1,721,076 research outputs found
IDENTIFYING THE ACTIVE FLOW REGIONS IN ACOUSTIC-HYDRODYNAMIC FEEDBACK MECHANISMS
No full textAcoustic-hydrodynamic feedbacks are a common theme in jet noise. Strong sound emissions are supported by fluid instabilities, whose core is not necessarily localized in space. A common example is the feedback-loop insta- bility of cavity flows, impinging jets or the flow past air- foils. The feedback-loop is composed of a convective in- stability, which is usually an instability of the shear layer, and an acoustic pressure wave or a hydrodynamic non- local effect. Despite the fact that such a mechanism is widely accepted, a precise identification of the most sen- sitive spatial regions underpinning the instability is miss- ing. Herein, we propose a non-local decomposition of the structural sensitivity, which allows us to precisely identify the most sensitive regions of the flow responsible for the closure of the feedback-loop. The systematic use of these techniques could be applied in the design of passive flow control devices
MIMO Technique applied to the Greek Theatre of Tyndari
No full textSpatial Multiple Input Multiple Output (MIMO) is an innovative technique developed to provide an appropriate filter array required to virtually reconstruct the perceived sound field at the listener's position, generated by a sound source with arbitrary and time-varying directivity. This is particularly useful when the sound source has a complex directivity pattern (e.g. the human voice or specific musical instruments). To mimic the directivity of a real sound source, a “DodecMIMO” was developed. This prototype loudspeaker is capable of simulating the directivity of any dynamic sound source as well as a standard omnidirectional sound source. Prior to field applications, the sound source was measured in an anechoic chamber to obtain its acoustic characterization. The first on-site application took place in the Greek-Roman theatre of Tyndaris (Sicily, Italy). The measured impulse responses (IRs) were analyzed and compared with previous acoustic studies performed with omnidirectional sound sources. The outcomes were used to compute the acoustic maps for each measuring point by overlapping the panoramic image acquired at each position with the acoustic response recorded by the microphone. This innovative technique is very well suited to create a virtual 3D audio reproduction, technically called auralization, which can faithfully reproduce a complex real sound source
Acoustic and energy characterization of buildings with the use of BIM
Full text linkThe acoustic characterization of buildings combined with energy performance represents in some countries of the European Union such as Italy a legal obligation. The Italian state, for public buildings, also obliges to comply with the minimum environmental criteria (CAM). These provide for the new buildings or existing buildings subject to total renovation a minimum performance level corresponding to class two according to UNI 11367 standard. In addition, the transposition of the European directives introduced the obligation to use the BIM for public works on the national territory. This work analyzes digital procedures based on the BIM model to characterize buildings from an acoustic and energy point of view through instrumental investigations
SOURCE ESTIMATION ALGORITHM IN NON COOPERATIVE BI-STATIC SONAR SYSTEM
Full text linkResearch and surveillance at sea may benefit from the covertness of some of the involved platforms. Within this context, the paper focuses on a non-cooperative Bi-Static sonar configuration: a specific active sonar system in which the surface transmitter and the underwater receiver do not communicate with each other and the covert receiver has no information about transmitter source depth, course and speed, waveform and frequency spectrum, etc. Environmental conditions (bathymetry, sound speed) in the whole surveillance area are assumed known. Within this framework, several works have been proposed and this paper delves into the preliminary phase of one of these: a target detection algorithm based on the joint source-target localization at the receiver through matching the multipath arrivals with model based predictions. Such algorithm requires the application of an unknown source deconvolution processing to extract the multipath arrivals, plus the inversion of the arrivals times. Unknown source deconvolution is performed through a novel algorithm in the time-frequency domain, which aims to estimate the characteristics of the unknown transmitted signal. The algorithm is described and finally validated with synthetic acoustic signals and applied to real data. Tests on real data, though at a preliminary stage, confirm the theoretical and simulated analysis
Geometric optimization of a multiple coiled-up resonators for a broad band acoustic absorption
No full textIn order to investigate new solutions to obtain high absorption at low frequency, the concept of quarter-wave acoustic resonators coiled-up metamaterial has been applied to limit the thickness thanks to the possibility of ”rolling” their channels. This study is based on a metamaterial made of n quarter-wave coiled-up resonators in parallel: once the minimum frequency of absorption (e.g. 100 Hz) and the second harmonic (300 Hz) have been defined, this frequency range is divided into n different resonators so that the envelope of the high-harmonics is such as to obtain a high broadband absorption. Starting from the study of a single resonator, an analytical model was defined to calculate the normal incidence absorption coefficient and then it was extended to systems with n-resonators in parallel using the parallel equivalent circuit theory. This models were validated with impedance tube measurements and then used for a geometric optimization: defined a metamaterial with 16 resonators in parallel (minimum frequency of 100 Hz), the geometry was varied to maximize the absorption coefficient. The optimized geometry was than simulated using a 3D FEM model to consider the real development of the channels. The results of both analytical and FEM models will be discussed
Effects of natural sounds on students' learning and classroom soundscape
No full textPrevious studies on the effect of acoustic exposure on cognitive abilities mainly explored the effect of anthropogenic sounds (e.g., traffic and chatting noise), intended as environmental stressors. However, recent soundscape research highlighted the potential of positive and desired sounds to increase student performance. In this paper, the effect of natural sounds (including birdsong in a
garden) on students' cognition and soundscape assessment was analysed by comparing students’ performance in a math task and affective responses when exposed to birdsong with those obtained in the classroom ambient noise. 229 pupils aged between 11 and 13 (grades VII and VIII) participated in the experiment, carried out with a class-wise paradigm. The data from the cognitive tests were analysed using Generalised Linear Mixed-Effect Models, while the soundscape data were compared using the Wilcoxon test. The results show an increase in calculation accuracy and perceived pleasantness and eventfulness when birdsong is played
Acoustical and Physical Characterization of Posidonia Oceanica Fibres for Sound Absorbing Applications
No full textThis article presents an experimental investigation on the acoustical and physical characterization of loose fibers obtained from Aegagropiles of Posidonia Oceanica, commonly called Posidonia Balls. These aggregates of fibers are formed near the beaches following the action of the sea waves, which remove the leaves of the marine plant Posidonia Oceanica, typical of the Mediterranean sea. Egagropili are very common on beaches, and can be a renewable resource for making sound-absorbing panels with low environmental impact. The aim of this research is to experimentally characterize the acoustic behavior of loose fibers obtained from Aegagropiles collected on the beaches of southern Sardinia. The characteristics have been studied as a function of the apparent density of the fibers, in order to optimize their performance. The five physical parameters present in the Johnson-Champoux-Allard model were also evaluated to obtain an analytical model of the acoustic behavior of the Posidonia fiber panels as a function of density and thickness
A field survey of acoustic conditions in primary and kinder school canteens
No full textCanteens usually have critical acoustic conditions resulting from the need to maximize the number of occupants while minimizing volume. Thus, in the absence of specific sound absorbing treatments, very high sound pressure levels are usually observed resulting in significant impairment of communication (with increased vocal effort of speakers and reduced speech intelligibility), and dangerously high exposure levels for workers. The present paper reports acoustic measurements carried out in a nursery school canteen having a volume of 211 m3 and seating about 50 children, and two primary school canteens having volumes of 650 m3 (seating 126 children) and 367 m3 (seating 107 children). Reverberation time was measured in each room as well as sound pressure levels during peak occupation (averaged over 15 minutes intervals), resulting in Leq, A spanning between 83 dB (in the nursery school) and 89 dB in the primary school with the smallest volume
Monaural and binaural perception of speech in complex scenes: recent results on the role of correlation
No full textThe perception of speech and its intelligibility have long been studied in relation to monaural factors (i.e. reverberation and clarity). On the other hand, especially in more complex settings, binaural hearing is necessary to achieve performance. The deployment of effective binaural processes is strongly influenced by the layout of source, masker(s) and receivers, by reverberation and also by the type of reflections in the impulse response. Scattered reflections are well-known to provide a spatially spread wavefront, a time-distributed response and uncorrelated binaural signals compared to specular ones. This work discusses the various effects mentioned above and presents some recent results on the role of correlation in this context
A preliminary numerical analysis of the noise radiated by a gearbox
No full textThe noise spectrum generated by a gearbox is characterised by harmonics which depend on the geometry of the gears, their rotational speed, possible axis eccentricity and imperfections of the teeth surface. The experimental investigation of the gearbox noise emission is a complex task, especially due to the difficulties in separating its contribution from the overall noise generated by other machineries of the testing train. Conventional laboratory tests evaluate the condition of full speed and no load, which is not representative of a realistic scenario, where the gearbox is subjected to a torque provided by the machinery it is connected to. This paper presents a numerical approach to predict the noise radiated by the gearboxes, through a multibody analysis in the time domain combined with a vibroacoustic analysis in the frequency domain using the boundary element method (BEM). The study investigates the influence of the rotational speed and the resisting torque on the radiated sound power. The results show that an increment in the rotational speed affects the position and magnitude of harmonic components, while an increment in the applied torque has a significant impact only on the magnitude of the radiated noise
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