Marcus Asteborg; Niklas Svanberg BEE06:05, pp. 124. TEK/avd. för signalbehandling, 2006.
The most common method to attenuate noise in ventilation systems today is passive silencers.
For these to efficiently attenuate frequencies below 400 Hz such silencers need to be large and a
more neat solution to attenuate low frequencies is to use active noise control (ANC). The usage
of ANC in ventilation systems is well known and there are several commercial products available.
ANC is not, however, used on a wide basis due to its often high price and poor performance.
Since the price is an important factor in ANC systems the expensive laboratory filters and
the amplifier that is currently used in the experimental setup at Blekinge Institute of Technology
(BTH) need to be replaced with cheaper ones, but without too much performance loss. For easier
implementation in ventilation systems the placement of the reference microphone is important,
the shorter distance from the anti-noise loud speaker the easier the ANC system is to implement.
But if the distance is so small that the ANC system is no longer causal the performance will
be decreased and if the reference microphone is close enough to pick up acoustic feedback from
the anti-noise loud speaker the performance will also be decreased. In this thesis the expensive
laboratory filters will be exchanged to cheaper alternatives, power and total harmonic distortion
(THD) measurements will be done on the amplifier that is driving the loud speaker and the
reference microphones position will be investigated with measurements on the group delay of
the system and the acoustic feedback between the loud speaker and the reference microphone.