues and ornate embellishments on the walls and ceilings. Although originally intended for visual effect, their influence on acoustics is tantamount. What is new is the systematic use of physics to investigate, predict, and create precise acoustic outcomes from these diffusing elements.
Bradley recently won a prestigious five-year $410,000 Faculty Early Career Development (CAREER) Award from the National Science Foundation (NSF) to investigate the behavior of sound energy in acoustically sensitive spaces, like concert halls. This is the first CAREER award received at Vassar—it’s normally a grant offered to faculty at top research universities. In particular, Bradley is looking at the diffusion potential of fractal surfaces. A fractal is a geometric structure that contains within it increasingly smaller scale copies of itself, a characteristic known as self-similarity.
“Think of the branches, twigs, and leaves on a tree, each a smaller replica of the trunk geometry,” says Bradley. “My research focuses on predicting and measuring the effects of the fractal design on acoustic diffusion. If we can accurately predict the scattering behavior of fractals, we’ll have a knowledge base for incorporating fractals into the design of acoustic diffusers. Fractals are particularly interesting because their self-similar characteristic theoretically allows for scattering across the entire human hearing frequency range, which is not something attainable by regular diffusers.”
The NSF grant will support the creation of 10 undergraduate summer research positions at Vassar, the acquisition of $150,000 in acoustics research equipment for the Physics and Astronomy Department, and collaborations with the IBM Hudson Valley Acoustics Laboratory in Poughkeepsie and the Institute of Technical Acoustics at RWTH Aachen University in Germany.
The grant will also extend a science outreach project Bradley had already initiated, an ongoing physics workshop for ethnic minority high school students from the Bronx.
“The Bronx is the worst borough of the city in terms of access to physics in public schools,” explains Bradley. “Through this program, underrepresented students who wouldn’t normally have access to a large variety of science topics are given an opportunity to learn and to see the potential in a science career. They also take a tour of Vassar’s campus and get a feel for what it’s like at a top-tier residential liberal arts college.”
Broadening participation in the sciences is an important cause for Bradley, who is of Mexican heritage and was the first in his family to attend college. “When I was in high school, we had rather limited access to advanced placement courses in the sciences,” he recalls. “My journey has been possible only because of the support of many mentors. I feel a deep sense of obligation to guide students who might not otherwise have the opportunities necessary to fully realize their dreams.”