The sound transmission loss across ventilation window under active noise cancellation

An experimental investigation on the use of active noise cancellation to improve the sound transmission loss across a ventilation window was carried out in this study. This experiment series was conducted inside a building acoustic test chamber consisted of two isolated rooms. One of these rooms was reverberant and was used as the receiver room, while the other was made semi-anechoic and it was where the sound source was located. The ventilation window was installed on a brick wall separating these two rooms. Active control with one reference signal, one error microphone and maximum three secondary sources was implemented using the EZ-ANC II MIMO controller. Six well separated microphones which spanned over the whole volume of the receiver room were used to capture the average sound pressure level inside the receiver room. The average sound pressure levels in the receiver room with and without the active control were used to describe the improvement of sound transmission loss by the active control. Among the various control combinations tested, sound transmission loss improvement was found at frequencies below 550Hz. The highest improvement was ~5dB and was found at ~200 Hz. The improvement decreased as frequency increases in general, but there was a dip at ~300 Hz. This dip was believed to be due to a longitudinal resonance in the ventilation window cavity.