147th ASA Meeting, New York, NY

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Active Noise Control

Kenneth Cunefare - ken.cunefare@me.gatech.edu
Department of Mechanical Engineering
Georgia Institute of Technology

Special Lay-Language Paper for the
75th Anniversary Meeting of the
Acoustical Society of America
May 2004

Active noise control, the suppression of an undesired sound by the action of another sound, has had a long and not entirely painless development arc over the past 70 years. From its genesis in the 1930s, to the current state with consumer products on the market, the development of active noise control has closely paralleled and depended upon the advancement of low-cost electronics.

The basic elements of an active noise control system include one or more microphones to sense the noise over some location of interest and a means to produce an appropriately tailored sound field that acts as a "control field." The control field is usually created through the action of a control system connected to loudspeakers. Applications of the concept have included control of noise produced by industrial fans, suppression of noise in heating, ventilation, and air conditioning ducts, reduction of vehicle exhaust sounds, creation of "quiet zones" within vehicle interiors, reduction of the noise levels inside aircraft and spacecraft launch fairings, and the now almost commonplace active noise control headsets in aviation and consumer use.

Active noise control was invented by Paul Lueg in Germany in 1932. His 1934 U.S. patent included three of the common concepts of active noise control: suppression of noise in ducts, cancellation by interference at points in space, and creation of protected "zones of quiet." While valid in concept, the available detection, processing, and sound generation systems of the day apparently did not lend themselves to development of a commercial product. There is some evidence that Lueg performed an experimental validation of the concept, but any detailed records are lost to us.

In the 1950s, Harry Olsen took up the question of active noise control systems, using an "electronic sound absorber" to experimentally demonstrate the performance of a system intended to create a zone of quiet about the head of an individual exposed to a sound field. One conceptual application of the system was to reduce the noise level at the passenger positions in vehicles. While the system worked, it (and its descendants) subsequently proved sensitive to the location of the listener's head; relatively small changes in the head position would lead to significant changes in the noise perceived by the listener. More successful systems used many actuators to control the sound at many microphones, which has led to actual use of the concept in passenger vehicles and in the SAAB 2000 and DASH 8-Q400 aircraft.

The late 1950s also saw the first implementations of what would later become the most common active noise control device, noise reducing headsets. W. F. Meeker of RCA performed a number of analyses and experiments on "active ear defender" systems for the Wright Air Development Center, with obvious interest in reducing the noise exposure of pilots. The first highly public civilian use of such a system was by Dick Rutan and Jeana Yeager in their around-the-world flight in 1986, using active noise control headsets supplied by Bose Corporation. Active control headsets are now a common accessory in
the aviation community, and are marketed to the public at large for reducing noise while flying on passenger aircraft, mowing the lawn, etc. In terms of commercial success of active noise control concepts, active control headsets would seem to hold the leadership position.

Active noise control in ducts received, and continues to receive, extensive attention. In duct systems, it is easier to implement an active control system, as one can use a microphone upstream in order to detect what is coming toward the system; this upstream sensing location provides an opportunity to know what the system has to handle, in order to reduce the noise levels that propagates downstream. For a time there was a commercial system available from Digisonix, now defunct, that could be customized for a particular application, be installed, turned on, and quiet would ensue. Having personally witnessed the effectiveness of this system, and the numerous potential applications for such a system in industrial and commercial facilities, I can only hope that a similar product will someday return to the market place. While controllers are available (e.g., Causal Systems; EZ-ANC), all-inclusive off-the shelf solutions including the necessary sensors and actuators are wanting.

Publications related to active noise control amounted to but a few per decade from the 1930s through the 1960s, but then rapidly accelerated into the tens, hundreds and thousands through the 1970s, 80s and 90s. Active noise control developments continue apace today, with many of the same applications in mind as evidenced in Paul Leug's original work. But what truly enabled the rapid advancement and attempts at commercialization of active noise control concepts has been the development of compact, light-weight, inexpensive electronics for detecting the noise to be controlled and generating the necessary control signals. For example, all of the electronics necessary for an active control headset are frequently packaged within the headset itself; yet they are compact and not too heavy for extended wear. For highly complex active control systems, the rapid advance in computational power and compact signal processors has been equally enabling.

But why is not active noise control more prevalent than it is today? This is a point that has long frustrated those who have labored to bring active noise control products to market, with explanations ranging from a willingness of individuals to expose themselves to noise, to a reluctance on the part of manufacturers to include active noise control systems in products due to increased cost or a lack of integration between audio systems and noise control. Additionally, cost and reliability concerns are also a barrier. In any case, the hard fact of it is that active noise control systems have not been able to make a significant impact on the noise to which we're all exposed on a daily basis, perhaps to the detriment of us all.

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