Active Noise Attenuation In A Natural Ventilation Window

The natural ventilation window investigated in this thesis refers to a spaced double glazing window, where the external and internal openings are staggered to create a natural ventilation path and prevent direct sound propagation. It has been found that the transparent micro-perforated membrane absorber (MPA) can be placed along the ventilation path to attenuate external noise, which is mainly effective at mid-high frequency, with better performance than a typical closed single glazing window. The low frequency noise reduction of such window is limited due to the MPA is mainly effectively at mid-high frequency unless the airspace is sufficiently large. This thesis employs active noise control (ANC) techniques to increase the low frequency transmission loss of such windows.An analytical model for the active noise reduction window (ANRW) system is firstly developed. A semi-infinitely long duct is used to simulate external free space, through which the ANRW is modeled as a five-cavity coupled system. The modal expansion method and coupled cavities theory are used to obtain the sound field inside the window and the room. ANC is then employed to obtain the complete analytical model for the ANRW.Numerical simulations are carried out to optimize the secondary source arrangements, where it is found that for the example single channel ANC system, the optimal position of the secondary source is at the external opening center, with which the frequency range of effective control is up to 390 Hz, and the extra noise attenuation (EA) at the room due to the ANC system is almost equivalent to the noise reduction at the error sensor. The frequency range of effective control can be extended by using a multiple channel ANC system. A 1:2 scale model of the ANRW is built in an anechoic chamber, with which the effects of the primary source, the secondary source and the error sensor positions on the control performance of a single and a dual channel ANC system is experimentally investigated. The results show that either for the single or dual channel system, the optimal positions for the secondary sources and the error sensors are at the external and internal opening, respectively. With the optimal configurations, the frequency ranges of effective control of the single and dual channel system are up to 820 and 1000 Hz, respectively. Moreover, the EA obtained with a normal incident primary sound field is higher than that obtained with an oblique incident primary sound field because the secondary sound field matches better with the normal incident primary sound field.Further experiments are carried out to investigate the noise reduction ability of two full scale ANRW prototypes. It is found that the direction of window openings and reflective ground have little influence on the noise reduction. When the direction of window openings parallels the reflective ground, the frequency range of effective control is up to 550 Hz. When the primary source is placed in a reverberation chamber, where multi reflections exist and the primary sound field at the external opening consists of sounds from all directions, the noise reduction ability of the ANRW decreases because the reference sensor can only receive sounds incidence from certain directions.