Mark A. DeSorbo
CORNING, NY—A sound analyzer readS 85 decibels on the A-scale [dB(A)] in typical heavy street traffic, while ring exhausts that pull particles, heat and fumes from Corning Inc.'s optical fibers register in excess of 88 dB(A) on the human ear-level equivalent.
A jackhammer ripping through the tarmac tops out at over 100 dB(A), so do the vacuums that Corning uses to pick up waste fiber. That's on the high side, considering that extreme
pain and hearing loss occurs at 140 dB(A), just 20 dB(A) more than what a roaring jet engine would sound like just a few feet away.
Noise that permeates Corning's ISO Class 7 (Class 10,000) cleanrooms is somewhat normal. Experts, like Colin Gordon, would agree that Corning's cleanrooms are not typical in the microelectronics industry and that air re-circulation systems are usually the noise culprit.
'Corning seems to have some terribly noisy production sources to contend with,” says Gordon, president of Colin Gordon & Associates.
Noise, however, is essentially a growing pain, a symptom of the demand that has forced manufacturers of all types to increase production. For Corning, it means more equipment, more clamor and noisy cleanrooms.
Enter David Sutton, a vibration and acoustics engineer. He has the task of reducing sources of cleanroom noise in order to bring the average sound level to 85 dB(A), which is Corning's noise level standard for a 12-hour workday. That's 5 dB(A) lower than what the Department of Labor's Occupational Safety and Health Administration (OSHA) mandates, and Gordon notes that OSHA defines limits as in a 'noise dose.”
Sutton expects to complete the noise production project by late spring. 'What prompted this was that people said it was too loud in some areas, and we found noise levels in certain areas exceed Corning's noise standards,” he says.
The primary area of concern is the production area, which is 60 feet wide, 180 feet long and 26 feet high. It houses 10 five-story towers where fiber is pulled from glass pre-forms. 'Within the towers, it sometimes exceeds 100 dB(A). People working in there have to wear earplugs, and they don't like to. To talk to someone, you have to pull them out in order to hear,” he says. 'Ideally, we'd like to enclose the towers, but people still need to get in there, and we'd have to exhaust the heat if we do enclose them.”
Aside from that, other noisemakers to quell include power supplies, compressors and ventilation that Sutton hopes to combat with silencers and acoustic batting. He's even considering using white noise to mask the racket.
Sutton's work was dictated long before he accepted the assignment. Noise and vibration in a cleanroom is sometimes not a priority in cleanroom design, and in some cases, it's completely overlooked.
'It is a critical pre-design issue, and noise control should be a consideration even before the first line is drawn,” says Dr. Kenneth Goldstein, principle of Cleanroom Consultants Inc. (Phoenix, AZ). 'The problem is that [Sutton] is trying to put a patch on after the fact, and that's extremely difficult. It's almost impossible to retrofit a cleanroom without downtime. There's no simple thing you can do.”
Most of the time, mechanical engineers are doing the bulk of the work in a construction project, and Goldstein says they are 'not versed in noise control. They are power distribution specialists.”
Sutton, who is also a mechanical engineer who has worked on many design-build projects, can vouch for that. 'Noise was the least of my worries,” he admits.
There are few people knowledgeable about the 'black art” of cleanroom noise and vibration control, Goldstein says. 'There are maybe a half dozen people in the world who deal with noise and vibration control in the cleanroom,” he adds.
Gordon, being one of those few people, disagrees that noise control in a cleanroom is 'black art.” Rather, it is a specialty within the microelectronics industry that many do not get involved in. The reason, he says, is simple: cleanrooms are specialty environments.
'You can't put carpets in a cleanroom to absorb noise,” he says, adding that cleanrooms use 100 times more energy than a normal office building, and the equipment in a cleanroom is highly sensitive to vibration and noise, which is 'another form of contamination.”
Keeping noise down to a dull roar in a cleanroom is a tough task to begin with. Cleanrooms are naturally noisy because of constantly circulating air. The sound level in a normal cleanroom is about 65 dB(A), says Gordon. That means normal voice-level conversation over a distance of six feet or more will be difficult or impossible. 'Communication will require raised voices or shouting,” he adds.
In the early stages of a design/build project, Gordon says his consultants typically ask clients what the cleanroom should be designed for and what types of tools will be used. 'We then work through the project with the engineers to make sure noise and vibration goals are met,” he says.
In up-and-running cleanrooms where the noise level is an issue, Gordon says cladding can be fastened around machines to absorb vibration and noise. Silencers can used to muffle the rattle and hums of ventilation systems and fans. 'You have to identify where the most noise is coming from. Batting or sound-absorbing materials are not acceptable in cleanrooms because of particle generation,” he says. 'To contain noise, one needs an airtight enclosure of steel, aluminum or some other solid material with enough surface weigh to resist excitation by the impinging sound pressures adds.”
Gordon also believes that using white noise to cover up noise will only make cleanrooms noisier.
Goldstein suggests that if equipment is outside of the cleanroom, like compressors, pumps and motors, it is possible to silence the noise, by building a sound enclosure around it. 'You can build a tiny house of batting—sound absorbing material—around the equipment,” he says, adding that heat generated within the enclosure must be exhausted.
In his search for noise, Corning's Sutton leaves no piece of equipment unturned. His latest victory involved vacuums with three motors attached to 55-gallon drums that generate a great deal of noise. 'Each of the motors has an on/off switch. It would get so loud, you couldn't tell if all three motors were on,” he says. 'So, I installed one switch for all three motors and a light to indicate when they are running. Every little bit helps.”