Purdue News

January 18, 1992 Noise Creates Sound Of Silence WEST LAFAYETTE, Ind. – Scientists at Purdue University are making noise to reduce, rather than increase, the din of everyday life. Researchers at Purdue's Ray W. Herrick Laboratories are among the nation's leaders in the development of active noise-control systems that can eliminate unwanted sounds from cars, airplanes, ventilation systems and other sources. "Sound in air is created by small fluctuations in air pressure," says Jim Jones, assistant professor of mechanical engineering. "Active noise control involves creating a sound that is equal in amplitude (volume) and identical in frequency (pitch) as the noise we want to eliminate," says Jones. "However, the sound waves are inverted, or opposite in phase, so when the offending noise increases air pressure, the noise-control system causes a corresponding decrease to cancel the sound. "It works like adding a positive and a negative number together to get zero." Jones says some people have referred to active noise control as "anti-noise" or "negative noise", but it is actually just an inverted form of the sound. Active noise-reduction systems use sensors such as microphones to detect the offending sound and send it to a computer. The computer analyzes it and generates the opposing sound, which is reproduced through appropriately placed speakers. Each spring the Purdue School of Mechanical Engineering conducts a short course in active noise and vibration control for scientists and engineers from businesses and other institutions. Jones says it was the first program of its kind offered in the United States. Robert Bernhard and Rahmatallah Shoureshi, both associate professors of mechanical engineering, teach the course with Jones and conduct research on active noise control at Herrick Labs. A simple demonstration of active noise reduction uses two loudspeakers, each emitting the same tone. The cones of the speakers vibrate in and out to create small changes in air pressure and generate the sound. The signal to one of the speakers can be inverted so that as its cone moves in, the other speaker's cone moves out. If the speakers are positioned face-to-face, the sound will be cancelled. Jones says active noise-reduction systems are on the verge of breaking into the market. "The electronic muffler will probably be the first large-volume application in the next few years," he says. "It's still too expensive, but volume production in the auto industry would drive the cost down to make it economically feasible." Current mufflers use a series of chambers and restrictions in a tube to reduce exhaust noise. While the car is quieter, the muffler also creates back pressure on the engine, reducing performance and fuel efficiency. Jones notes that an electronic muffler would eliminate exhaust noise without the increase in back pressure, resulting in a quieter, more efficient car. "As an added bonus, you could tailor the sound of your car," says Jones. "A flick of a switch could change the engine sound from highly muffled to the deep, throaty roar of a sports car." Jones says traditional, passive noise-control methods still will be used in conjunction with the new active devices. Passive techniques isolate the noise source from listeners with vibration mounts, baffles, walls and barriers of sound-deadening materials. While passive techniques perform well at high frequencies, active systems work best at lower frequencies. Jones says the next generation of turboprop airplanes may use active noise reduction. Microphones near a passenger's head would monitor engine and propeller noise. The signal would be processed and played back, effectively canceling noise around the passenger. The system, he says, could cut the weight of the aircraft by reducing the need for heavy wall treatments that would otherwise be used to muffle sound. Another common source of annoying sound is from heating, ventilation and air-conditioning systems, and Jones says active noise control is ideal for canceling noise generated by air flow and fans. The Purdue lab has a large section of ductwork attached to an adjustable fan for experiments to make forced-air ventilating systems quieter. Research is also under way on the next generation of active noisereduction systems. Jones says rather than using speakers to cancel unwanted sounds, an entire structure such as an airplane fuselage can be precisely deformed to cancel out noises generated along the surface. "We can reduce noise in a structure by embedding devices that vibrate the structure to cancel the noise," says Jones. "Other researchers are working on materials that can be deformed yet still be strong enough to take the stresses of applications like flying and landing an airplane." As the cost of producing active noise-reduction devices comes down, Jones says, they may even be used in household appliances. Before too long, there may be a speaker in your refrigerator humming along with the motor to make the kitchen a quieter place. Purdue News Service: (765) 494-2096; purduenews@purdue.edu