Science / Medicine : Sounds...
Today’s world is just too noisy. Yet if current research on developing computer-directed active noise control (ANC) systems succeeds, there will be a whole lot more noise in the air--and the world will be a quieter place because of it.
Already, ANC systems are quieting industrial air conditioning, heating and ventilating fans, magnetic resonance imagers, high-end refrigerators and grain-loading equipment. By 1993, several luxury cars will be equipped with ANC mufflers, and from there, the list of potential applications becomes nearly endless.
“We’re talking about a technology that could make a significant impact on reducing unwanted, and oftentimes dangerous, noise in the home, on the street and on the job,” said Irene Lebovics, president of NCT Personal Quieting, based in Stamford, Conn.
Active noise control is the seemingly contradictory process of using noise to get rid of noise. It is based on the fact that when equal but opposite sound waves collide, they cancel each other, leaving behind nothing but silence. “In principle, the idea of active noise control is simple,” said James D. Jones, assistant professor at Purdue University’s Ray W. Herrick Acoustics Laboratory. “In application, though, it’s a very complex and difficult problem.”
The basic ANC system consists of two microphones, a computer and one or more speakers. The first microphone picks up the offending noise close to its source and feeds the signals into the digital processor chip. The processor is programmed with a computer model that mimics the source of the noise, be it a fan duct, muffler pipe, airplane cabin or whatever.
The model then projects what the sound waves will look like downstream from the input microphone and generates the appropriate “anti-noise” that will come out of the speakers located some distance away from the input microphone. The second microphone sits just past the speakers and provides error correction feedback to the model, which then adjusts its signals to improve noise cancellation.
In an airplane or a car, for example, ANC loudspeakers might be placed in the headrest, canceling out the noise while the passenger is seated. With a muffler, the speakers would be placed at the exhaust pipe, quieting the car before anyone can hear it.
Except for the few ANC systems in place, sound deadening today is accomplished by using passive barriers: soundproofing materials such as plastic foam, masonry or air baffles that gradually reduce vibrations and thus noise. Passive systems, however, deaden only high-frequency noise, while the biggest industrial problem is low-frequency sound. In addition, passive soundproofing is bulky and heavy, which limits its usefulness in many applications, including in motor vehicles, aircraft and headphones.
ANC is certainly the most promising silencing technology to appear since foam insulation. Actually, it’s a technology that was first studied in the 1870s, but what distinguishes today’s noise controllers from those built even five years ago is the availability of powerful, low-cost computer chips that not only convert sounds into digital signals but analyzes those signals as well.
“A decade ago, we would have needed a $100,000 minicomputer to do the signal processing that an ANC device needs to achieve effective noise cancellation,” said Larry J. Eriksson, vice president for research at Nelson Industries Inc., whose Digisonix division makes ANC systems. “Today we can do the same processing with a single chip costing less than a thousand dollars, and next year we’ll be able to do more for even less.”
Credit computer engineers for creating these advanced chips, but honors also must go to the acoustic scientists who have harnessed these chips’ power. By studying the sound-producing characteristics of hundreds of materials and thousands of environments--from something as simple and benign as a heating duct to a system as complex and hostile as an engine exhaust manifold--these researchers have developed a variety of computer models that can predict how sound waves will behave and generate the corresponding opposite sound waves, all in a matter of a few microseconds at most.
Engineers are now trying to capitalize on these advances. One application with life-saving potential is in quieting magnetic resonance imagers. While physicians have been thrilled with MRI’s diagnostic capabilities, their patients have been less than delighted with the tremendous amount of noise these machines produce. Some patients, in fact, balk at having an MRI scan performed because of the noise. But by using an ANC headphone that produces a zone of silence around a patient’s head, the scan becomes a much more pleasant experience.
In the future, ANC headphones could also find use on construction sites, inside helicopters and even in the home--imagine a headset designed to tune out barking dogs or screaming children but still allow you to hear the telephone ring. Eventually, a similar system could be built into a car’s headrest, cutting road noise to a bare whisper without blocking out sirens, car horns or even conversation.
The next product that should hit the marketplace is the electronic muffler, slated to appear on cars in 1993. The auto industry is eager to use ANC mufflers because the device improves engine performance and power while cutting fuel consumption 2% to 5% by eliminating the back pressure that a conventional muffler produces. While 2% may seem to be a minor improvement, auto manufacturers consider that a big gain.
Cars and trucks, though, represent but a small part of the potential muffler market. For example, CSX, the transportation conglomerate, had a severe noise problem with its grain loading equipment. An NCT electronic muffler not only cut noise levels by 80%, bringing them well below government noise standards and eliminating employee complaints, but also saved CSX “a substantial amount of money by improving fuel efficiency by 20% and cutting equipment costs,” according to Lebovics.
In the long run, though, the best way to cut noise in some applications may be to eliminate the machine vibrations that generate sound waves. A variant of ANC may be able to do just that by generating “anti-vibrations” directly onto the noise-producing object. “Not only would this cut noise,” said Purdue’s Jones, “but it could improve an object’s durability since vibration is an important cause of wear and tear.”
He and his students are studying the use of piezoceramics actuators to dampen vibrations. Piezoceramics are materials that contract and expand when stimulated with an electric signal. One source of such a signal could be the same digital processors, run by the same filters, used in ANC systems. “If we can get this to work, it may be possible to install piezoceramic actuators directly on a surface that vibrates and control at least that part of the vibration that is capable of generating acoustic radiation,” Jones said.
Active manipulation of vibrations and sound waves has clearly reached a point where success is breeding still more success. “Like any new technology, we have to be careful not to sell it as a panacea for all noise and vibration problems,” said Eriksson of Digisonix. “But at the same time, it’s clear that this field is taking off. It’s going to be interesting to see what happens over the next decade.”
Sounds of Silence: 1. Input Microphone: The input microphone picks up offending noise close to source. It feeds that signal into a digital processor chip. 2. Controller: The processor is programmed with a computer model to mimic the sound of some object-a fan duct or muffler pipe, for instance. 3. Loudspeaker: The model then projects what the sound waves will look like downstream form the input microphone. It generates the appropirate “anit-noise” that will emerge from speakers located away from the input microphone. 4. Error Microphone: A second microphone, located near the speakers, provides error correction feedback to the model, which then adjusts its signals to improve noise cancellation.
