Fredrik Tibblin; Tomas Bengtsson , pp. 39. TEK/avd. för signalbehandling, 2005.
Active headsets have been on the market for years and now it is time for something else to enter the stage. The technology of active noise reduction can for exmple also be used in ventilation systems for reducing ventilation noise. An active control system principally consists of four important components, namely the downstream error microphone, the upstream detection or reference microphone, the digital signal processor (DSP) and a loudspeaker.
The upstream reference microphone listen to the primary ventilation noise and sends it to the DSP which makes a phase shift to the signal and sends it to the loudspeaker. The loudspeaker sends out the opposite waveform of the incoming one and reduce the primary ventilation noise. To adjust the sound radrated from the loudspeaker so low reidual soundlevel is obtained a downstream error microphone is used. The residual sound is a sum of the primary ventilation noise and the secondary sound generated by the loudspeaker.
In noice reduction systems today there are not many systems that use the active technology. Passive silencers have low noise attenuation at low frequencies, approximately under 250 Hz, and that is why the active technology should be used. The aim with a silencer based on active noise control is to improve the reduction of the low frequency noise below 250 Hz.
In this project we also have a passive component that reduce the higher frequencies. The active control system should make the system better in the low frequency range.
The use of both active and passive noise reduction at the same time makes an even better system and the goal with this work was to implement a system with high performance to a low cost.