Summer 2013

Chamber of silence

New lab is one of the quietest rooms on Earth


Feature Image
Jonathan Cohen
Ron Miles, distinguished professor of mechanical engineering, researches ways to make hearing aids able to better distinguish particular sounds. The new anechoic chamber (a room without echo) will let him test tiny microphones in an atmosphere in which sound waves can not bounce off anything.

When Ron Miles was young, he would go with his mother to fabric stores, where he spent hours wandering aisles flanked by bolts of cloth. But it wasn’t the endless satins, tulles and twills that drove him crazy: it was the sound — rather, the lack of sound. “I hate fabric stores; the sounds just get sucked up in the bolts of fabric,” says Miles, distinguished professor of mechanical engineering and associate dean of research in the Thomas J. Watson School of Engineering and Applied Science. Yet, his new one-of-a-kind laboratory is among the quietest places on the planet. “It is way beyond the sound of a fabric store,” he chuckles. “But I do like this.”

Miles is an expert in acoustics — the science of sound. His background in noise control began at Boeing, where his job was to stop the roar of the engines from entering the aircraft. Today his focus is much, much smaller; he’s building a better hearing aid.

When our ears work well, they are exceptional at distinguishing speech from background noise. For people who use hearing aids, however, multiple sounds can become cacophony. It can happen on busy streets and at places such as airports, where flights are called simultaneously. “It’s the cocktail-party effect,” Miles says. “You’re struggling to understand the person in front of you through the mass of competing voices and sounds. Everyone has this problem, but it’s maddeningly frustrating if your ears aren’t working well.”

Miles has invented a tiny microphone that can filter unwanted sounds. The device is so sensitive that regular laboratories are not quiet enough for testing, so for years he has traveled to other universities to find spaces devoid of sound reflections. But even when testing took place at 3 a.m., Miles’ microphones were too sensitive to be accurately measured.

This spring, after a decade of planning, he has his own lab — the ultimate in quiet rooms, known as an anechoic chamber. It is a controlled environment in which he and his colleagues can study how sound radiates along a path from a source (such as a loudspeaker) to a microphone, free from reverberations.

Suspended underground

The chamber is isolated underground, 2½ feet beneath the floating cement pavers at the entrance of the Engineering and Science Building. Fire doors open into the horseshoe-shaped space, housing computers and equipment, that wraps around the outside of the chamber. A second set of thick doors is part of the shell that Miles and the facilities team at Binghamton designed to encase the actual chamber. “It’s the mass law of acoustics engineering,” Miles says. “You stop sound with really heavy things.” So the room has multiple layers of drywall, thick insulation and a lot of concrete. A gap in the floor where it meets the walls around the chamber and the springs below — like the shock absorbers of your car — keeps the chamber mechanically isolated from the building.

The third set of doors is the heaviest, weighing 260 pounds apiece, with rubber guards that fight against the floor to create an airtight seal. Inside the 250-square-foot anechoic chamber is yet a fourth door, the inside covered, like the walls and ceiling, with approximately 600 fiberglass wedges. They are also under the wire mesh floor. When the chamber is closed, there is nothing for sound to bounce off, as the walls absorb the waves.

The chamber was custom-designed by Eckel Industries to meet Miles’ research requirements. “The frequency range determines the size of the chamber; the lower the frequency you want to measure, the bigger the chamber needs to be,” he says. “Then how low a frequency you want to work at determines how big the wedges are.”
As verified by a standardized test, the walls absorb all sound down to about 80 hertz, which Miles says is plenty low for his research. Speech is between about 500 and 5,000 hertz, and hearing varies from 20 hertz to 20 kilohertz — “when you’re young and haven’t destroyed your ears yet,” he says.

In addition to blocking sounds from outside, the room absorbs sounds that start inside.

“If you got locked in, you could scream and no one would ever hear you,” Miles says. Though the chamber might sound like the ultimate sanctuary for peace and quiet, the room can be unnerving after the novelty wears off.

“It’s not a place you’d want to hang out in. All of a sudden you’re in an environment where there is no reflection of sound, and your usual perception is gone,” he explains. “Your ears will sometimes adjust their sensitivity level, and they start to oscillate. Or you will perceive sounds that aren’t really there. It can be creepy.”

For microphone research, it’s fantastic.

New direction for microphones

Miles’ research is a new approach to designing and fabricating the tiny directional microphone that is key to improving conversation for the hearing impaired.

A directional microphone produces more output when sound comes from certain directions. When a microphone is oriented toward the sound source — like a voice — it picks up that voice clearer than competing noises from other directions.

Traditional hearing aids use a pair of independent microphones and calculate the difference in signals to each. For example, if two microphones are parallel and a sound comes from the left, it will hit one microphone in the pair before reaching the other. The difference in the two signals creates a directional sensor.

Because the microphone unit is so small, about the size of a Tic Tac, it is limited by the noise floor — the quietest sound that can be detected without the differences in the signals being overcome by the microphones’ own noise.

“Being small is an engineering hassle,” Miles   explains. “You want the hearing aid to be small, so you want the microphones to be really close together. But the closer together they are, the less difference there is, and the signal gets smaller and smaller. As the signal gets smaller, it gets buried in the inherent noise of the system.”

Miles’ design uses a microphone diaphragm that responds to pressure gradients by rotating about a central pivot. When a sound wave causes the diaphragm to rock about the pivot, the motion is converted into an electric signal. Using this unique approach to microphone diaphragm design makes the unit smaller and quieter, thus lowering the noise floor and improving the ability to detect directionality at frequencies lower than anything currently available in hearing aids.

His research, funded by the National Institutes of Health, has been published by the Journal of the Acoustical Society of America.

A new level of testing

In May, the highly anticipated microphone-positioning system was installed in the chamber.

A boom, attached to a steel grid, allows Miles and his colleagues to move microphones on five axes to nearly anywhere in the chamber within a fraction of a millimeter. “It will enable us to scan around sound sources and get an accurate three-dimensional sound map of any kind of machine,” he explains.

According to Steve Kawamura, field sales engineer for Brüel & Kjaer Sound and Vibration Measurement, most positioning systems run on only two axes. “I cannot think of another university in the United States that has a chamber of this caliber. It’s one of the best I’ve seen, among even those in private Fortune 500 manufacturers,” he says. “We make one of the quietest microphones in the world, and this chamber is pushing the limits of its capabilities.”
 
Miles foresees using the positioning system to map how the body affects sound. “We can measure a sound field in a gazillion positions around a mannequin and then use that information to figure out how sound varies and flows around the head, in turn, improving the design of hearing aids by better orienting our differential microphones.”

Moving from the lab to life remains the challenge. “Commercializing is harder than inventing. I have no trouble dreaming up ideas. The hard part is figuring out the day-to-day engineering grunt work to make it manufacturable in large volumes,” Miles says. “There’s still not anything man-made that can compete with our ears when they’re working well.”

Not yet, anyway.

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Step inside the anechoic chamber.