Science Works to Muffle the Sounds No One Wants to Hear
Taking a cue from children who clamp their hands over their ears to block out parents’ scolding, efforts to reduce noise have long focused on trying to muffle unwanted sounds by using barriers and insulation.
But now scientists are on the attack, using noise to fight noise.
“In active noise cancellation, you have to make a noise just as loud as the noise you are trying to cancel. What makes it work is that the sound waves are mirror images of each other,” says Larry Eriksson, an acoustic researcher.
Eriksson likens the situation to a lake where the crest of a wave traveling in one direction meets the trough of an identical wave heading the opposite direction, “creating a smooth surface.”
The concept of noise cancellation is not new. In the 1930s, a German engineer obtained a patent for a system to cancel unwanted sound by generating anti-noise--sound waves exactly 180 degrees out of phase with the source.
However, Eriksson says such systems were not practical because they could not adapt to changes in the noise source or surrounding environment.
“These changes occur very rapidly and you really need computers to constantly monitor the efficacy of the sound-cancellation system,” says the audiologist, who works for Nelson Industries of Stoughton, Wis.
Used on Fans, Pumps
The advent of microcomputers and an improved understanding of how to use the new technology has enabled active noise control to become a commercial reality over the last two years.
Active noise cancellation is used to quiet irritating sounds from fans, ducts, industrial pumps, medical equipment and airplanes.
Systems are also being devised to reduce the noise of automobile exhausts and to create “quiet spaces” in certain work areas. Researchers are also trying to come up with ways to quiet the sound of sirens for people inside ambulances and to insulate entire rooms from outside noise.
The easiest sort of noise for such systems to cancel is low-frequency sound from a repetitive source. The pitch equivalent to A above middle C on a piano “is the upper end of where the products work,” Eriksson said.
Passive noise control, which restricts the flow of sound waves with mechanical devices such as mufflers, tubes, baffles and acoustic filters, works more efficiently than active control on high-pitched sounds. But passive control is a poor method of quieting low-frequency noise because the equipment to silence the large waves is cumbersome and takes up too much space.
Most active noise control systems consist of a microphone to detect the bothersome sound, microchips to process the information and electronically driven loudspeakers to generate sound waves to cancel the noise.
Under current technology, the anti-noise generally is broadcast in a somewhat confined area such as a duct or transmitted through earphones placed on a person’s head.
Businesses using such systems to cancel out noises from large fans and ventilation ducts include foundries, plastic processors and incineration facilities.
Can Be Damaging
“Generally, most of the problem with industrial fans is the noise they cause in the surrounding community,” Eriksson said. “Large fans are often located on the roof and can produce high sound levels often audible a half-mile to a mile away from the plant.”
“In some cases, noise can be damaging to the hearing. In many other cases it is merely annoying or bothersome. But if it is keeping you from sleeping at night, it certainly is detrimental to the quality of life and health in the long run.”
A medical setting may be the place where many people will have their first encounter with active noise control.
“It sounds like a jackhammer and it sometimes goes slower and sometimes faster. . . . The noise is very loud and you can’t predict when it’s going to go on or off,” says Susan Shapiro-Bikoff, describing her recent experience in a diagnostic device called a magnetic resonance imaging scanner.
“You’re in a tunnel and if you’re at all claustrophobic, the noise is going to increase your fears,” she says.
The source of the MRI’s low-pitched, thumping sound is a 12,000-pound superconducting magnet used to make moving X-ray-like images of a patient’s body during a procedure lasting about a half-hour.
If the MRI used on Shapiro-Bikoff had been equipped with an active noise-cancellation device, her examination for possible back injuries might have been less traumatic.
A firm called Noise Cancellation Technologies Medical Systems plans to install a device to reduce noise at an outpatient MRI center in Houston this summer. The anti-noise frequency will be transmitted to patients via headphones and the resulting silence will be filled with relaxing music.
Vibration Wave Program
Michael Parrella, chairman of NCT Medical Systems, said his Columbia, Md., firm is also developing equipment designed to cancel vibration waves in a method similar to noise cancellation. Such technology could help increase the accuracy of very sensitive equipment, such as electron microscopes, where even slight vibrations can affect results.
A development that could have more impact on the life of the average person is an automobile “muffler” employing active noise cancellation. The manufacturer recently applied for a patent on the device and expects to have the first product available by next year.
Current mechanical mufflers force gases back into the engine’s manifold, resulting in a loss of “8% to 20% of horsepower,” Parrella says. Active noise cancellation would rely on electronic devices and not create any back-pressure.
Saving Offsets Cost
Although he concedes electronic “mufflers” probably would cost more than the old, metal standbys, Parrella says he expects the price difference to be offset by savings on gasoline.
Airplanes and helicopters may also benefit from the fledgling noise-cancellation technology.
The pilots of the Voyager, an experimental aircraft that made a record-setting, nine-day flight around the world in December, 1986, wore headsets equipped with sound-cancellation devices to avoid hearing loss and to reduce fatigue.
The headsets generated anti-noise to cancel engine noise that cannot be reduced by traditional headsets, which use foam rubber or similar material to insulate the ear from sound.
The maker of the new device, Bose Corp. of Framingham, Mass., claims its invention “allowed the pilots to hear radio communications more clearly and to reduce noise-induced hearing loss.”
Significant Innovation
Dr. George Julian, flight surgeon for pilots Dick Rutan and Jeana Yeager, said the noise-cancellation system was one of the “most significant innovations that were part of the Voyager project.”
The entire noise-cancellation system--microphones to monitor sound at the user’s ear, microchips to process the information and speakers to generate an out-of-phase cancellation signal--is contained in the ear cups of the headset. Passive noise-reduction equipment was also incorporated to reduce high-frequency sounds.
Protecting the hearing of soldiers aboard military aircraft has long been a major challenge to noise-control experts.
“Noise has been a real problem. We are paying a lot of disability (benefits) to people with permanent hearing damage,” said Dan Schmidt, a Navy aerospace engineer. “Right now, there exists an immediate need in a Navy helicopter to protect the air crew from noise.”
Schmidt says an experimental helicopter, called MH-53E, was designed with an extra rotor blade and a third engine to improve its effectiveness in mine-sweeping operations--changes that have made it “a real noisy aircraft.”
“The MH-53E program is going to be canceled unless we can find a solution to the noise problem,” Schmidt said.
Copter Noise a Hazard
Studies have shown that crew members riding in the helicopter for more than a few hours could incur permanent hearing damage, Schmidt said. To communicate over the helicopter’s mechanical noise, Schmidt said, crew members currently turn the volume control on their radio headsets all the way up “and the signals start to break up and make the noise problem even worse.”
Preliminary tests using a noise-cancellation headset, patterned after the Voyager gear, on MH-53E crews have been “real favorable,” the engineer said. The headsets will probably cost the Navy between $300 and $500 and will be built to withstand shock, salt, fog and other conditions experienced in “everyday flight,” he said.
Studied for Other Craft
The Navy is also looking into using the devices in other aircraft, such as the AV8-B Harrier, a jet fighter that takes off vertically; and the OV-10, a turboprop jet used for observation.
Gary Koopmann, director of the noise-control laboratory at Pennsylvania State University, calls active noise cancellation “a powerful new tool to treat noise problems.”
“Traditionally, we just had sound-control treatments involving damping materials. In addition, low-frequency sounds have always been difficult to deal with.”
However, Koopmann says he does not expect active control to completely supplant passive techniques. Under current technology, he says, it would be hard to use active noise cancellation to control high frequency sounds or reduce noise in large areas, such as auditoriums or airplane cabins, with a wide range and source of sounds.
