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To Quiet a Whirring Computer, Fight Noise With Noise
By ANNE EISENBERG
Published: May 27, 2004
HE constant drone of a computer cooling fan can be annoying. But a professor at Brigham Young University has taken an unusual step to mute this noise: more noise, produced in just the right quantities from tiny loudspeakers that surround the fan.
"We make anti-noise," said Scott D. Sommerfeldt, a physicist who created a noise suppression system with his students. It is the latest example of a technology called active noise reduction, or noise cancellation, well known from its use in headphones designed to block out the low rumble of jet engines.
The sound waves engineered by Dr. Sommerfeldt are out of phase with sound waves from the fan and thus they cancel each other out, substantially reducing fan noise.
Dr. Sommerfeldt's system has four miniature speakers and four even tinier microphones set in a ring around the computer fan. The microphones and other sensors detect the noise of the fan blades and, with the help of digital signal processing and algorithms, radiate opposing tones from the speakers. The whole system can be tucked into the same space that a conventional computer cooling fan would occupy.
Noise-cancellation technology has been in development for more than 50 years, Dr. Sommerfeldt said. In typical headphones, microphones in the headset detect noise; the speakers in the earcups counter with anti-noise.
Dr. Sommerfeldt's target was not the roar of aircraft, but the hum of office machinery. To counter this noise, some active noise reduction systems place microphones and related gear in the middle of a room to calculate the amount of cancellation needed.
But Dr. Sommerfeldt wanted to put the system within the computer casing itself so that it might one day become a built-in feature of personal computers.
Trying to cancel noise from within the computer turned out to be tricky, though. "You are asking for trouble if you place the mikes right next to the fan," he said. While it may be possible to reduce the sound substantially at the microphone, he said, the noise level could increase farther away, where someone might be sitting.
To solve the problem, Dr. Sommerfeldt built an aluminum enclosure that mimicked a typical computer chassis and put a standard cooling fan within. He and his students placed microphones and speakers around the fan, analyzing the nature of the sounds they would have to suppress and modeling systems to counter this sound.
"The fan is not an easy source of noise to control," said Gerald C. Lauchle, a professor of acoustics at Pennsylvania State University and a colleague of Dr. Sommerfeldt. "Many interacting mechanisms create the noise."
The specific sounds Dr. Sommerfeldt decided to counter were those made by the blades of the fan as they rotate and push air past obstructions like fan supports. The pushing is periodic, dictated by the number and speed of the blades.
In the experiments, sensors mounted near the blades of the fan recorded the motion, and the information was fed to the digital signal processor along with the data from the microphones that were monitoring the overall noise. Then algorithms developed by the group adjusted the amount of canceling sound waves sent to the speakers so that the basic tone of the blades could be suppressed, as well as overtones or harmonics of the basic tone.
"The basic tone is distinct," Dr. Sommerfeldt said. "It sounds like one you could hear if you hit a piano key."
To find the pattern that worked best, the experimenters set up systems with one, two, three and four speakers. "We went with smaller loudspeakers and more of them," Dr. Sommerfeldt said. "The best configuration turned out to be four speakers spaced around the fan."
The group measured the reduction in sound at the fan and at various points at a distance, said Brian B. Monson, a graduate student of Dr. Sommerfeldt.
The experiments used two fan sizes, with blade diameters of about three inches and two-and-a-half inches. The smaller unit allowed the researchers to fit the entire assembly of fan, speakers and mikes into the space that would normally be occupied by a standard cooling fan.
Both systems resulted in a substantial reduction of sound, Dr. Sommerfeldt said, ranging from about 10 to 20 decibels for the basic tone and for the three harmonics.
"This work is marvelous," Dr. Lauchle said of Dr. Sommerfeldt's results, "because it's a new way of controlling that tonal fan noise."
Although the noise of the fan has been toned down considerably, it still emits a faint hum. "We'vê dealt with the constant pitch sounds," Dr. Sommerfeldt said, "the tonal noise that sticks out above the smear of frequencies you get from random excitation" of acoustic waves. But the random noise remains.
"We'vê taken the process a good step," he said, "but there are still improvements to be made."
Robert L. Clark, a professor of engineering at Duke University who is an expert in active noise control, said he was pleased with Dr. Sommerfeldt's success. "Those fan tones tend to be very annoying," he said. "It's good that he's reducing them significantly with this new method."
Still, it remains to be seen whether consumers will accept the additional costs should the system be developed commercially, Dr. Clark said. "It's not an overly expensive system," he said, "but there are the prices associated with all those components."
"It's going to be a matter of what the consumer will be willing to pay for a quiet computer," he said.