Implementation of Active Noise Control into Different Fan Applications

F3 - Active Noise Control & Noise Analysis

Axial and radial fans used for instance in various air conditioning systems, often are annoying due to their noise emissions.
This paper presents the implementation of Active Noise Control (ANC) into different HVAC fan applications with the objective of noise reduction. Therefore a commercial broadband feedforward ANC system (S-Cube Development Kit, Silentium) is used, which comprises a reference microphone, a loudspeaker, an error microphone and a control unit in which an adaptive algorithm is implemented. Due to the application of the virtual microphone theory, the error microphone is only needed during the calibration process of the system, where acoustic transfer functions are measured.

The present paper deals with the application of ANC to different systems containing a diagonal fan of axial type with 172 mm diameter and a radial fan of 566 mm diameter. ANC was used on the suction side as well as on the pressure side. The achieved noise reduction varies considerably for the different fan applications. In the best case the overall sound power level is reduced by about 6 dB(A) with decreasing certain frequency peaks like the blade passing frequency by up to 17 dB. In the worst case nearly no acoustic benefit by means of ANC could be found.
The achievable noise reduction depends on several factors. For the application of ANC the noise spectrum should have maximum levels in the lower frequency region up to 1000 Hz. For an effective noise reduction a combination of passive and active acoustic methods is important. The passive means absorb the high frequency sound of the spectrum, while the active methods reduce the low frequency noise. Good ANC performance results from good coherence between the two microphone signals, which often is difficult to achieve, because on the one hand the reference microphone should measure the characteristic sound, on the other hand it shouldn't be positioned directly in the flow because of flow disturbances which could affect the microphone signal. If an ANC system is implemented into an application it needs a certain installation space for maximum noise reduction. Often this space isn't available - so a compromise between available space and noise reduction has to be found. Eventually ANC reduces just air borne noise - structure borne noise at other locations of the device can't be reduced.