The main noise source in heating, ventilation, and air conditioning systems is usually a ventilating fan. Noise, generated by the ventilating fan is transmitted through the duct into the living and working environment. A typical fan noise spectrum consists of a broadband noise, which is superimposed with pure tones. Different methods are available to reduce a transmission of such noise from the ventilating fan into the living and working environment. In this article it is demonstrated how a feedforward active noise control system can be implemented together with a side branch resonator. Effectiveness of the feedforward active noise control system depends on the quality of a reference signal, which should be in a perfect correlation with the primary noise. An acoustic feedback is the main problem of feedforward active noise control systems in ducts. A combined method uses a single loudspeaker to work as a dipole source and a side branch resonator to reduce the acoustic feedback. A side branch resonator reduces noise transmission in a narrowband frequency range as well. In this article, a theoretical background of a dipole source with a side branch resonator is presented, along with some measurement results and simulations of active noise control.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 11839771Theme of the master thesis is noise radiation from cooling towers of the Power Plant Šoštanj to surroundings. The noise emission from the power plant is at most the consequence of operation of the cooling towers producing noise with characteristics of the white noise. The first part of the master thesis refers to presentation of performed measurements of noise of water drops falling down from different heights in a free sound field and measurements of noise of water jets, which were performed in Laboratory for Pumps, Compressors, Fans and Technical Acoustics at the Faculty of Mechanical Engineering, University of Ljubljana. This represents a basis for understanding noise generating mechanisms within cooling towers in a power plant to surroundings. Emission of noise of cooling towers is at most generated by sprinkling of the water jets. At different dia eter, distance and height of the water jet falling down in a basin the emitted noise was measured in a free sound field. For examination of the noise generated by falling drops a frequency range between 1 and 10 kHz was used. The second part of the master thesis refers to modelling of generation and spreading of the noise from cooling towers on the basis of Elis model. An expected sound pattern of so generated noise has been taken into consideration. Comparison between measured and calculated levels of noise generated by cooling tower has shown that, despite some simplification used in the model, the results match well.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 12044315Speech intelligibility of some lecture rooms at Faculty of Mechanical Engineering in Ljubljana is poor, namely less than 75% of Lecturer words can be correctly understood. Acoustical analysis of these lecture rooms is necessary. The Acoustics of lecture room depend on its dimensions, reverberation time, signal to noise ratio etc. Most important parameters of room acoustics for the purpose of the speech intelligibility assessment were identified. In some lecture rooms acoustical impulse responses were measured. Different software was used. Results were analyzed and lecture rooms were ranked according to speech intelligibility. Deviations of measurement results were analyzed.
D.10 Educational activities
COBISS.SI-ID: 11953435