Randomized ray tracing in room acoustics can be used to compute echograms, but the results computed at the receptor are affected by statistical errors [1]. To decrease these statistical errors, the usual solution is to increase the number of rays, but this also increases the computation time. Another solution is to increase the receptor size, but this lowers the spatial resolution of the computed sound field, which is particularly important for the early part of the echogram. In order to decrease the computation time and keep a sufficient resolution, the method proposed in this paper is based on a progressive modification of the receptor size (spherical type) as long as the ray's travel grows. At the same time, the number of rays into the room can be decreased as the sound ray lengths increase while keeping the statistical errors more or less constant. The consequence is that the sound field spatial resolution evolves from “precise” at the beginning of the echogram to “rough” at its end. The first applications of this new method show a significant gain of CPU time. [Broad spectrum diffusion model for room acoustics ray-tracing algorithms, J. J. Embrechts, Jnl. Acoust. Soc. Am. 107(4) 2068-2081 (2000)].

In a ventilation duct network, two types of silencers can be used: passive (medium and high frequencies) and active (low frequencies). Active silencers have a loudspeaker (control loudspeaker) that generates an anti-noise. Usually, this system is placed close to the fan and far from the duct air opening. However, when the system is placed close to an air opening, the control speaker can not only be used to reduce the fan noise, but also as a background speaker to play music, information, alerts (the target signal) ... This concept has been implemented in an active noise control (ANC) system based on the adaptive feedforward normalized FxLMS algorithm which uses a reference and an error microphone to pick up respectively the noise to attenuate and the residual noise after control to be minimized. The algorithm is modified so that the target signal does not contaminate the reference nor the error signals. The new algorithm is tested in real time in a short duct with different type of noises and target signals.