Active noise control

Under anti-noise (also called " active noise cancellation ", English Active Noise Reduction [ ANR ] or Active Noise Cancellation [ ANC ] ) it is colloquially sound that is produced artificially in order to wipe out by destructive interference of sound. For this, the generation of a signal is desired, corresponding to the interfering sound exactly opposite polarity.

Energy conservation

We have to distinguish two different problems:

• Exuding a noise source from sound, attempts by " anti-noise " from a remote location to achieve that by destructive interference, the resulting amplitude (and thus the sound energy ) in some limited space areas is lower. In adjacent areas, however, increases the sound energy, as energy is absorbed at any point, but simply redistributed. It has not been successful, a plurality of wavelengths large volume to calm overall. Success was far only the active sound attenuation in ducts ( linear problem ).
• In a sufficiently small confined area having dimensions smaller than λ / 4 ( such as between the ear and headphone) no interference is present in the strict sense, but the membrane of the headset acts as a movable wall, which provides for substantially constant total pressure (see acoustic short circuit ). Must be distinguished from the acoustic insulation, is converted in the incident sound into heat energy. In this way, only the reflection is prevented.

Application Headphones

With headphones with active noise reduction, the ambient noise is measured and calculated therefrom using the acoustic transfer function of the headphone, the proportion who would still remain on the ear with a built- in microphone. Then against a pole headphone signal is generated for this part for compensation. On tympanic membrane of the sound from the outside and the signal from the receiver meet together as a sound. The sound pressure level is significantly reduced. In addition, a target sound (voice, music) can be played through the headphones.

Also, headphones with active noise reduction systems can not completely eliminate noise. Firstly, every person has a different inner and outer ear shape, so that only one laboratory carried out precise adjustment would allow a complete neutralization. On the other hand also transmits the skull bone sound to the eardrum ( " body sound "); this proportion can not be influenced with Active Noise Reduction.

ANR is particularly suitable for the attenuation of low frequencies, where wavelengths are large compared to the dimensions of the headset. Here, only the sound over the entire surface of the headset on the same phase, so that it can be completely attenuated by an anti-phase signal. At high frequencies, this method has little effect, since different phase positions occur then the headphone surface and these are also dependent on the direction of incidence of the sound. In extreme cases, it may also occur at high frequencies pointwise amplification of the sound. Since a tight -fitting headphones can attenuate high frequencies well, low frequencies but only very poorly, ANR can be used to improve the lack of attenuation of headphones at low frequencies.

Occasion to develop such headphones was the need for noise protection for pilots. ANR-Kopfhörer-/Mikrofonkombinationen (headsets ) can in aviation - are used to ensure fatigue-free an environment as possible for cockpit staff - for example for helicopter pilots. The rotor noise contains extremely strong low frequency components, which are only slightly damped via a headset. Thus be due to the masking effect, higher-frequency signals, which are required for voice communications, concealed, so that the volume of the headphones must be raised very considerably to ever air traffic control yet to understand. Are now damped by this method, the low-frequency rotor noise, the communication headphone level can be significantly reduced.

ANR headphones require their own power source as opposed to simple headphones. This suffices to small battery (for example, AA or AAA ) or a battery because the ANR electronics only require a little electrical power.

Application Beschallungstechnik

In the active noise cancellation sound technology is used, for example, at major events since the subwoofer designed for such events cause very strong vibrations at correspondingly low tones. In this application, a loudspeaker, an anti- noise signal ( usually stage ) oriented in the opposite direction of the other speakers, the processor then generates controlled so as to reduce the acoustic load in a given area.

Other applications

The anti-noise - principle is used in combination with conventional silencers, in ducts and pipes for the damping of low frequencies in a small space. Conventional (passive ) Silencer require large volumes to still be effective even at low frequencies. A new application was found in automotive technology where the passenger compartment of cars with Active Noise Cancellation can be " reassured " about the Audi A8 or the Honda Legend.

Is or has been researched in other applications, such as

• For the insulation of aircraft engines
• To reduce the noise level in aircraft (Bombardier Q Series Dash 8, SAAB340Bplus, SAAB2000, Raytheon Beech King Air 350, Raytheon Beech 1900D, Raytheon Beech King Air 90, 200, 300 and Twin Commander)
• To change the sound of car engines
• To reduce the low-frequency hum of power transformers in substations,
• To improve the sound insulation of windows,
• To the creation of zones of peace in open-plan offices,
• To combat high sound levels in printing machines.
• For the reduction of noise from wind turbines
• To reduce the noise in magnetic resonance imaging
• For the suppression of standing waves in the cuboid rooms, the music or film sound reproduction occur when, see double bass array.

In almost all applications, a similar effect at a lower cost and without any additional power supply in other ways can be achieved. Therefore, the application is usually limited to special applications.

Similar procedures

A similar method is used in the high voltage direct current transmission line Baltic Cable to suppress high-frequency noise that arise during converter operation.