Mark A. DeSorbo
CORNING, NYAFTER ABOUT TWO months of evaluation, David Sutton, a vibration and acoustics engineer at Corning Inc., made several recommendations to management, from enclosing vacuum canisters to wrapping HVAC ducts, as part of a company-wide effort to decrease noise in the optical fiber manufacturer's ISO Class 7 (Class 10,000) cleanrooms.
“It looks like we'll see an 8-decibel (dB) drop on the vacuums due to the enclosures, and with the fiberglass wrap around spindles, guards and motors of process equipment, we're predicting a decrease of 5 dB,” Sutton says.
The redesign of some air vents is also part of the noise reduction effort. “Right now, the vents are narrow and when air comes through, it generates a lot of noise. So, we're going to look into opening them up,” he says.
Sutton began looking for cleanroom noise culprits last February in an effort to bring the average sound level to 85 dB on human ear-level equivalent A-scale [dB(A)]. [See “Turn down that cleanroom!,” CleanRooms, March 2000, p. 1]. The 85-dB(A) level is Corning's noise level standard for an 8-hour workday. That's 5 dB(A) lower than what the Department of Labor's Occupational Safety and Health Administration (OSHA) mandates and is regarded as a “noise dose.”
In areas around vacuums that Corning uses to pick up waste from its optical fiber process equipment, Sutton used a sound analyzer to measure noise levels as high as 100 dB(A), which is about the same amount of raucous generated by a jackhammer ripping through tarmac.
“In some areas it could be that high, which contributed to the overall level of noise in the cleanroom,” he says. And 100 dB(A) is on the high side considering pain and hearing loss occurs at 140 dB(A), or 20 dB(A) more than a roaring jet would sound like a few feet away.
Controlling noise in a cleanroom can be a difficult task, says expert Colin Gordon, president of Colin Gordon & Associates (San Mateo, CA). In fact, the sound level in a normal cleanroom is about 65 dB(A). At that level, Gordon says, communication can be difficult or impossible at distances of six feet or more.
While noise and vibration goals should be outlined in the early stages of a design and build project, there are still plenty of methods to combat it, Gordon says, adding that Sutton's use of fiberglass wrap is not enough.
“If you can pinpoint individual sources, you can then take steps to quiet them. If it's coming from the casing of a machine, there is no shortage of techniques to attenuate sound, but fiberglass itself does not absorb noise,” he says. “You have to use something solid, like barium-loaded vinyl. You put a layer of fiberglass, then barium-vinyl and then you have noise control.”
There are other simple steps for controlling noise as well, Gordon says. Sometimes, fans run at faster speeds than necessary and a lower setting can cut noise without having an effect on cooling. Excess noise can also be eliminated in selecting an air compressor as well. A reciprocating compressor, like a car's engine, creates its share of noise and vibration, he says. “A screw-compressor is quieter, it gulps air in one end and exhausts through another,” Gordon says.
But if wrapping, enclosing and turning a fan down do not bring the noise level down to a dull roar, it may be time to look at the process. That is the crossroads Sutton is expecting to come to over the next few months as he continues to beat back the noise.
There are two sides to combating noise, Sutton explains, adding that it is easy to wrap and enclose. “Then there's the process side, which is often difficult because you don't want to affect the product. The key is maintaining a low noise level when process and production is upped,” he says. “We want to cut the dB(A) wherever we can because as the process level changes, we don't want to have to come back in here and do this again.”