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Good vibrations -- and bad
From brakes to mine detectors, acoustics is all

By Bari Walsh

One of the most annoying problems in automotive design is also one of the most mundane: screeching brakes. After decades of study, car manufacturers have yet to come up with a thorough explanation of why brakes, even perfect ones, can get so noisy. Meanwhile, gobs of money -- some estimates are as high as $100 million a year in North America alone -- are spent in repair shops in a quest to rid brakes of their squeak, usually to little effect.

	Greg McDaniel. Photo by Kalman Zabarsky

It's only natural that Greg McDaniel should be attracted to a problem like brake squeal. An assistant professor in the College of Engineering's department of aerospace and mechanical engineering, McDaniel studies structural vibration and acoustics. That means analyzing data in an effort to help improve the design of car brakes, the efficiency of undersea mine and ship detectors, and even the diagnosis of tumors in the human body. In short, "if it vibrates or radiates sound," he says, "I'm interested."

More than crunching numbers, he looks for unexpected answers that will help solve real-world problems. In his work on brake squeal, funded by the Ford Motor Company, he approached a well-researched but stubborn problem from a fresh angle and came up with some surprising ideas that will likely lead car manufacturers in a new direction.

Brakes like potato chips

McDaniel attracted Ford's attention soon after he arrived at BU in 1996. His interest in both vibrations and acoustics -- he'd done structural acoustics for the Navy following his doctoral work at the Georgia Institute of Technology -- led Ford to fund a study he describes as "how brake vibrations create the actual squeal: what the path of energy is from the shaft into the brake system and finally to the listener's ear.

"In other words," he continues, "what creates the sound? You'd think they'd understand that by now, but nobody does. All the work had been done on the vibration itself, looking at why the brake vibrates in the first place. But there was this other question of how those vibrations translate into noise."

Braking systems differ slightly in design, but they have a certain basic structure: pistons apply pressure to the brake pads, which squeeze the disc-shaped rotor to stop the wheels from spinning. When brakes vibrate, McDaniel says, the rotors resemble the wavy shape of a potato chip -- either regular or crinkled, depending on the surface area of the rotor (automotive engineers sometimes make the surface irregular or nonsmooth in the hope of preventing squeal). "If you looked at the waves from above, as if in a movie," McDaniel says, "you'd see waves flowing around the circumference, shedding acoustic energy. It's a spiral effect, with waves moving around and out."

A vibrating structure like a brake rotor creates sound by compressing the air around it. That happens most efficiently when the waves produced by the vibrating object match the acoustical wavelengths in the air. To McDaniel's surprise, he found that brake systems are nearly unparalleled in their acoustical effectiveness. "It turns out that the vibrational waves [generated by the rotor] approximately match the wavelengths in air," he says. "It's a coincidence, and it's responsible for the fact that we hear the squealing so well."

The sound of musical instruments

"The brake rotor appears to radiate sound better than guitars or violins," McDaniel adds. "If you look at the patterns of vibrations from those instruments, they don't match the wavelengths of sound in air. It's a funny fact -- musical instruments appear to radiate sound less effectively than the thing that stops your car."

The answer to squeal lies in changing the way the rotor vibrates by designing a more symmetrical braking system. "Lots of things on cars vibrate," he says, "but they don't all produce sound, thank God." He developed a stationary brake system that simulates the functions of a real-world brake system, and his model will help manufacturers carry out this redesign effort without having to contend with the difficulties of inducing squeal in the laboratory and of measuring velocities on a moving surface. McDaniel won the Ralph R. Teeter award in 1999 from the Society of Automotive Engineers for his work.

His interest in vibrations extends far beyond brake pads. Last year he received one of the National Science Foundation's elite CAREER awards, which fund the work of promising young investigators for four years. With that grant, he's working on a new technique to significantly reduce the time it takes to analyze vibrations of complex systems. And in a project funded by the Navy, he's applying the principles of causality -- simply, that there can be no effect without a cause -- to the analysis of underwater acoustic fields produced by vibrating objects. The goal is to take sonar imaging to a new level -- to develop a way to create dynamic images of submerged objects "so we can tell real mines from counterfeit mines, or so we can diagnose cancer tumors," he says.

Like a child with his first chemistry set, McDaniel still sees mystery and wonder in science, and he wants his students to pick up on the possibilities that good scientific research can offer. He's inspired by a quote from science fiction writer Arthur C. Clarke: "Any sufficiently advanced technology is indistinguishable from magic." Being a good scientist, he believes, means "making something happen that most people believed couldn't happen -- something that's so totally new or different, you wouldn't have believed it could be done. People have always studied brake squeal, but only recently did we learn that it's a problem because brakes are actually designed to make sound. Maybe after a year, you'll never hear another squealing brake!"

14 September 2001
Boston University
Office of University Relations