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Passive sonar is discreet and does not disturb marine life because it listens only to the sounds emitted by noisemakers, which send out acoustic signals that travel through the ocean to be picked up by the sonar's antennae. These antennas are disturbed by noise from the vessel and the environment. The signals received by the antenna sensors are processed by appropriate algorithms, the outputs of which are displayed so that an operator can decide, with the aid of audio listening, whether detection is worthwhile. The article describes each stage of the information flow, from the sound source to the operator. This is also the order of the terms in the sonar equation used to estimate range. Three examples from real situations at sea demonstrate the use of this formalism.
Active noise control is a process that detects and analyses the pattern of an incoming noise and then generates an « anti-noise » mirror signal to cancel it out. In the 70s, the physical bases of active control were established following the foundational works of Maurice Jessel and co-workers. The developments in the 80s of digital signal processing allowed for the implementation of adaptive active control systems. This progress led to the birth of the first industrial applications of active control and, from the 2000s, to the marketing of numerous products. This article first describes the architecture of the systems. It then details the applications of active control and the main industrial achievements resulting from it.
Digital aeroacoustics is a discipline which allows for the prediction of noise radiation induced by the movement of a solid body in a fluid or to fluctuations ( turbulence, swirls) within the fluid. Growing rapidly due to the advances on calculation algorithms and the increase in the power of computers, it is closely linked to computational fluid dynamics (CFD) which provides data on noise sources. Although these two approaches are complementary they oppose in several aspects. Indeed, digital aeroacoustics has its own specificities due to the characteristics of acoustic waves and their propagation and is organized around several simpler basic equations which are often linearized.
This article deals with aeroacoustics in aeronautics whose basis were established in the middle of the last century. Still today, reducing the extremely annoying noise of aircrafts remains a priority in air transportation. The main equations of aeroacoustics are firstly reviewed. The obligatory acoustic certification in order to obtain permits is then largely detailed, notably via the role of standards in the aeronautic sector. This article then focuses on jet transportation aircrafts, whose turbojet engines are largely responsible for noise nuisances. This article concludes on other examples from the aeronautical sector, such as propeller aircrafts and helicopters.
The study and analysis of vibrations (or signals) has grown considerably in recent years, due to the development of increasingly sophisticated techniques and a wide range of needs in various fields: mechanics (transport, machinery, etc.), acoustics, optics, transmissions, etc. The study and analysis of vibrations (or signals) has grown considerably in recent years, due to the development of increasingly sophisticated techniques and a wide range of needs in various fields (mechanics (transport, machinery, etc.), acoustics, optics, transmissions, etc.). Current concerns can be broadly divided into three categories. The first, which is theoretical, concerns the dynamic calculation of structures as an extension of strength of materials and continuum mechanics.
Experimental modal analysis aims to identify the natural frequencies, damping ratios and mode shapes of a structure under given boundary conditions. Developed in the 1960s-1990s, experimental techniques and identification methods are now mature and available from industrial suppliers.Structural dynamics is nevertheless a complex discipline.This article aims to cover all the important knowledge to acquire a good mastery of experimental modal analysis.
Only two methods allow for the protection against noise, one consisting in the reduction at source and the other preventing its propagation towards the protection zone. Attenuation techniques involve the placing, on the trajectory of the wave, of a set of secondary sources emitting a second wave of the same amplitude but of opposite signs. Finally, via destructive interference, the resultant is nil in this active method. After having recalled physical and acoustic basis, this article develops the concept of active control, its two command strategies in open or closed loop and the various means to achieve an optimal filtration.
Room acoustics is one of the areas of acoustics that attracts most interest from the public and from acousticians themselves. It differs from most other fields of acoustics in that sound is considered in its positive aspect: communication, culture, music, whereas in many fields of acoustics it is considered in its negative aspect, i.e. noise that causes discomfort, which unfortunately it often does. We like to associate room acoustics with a long-practiced art form, sometimes shrouded in a certain mystery, as evidenced by the myth of ancient theater acoustics, which some acousticians keep alive.
Unlike other studies carried out in this field, the study of the acoustics of rooms cannot be mathematically modelled in order to describe the propagation of sound. This article presents the linearization process of a complex set of variables, which, unlike in traditional models, does not involve prioritization or traditional parameterization. It includes a methodological scheme involving six proposals concerning the virtual distribution of sources, the search for a compromise criterion between clarity and homogeneity, the choice of a frequency band, the determination of the opening angle of sources and the delineation of the listening plane. The implementation of the model is analyzed in the case where sources are predetermined and in the case where only the site is known.
The acoustic study of a room is carried out via an approach by models. This does not only involve acoustic knowledge. In this article, this approach is illustrated by three examples. The first one, a simple optimization attempt based on a traditional model, is an intermediary stage between the systematic approach and the linearized approach. The second one, based on a simple comparison of two calculation formulae of a reverberation length, is a typical bifurcation scheme involved in most models. The third example illustrates how the failure of a traditional model can be addressed in order to achieve suitable adequacy between measurements and predicted values.
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