André L'Espérance

Heuristic model for outdoor sound propagation based on an extension of the geometrical ray theory in the case of a linear sound speed profile

Abstract In the case of outdoor sound propagation at relatively short ranges, it is reasonable to assume that sound follows straight ray paths. At longer ranges, refraction due to temperature and wind gradients results in ray paths that are curved, and calculations using straight rays are no longer valid and lead to erroneous results. In addition, atmospheric turbulence plays an ever-increasing role by degrading the coherence of the sound field. In this paper, the theory valid for propagation from a point source above an impedance plane is extended to include the effects of curved rays and the loss of coherence due to atmospheric turbulence. The model assumes a sound speed gradient that varies linearly with height above the ground. This assumption allows an analytical determination of the travel times of the curved rays and the modified angle of incidence of the ground-reflected paths. The additional reflected rays that may appear in the presence of a positive gradient are included in the calculation. The assumption also permits the determination of the position of the shadow zone in the presence of a negative gradient. The total sound pressure level is computed by summing the contribution from all the rays. In this summation, a fluctuating index of refraction is used to take into account the partial coherence between the rays caused by the effects of atmospheric turbulence. If the receiver is in the shadow zone, a diffraction theory based on a residue series solution is used to compute the sound levels. The results of calculations using the model are compared with experimental results.

Sound propagation in the atmospheric surface layer: Comparison of experiment with FFP predictions

Abstract This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases.

Active noise control in large circular duct using an error sensors plane

Abstract Active Noise Control (ANC) of higher order modes in a circular duct is studied. To overcome the limitations of a modal approach to locate the error sensors, an alternative strategy, referred to as the error sensor plane concept, is proposed. The basic objective of the error sensor plane concept is to create a quiet cross-section in the duct so that the noise from the primary source cannot propagate over this section. To create this quiet cross-section, a network of error microphones is located in the cross-section of the duct. To determine the required number of error microphones and their location in the quiet cross-section, a model simulating a multi-channel ANC system for a circular duct is developed. Using this model, it was deduced that to achieve good control performance, the maximum distance between adjacent error sensors had to be minimized, as well as the distance between the error sensors and the duct wall. To determine the optimal location of the error sensors, the k mean algorithm is implemented. A scale model was used in a laboratory setup to verify the efficiency of the proposed strategy to control high order modes in a circular duct. The obtained results demonstrate that, for the control to be effective, the maximum distance between each error sensor, and the limit of its zone of influence should be less or equal to 1/3 of the wavelength for the frequency considered, i.e. when D max / λ cf

Actuators positioning for multichannel active control system in circular ducts

Abstract In the particular case of multichannel control, the acoustic power limit of loudspeakers may be an important restriction for active control application when the noise levels are very high. The goal of this paper is to present the different parameters that allow to minimize the power used by the control sources for a multichannel active control system in a circular duct. An experimental study of the longitudinal distribution pressure field for the particular case of higher order modes, as well as some experimentations with active control for different geometric conditions have been done to analyze this problem. The results of these experimentations have allowed us to understand that the most influential phenomenon that determines the control sources optimum position are the reflection from ducts closed end. With this result in mind, a simple and efficient methodology of positioning has been developed. The efficiency of this positioning method has been proved from experimental tests.

Sound propagation in a wind tunnel: Comparison of experiments with FFP and residue solution

Abstract Several models and solutions have been developed for propagation in the presence of refraction and ground of finite impedance. These include the fast field program (FFP) and the residue solution. In this paper, the results obtained with these theories are compared with scale model sound propagation experiments performed in a wind tunnel. The data are obtained in the presence of relatively steep wind speed profiles which have stable gradients and provide a good test for the theories. In addition, a single procedure to approximate realistic profiles by an equivalent linear profile is discussed.

Upgrade of a multi-channel active noise control system for an industrial stack

Active Noise Control (ANC) has been studied in the 90s as an innovative way to reduce the noise in specific situations. Some applications are well known today and found commercial success such as noise-cancelling headphones. However, the use of ANC in industrial applications is more complex, thus being an uncommon solution in this field. The use of ANC for industrial stack noise is one of these applications. One of the first large-scale implementation has been set up at the end of the 90s. This system was a 10-Channel ANC system installed on a 1.8 m wide chimney to attenuate a 320 Hz pure tone. At that time an 8 dB noise reduction was achieved at error microphones. In 2011, it has been decided to upgrade the system with the latest generation of Digital Signal Processor (DSP) allowing a real-time optimization and a better tracking speed. This paper describes the overall system and the updated multi-channel controller developed for this application. It also presents the improvements, the achieved noise redu...

A long‐range outdoor sound propagation model based on an analytical determination of the acoustical ray paths

For short‐range sound propagation problems, the acoustic energy reaches a remote receiver via a straight direct ray and a ground‐reflected ray and the total pressure at the receiver can be precisely computed considering the geometrical spreading, the atmospheric absorption, and the ground effect [Piercy et al., J. Acoust. Soc. Am. 61. 1403 (1977)]. For long‐range sound propagation problems, additional effects have to be considered: mainly, the refraction due to temperature and wind gradients that curve the rays and the turbulence that destroys the coherence between the rays. To evaluate the effect of refraction, a constant linear sound‐speed gradient is assumed. This assumption allows an analytical determination of the curved rays and it also permits the determination of additional reflected rays that may appear in the presence of positive gradients, or of the position of the shadow zone in the presence of negative gradients. The total sound pressure at the receiver is computed by summing up the contribut...