Research & Innovation

Sound Science: Acoustic Researcher Builds Unique Echo-Free Chamber

You don’t know quiet until you’ve stood inside an anechoic chamber. As the heavy door swings shut on the one in the Noise and Vibration Control Laboratory at Stevens, the rumble of traffic, hum of ventilation fans, and chatter of student voices fall away.

Settle in a moment and you’re left only with the sound of your heart beating and the fluid pulsing in your ears. It’s a subtly strange experience and a reminder of the scarcity of silence in our daily lives.

Stevens has one of the few anechoic — meaning “no echo” —  chambers in the region. The echo-free environment allows precise measurements of sound qualities such as amplitude (loudness), frequency (pitch), and direction. Researchers of various disciplines as well as product manufacturers of every stripe come here to study sound in its purest form.

An Echo-Free Chamber

They are greeted warmly by Marehalli Prasad, professor of mechanical engineering and director of the Noise and Vibration Control Laboratory. Prasad, who has been at Stevens for 35 years and is the only faculty dedicated to acoustics in the mechanical engineering department, remembers building the anechoic chamber with his bare hands.

In the early 1980s, Prasad was collaborating with IBM on a research project on acoustic radiation—in those days, the company was interested in noise and vibration reduction for their bulky, clattering computer machines. IBM provided Stevens with the parts for an anechoic chamber through their Technical Gift Program. Prasad and his graduate students assembled the parts at its current site in the annex of the Edwin A. Stevens Building.

Inside the anechoic chamber, a room roughly 15 by 18 by 10 feet, the walls and ceiling are covered with two-feet-long high-density fiberglass wedges, protruding like space-age stalactites. The wedges serve to absorb sound instead of reflecting it back. Prasad demonstrates their effect by clapping his hands at various distances from the wall; the closer he is, the more dampened the sound of his clap, the wedges seeming to soak up the sound before it can reach your ears.

To prevent outside sound from transferring in, the chamber walls are insulated by more fiberglass sandwiched between two layers of steel. The entire setup rests on a heavy concrete base, which is separated from the ground by hundreds of compressed fiberglass cubes to isolate the chamber from vibrations. To step inside is to leave the noisy world behind.

Visitors to the anechoic chamber come to find out what their products really sound like. Prasad has hosted representatives from companies that make blenders, toilets, and window shades. The military has tested methods to mimic machine gun sounds used in training exercises. (That was loud enough to be heard outside on the street and alarmed some passersby, recalls Prasad.) One time, a fried chicken company came to make sure their product was as deliciously crunchy as their competitor’s.

Why Noise Matters

Noise is any unwanted sound, from the honking of car horns to the scratching of nails on chalkboard. Prasad considers noise a modern epidemic and is quick to enumerate its many detrimental health effects.

The most obvious danger is cumulative hearing loss, which depends on the sound’s loudness and duration. Noise levels are given in “A-weighted decibels (dBA), ” which measure not only the loudness but also how the human ear perceives different sounds, with more sensitivity to higher frequencies. Sustained noise of 90 dBA (akin to riding in a subway) for more than eight hours will produce lasting hearing damage, and for every additional five dBA, the safe exposure time is cut in half, explains Prasad. That means it only takes a few minutes at a loud rock concert (120 dBA) to sustain lasting damage.

Moreover, noise stresses the whole body, increasing heart rate, constricting blood vessels, and spurring the release of stress hormones. Prasad likens the invisible yet pervasive danger of noise to that of second-hand smoke—and calls for a similar campaign for more public awareness.

In New York City, noise is the number one complaint called into 311. Efforts to regulate unwanted sounds date at least to Ancient Rome when rattling chariots were banned from roads at night. Noise is a major quality-of-life issue, and yet, according to Prasad, it is too often an afterthought in the design process.

“The first thing is to recognize that noise is a problem,” he says. “Then you figure out where in the design the noise is coming from and finally how to reduce that noise.”

Teaching Awareness

Prasad’s research focuses on noise reduction in the design of mufflers, corrugated pipes, and other machine components, but his interest in noise and acoustics range far and wide. “Acousticians are a kind of nomadic people,” he says.

Early in his career, he worked on helicopter design for an aeronautics company in Bangalore, India. He’s traveled to Romania as an expert consultant for the United Nations Industrial Development Organization to advise the country’s electrical industry on noise control methods. Some of his recent publications explore how hospital noise affect patient delirium and the acoustics in Hindu worship spaces.

And in his four decades at Stevens, teaching has always been a priority for Prasad.  He has taught more than 1000 students, advised dozens of senior projects, and is often invited to high school and middle schools, where he easily wins over young audiences. He sums up his approach to teaching as “going to the basics, and where possible, show and tell.”

Prasad is always ready for an impromptu demonstration. He keeps in his office a stash of acoustically interesting knick-knacks collected over the years. Selecting a small flask, he blows air across the opening to show how air resonating in the neck produces a tone.  This is Helmholtz resonance, he says, and the same principle allows resonators built into aircraft panels to attenuate noise. He hits a wooden figurine of a frog with a stick, then again with his fingers covering the frog’s gaping mouth, demonstrating how a cavity amplifies sound—the principle behind the design of many musical instruments. He stretches out a Slinky on the table and pulses the two ends towards each other, showing how the two waves cancel out each other in the middle—like the destructive interference used in noise-canceling earphones.

Last December, Prasad was elected a Fellow of the Institute of Noise Control Engineering, a professional organization that promotes noise control solutions. In August he received the Institute’s Outstanding Educator Award for his “unique contribution to the education of future noise control engineers.”

“One of the major things to do in noise control is education,” says Prasad. “People say ‘what you don’t know doesn’t hurt you’—that’s not true in the case of noise.”