Simulated free field measurements for active cancellation of noise entering a window

Environmental noise propagating through an open window may be cancelled by a sparse array of loudspeakers placed in the window frame. This approach towards global noise cancellation mitigates noise pollution at its source while retaining the light and ventilation received from an open window. To perform global noise cancellation measurements in a non-anechoic environment, a simulated free field approach was developed. Using repeated randomized stepped sine waves, where each frequency has an integer number of wavelengths, discrete frequencies are time gated to negate reflections and then are concatenated together to yield a pure tone. With a dense concentration of discrete frequencies, intermittent data may be interpolated via a spline interpolation. Results for this approach will be presented in two parts: First, the transfer functions of individual loudspeakers are measured and are compared to conventional measurements performed in an anechoic chamber. Second, the global noise cancellation produced by the sparse loudspeaker array was measured with and without the time gating approach to determine the effectiveness of the method.