Noise isn't just irritating, it can also be costly. It can lead to mistakes in production because it can affect how machines work. That's why researchers are finding ways to reduce noise at its origin.
Engines can be loud - loud enough to make a whole bus vibrate so much that the windows shake. Even small cars are noisy. The solution for some is to turn up the radio to drown out the engine. But there are better solutions than just trying to be louder than the noise. By producing similar but inverted sound waves, the disturbing noise can actually be canceled out.
The technology that enables active parts to react to outside influences is called Adaptronic. A good example of how such a system would work is inside the body of a car, according to Heiko Atzrodt, an engineer at the Fraunhofer Institute for Structural Durability and System Reliability LBF. Cars nowadays rest on a sub-frame which carries the suspension. The individual wheels are driven by a movable shaft to which the engine sends energy. So different noises are produced - by the engine, and by the wheels when they are turning.
Producing vibrations with special ceramic
Three things are needed to produce suitable inverted waves, which cancel the noise: A sensor is required to detect the noise, a measurement technique to evaluate it, and an oscillator to produce the vibration for the inverted sound waves. A case containing these elements would be screwed directly onto the suspension, or wherever it is required - where a vibration damper, rubber or spring normally go.
For example, a piezo ceramic, which can produce electricity from movement, could be used to make the oscillator. Piezoelectricity is produced by certain materials (like some types of ceramics) which have naturally accumulated electric charge and can release it in response to mechanical stress. Piezo ceramics are well suited to be adaptronic actors, which adapt and react automatically, because they would produce electric impulses in direct response to the movement of the car.
"Piezo ceramics can only make short movements, but they exert a lot of force," Atztodt told DW.
So they are especially suited to equalizing noise from high-frequency waves. If the engineers want to balance noise from low-frequency waves, electrodynamic actors can also be used. They have a coil like an electromagnet in which a magnetic field is created when the current flows through it.
Wafer-thin sensors detect waves
But how is the frequency for inverted waves worked out? Saskia Biehl, who heads the micro- and sensor technology group at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, developed special wafer-thin sensors which can, for example, be installed as a washer or even cast in a metal block. If you exert pressure on these discs, they immediately send a signal to the control electronic.
These highly sensitive sensors are used, for example, in machine construction. Today, automated cutting and milling machines or robots have an extremely high level of precision. Through motor vibrations and movements of the arms and the grippers, the machine can nullify its own precision. In the worst case, it can produce defective goods if the vibrations tamper with the result.
These noise dampers can also be used for laser or highly-sensitive optics production factories. They are then, for example, built into the laboratory working table to cancel out noise, which might come from a passing train or a truck on the street.
Speakers at a construction site
Company WaveScape Technologies has developed a sound generator for construction machines so that construction sites will be quieter in future. It has a loudspeaker and can produce inverted waves which swallow the noise from the machines.
Compressors and generators could therefore become considerably quieter. The principle is the same as for high-quality headphones that are able to blend out aircraft noise - except that these will be used directly at the source of the noise.
And engineers can successfully combat noise even without electronics. The Fraunhofer Institute LBF presented a centrifugal pendulum at the Hanover Fair 2013. The bulky part of the pendulum is stuck on the shaft of the engine - of a large ship, for example - and can lower the noise the engine produces.
The advantage of such a centrifugal pendulum is that its frequency adjusts automatically, without electronic assistance, to the speed and thus to the vibrations of the engine, because it has a natural frequency that relies on centrifugal force.