Fraunhofer Adaptronics Alliance

Active Noise Control

Image 1: Acoustic demonstrator
Image 2: The principle of passive (left) and active (right) noise reduction

Noise harms human health

Our environment is marked by noise emitted by planes, traffic, machines – an increasing number of people suffer from that. Experts affirm noise to be an environmental pollution. Reduction of noise thus is an important challenge for environmental technique. The usual approach for noise reduction consists in passive insulation. Structural vibration provoking noise radiation can be reduced e.g. with insulating foam. However this approachis increasingly limited due to technical and economical reasons.
Actively influencing the dynamics of the noise emitting structure hence is an innovativetechnology for optimized noise reductionincluding all areas of everyday life.

Active system

Active systems are implemented to reduce noise at its source: An actuator induces inverted vibrations to the structure compensating the sound waves. Noise emitted by surface structures – e.g. engine hoods or motor way sound barriers – are application fields for this technique. This proposed principle of noise reduction is referred to as Active Structural Acoustic Control (ASAC). It can be taken as a representative for the structuraltechnology Adaptronics.

Acoustic demonstrator

Noise transmission and the active reduction is demonstrated on a rigid box made of plexiglass with a thin metal plate mounted on one side. The five plexiglass walls are constructed in a way that no noise transmission is permitted, i.e. they are soundproof. A speaker inside the box serves as an acoustic source. Once switched on, the emitted sound waves excite the metal plate, thus the plate vibrations transfer the noise outside the box. A piezoelectric wafer applied to the plate transforms the mechanical vibration into an electric signal. A second piezoelectric wafer serves as an actuator to induce the compensating vibrations. The actuator signal amplitude and phase relative to the sensor signal can be adjusted by the means of the two controllers. Once the actuator signal is adjusted to its optimum, the noise transmission is almost completely compensated – silence results outside the box while inside the acoustic source still produces noise.

Image 3: Experimentally extracted mode shapes of a metal plate at 162 Hz
Image 4: 354 Hz
Image 5: 528 Hz