May 27, 2004
WHAT'S NEXT
To Quiet a Whirring Computer, Fight Noise With Noise
By ANNE EISENBERG
THE 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've 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've 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.
aber ich bin mir auch nicht sooo sicher, dass ich das mit sicherheit für wahr sagen könnte, was ich mir vorstellte..
