Active noise control works - but where is it going?

Larry Riddle

Many people have heard of active noise control, but with the exception of headphones, few people have experienced its benefits. Why has this happened?

The short answer is cost. Unlike over-hyped technologies, active noise control works. The obstacle has been to make active noise control a cost-effective solution to noise problems. This obstacle has been far more difficult than many people expected. When a technology is more expensive than its benefits are generally worth, only a small market exists for the technology.

Now, new technologies are usually expensive to start out. Early users of a technology pay a higher price for being first, with a competitive advantage as the benefit. Technology developers gain valuable experience working with early customers. This experience and know-how yields product improvements that provide more economical solutions for future customers. In this way, the technology spirals into wider and wider use, as the cost for implementation falls.

So, what happened to active noise control? Why don't we see active noise control technology in our daily lives? The answer lies in the fact that most consumers are reluctant to pay more than 100 to 2000 dollars for noise reduction solutions. This cost range has been difficult to achieve with effective active solutions. The notable exceptions are noise canceling headphones and some dishwashers.

The result is that other applications of active noise control have been in industrial situations, where the benefit for active noise control is driven by hearing safety concerns. In some cases, improving speech communication has been important enough to use active noise control . Even in industrial markets, active noise control technology spirals have been rare. The result is that the pervasive use of active noise control still waits for the "killer-app" that can propel it out of the one-of-a-kind, custom project cycle that best describes how much of active noise control technology is applied today.

So, where has active noise control been successfully applied? A partial list:

• Exhaust mufflers for internal combustion engine
• Headphones for aircraft use
• Headphones for auto racing
• Ambulances
• Dishwashers
• Vibration isolators for engines
• Vibration isolation to protect electronics from vibration and shock
• Aircraft cabin noise reduction

These products are successful because they have taken active noise control technology and reduced it to practice. Another common element is that with the exception of cabin noise reduction, the solution is 'simple'. By simple, I mean that the solution does not require an extensive number of microphones, sensors, cables, etc. In many cases the controller that produces anti-sound commands can be built with analog circuits, leading to lower costs and portability.

The exception to this rule is aircraft cabin noise reduction. In this case, numerous speakers (or headsets) and microphones are used. In the noise reduction system built into new Boeing aircraft, it appears that the noise reducing apparatus has been integrated into the entertainment system. This has the benefit of reducing the cost of the noise reducing system, since it is being added to an existing acoustic system. The existing entertainment system uses headphones, signal processors, and cabling that can be leveraged by the active noise control (ANC) design.

So where should we go in ANC technology development? I think that continued development of simple solutions is best done through education. Present commercial efforts are mired in patent issues and return on investment questions. Through education, engineers can be trained in the use of active noise control. When these engineers are confronted with design problems, they will be equipped to find niche solutions using active noise control.

Researchers in this field should focus on two approaches. The first is the development of integrated active control solutions for complex noise problems. Automobile audio systems, with integrated personal communications systems and active noise control technology could improve driving enjoyment and safety. A second approach is the search for the 'killer application'. As an example, are there new ways of communicating that rely in a fundamental way on ANC technology? We will have to wait and see.