Carlos de la Colina

Simplified calculus to estimate the acoustical absorption of nonplanar materials

The absorption curve, in function of the frequency, in hard‐backed layers of granular materials, shows undesirable series of maxima and minima related to the layer deep. In order to increase the efficiency of these absorbing materials, it is necessary to smooth their frequency response curve. It is obvious that one of the easiest solutions consists of modifying the layer surface with selected profiles (e.g., wedges). For these situations, a simplified method of analytical calculus for the valuation of the absorption, in function of the frequency, has been developed. The validity of the theoretical approximations has been checked against experimental measurements in different test samples by means of a standing wave tube. Additionally, the application of this study to the design of absorbent acoustic noise screens, against traffic noise, made with recycled rubber crumbs of tires, is also presented. [Work supported by a LIFE project.]

The role of the physical factors involved in sound-attenuation characteristics of ear muffs

Abstract This paper deals with the assessment of the factors involved in sound-attenuation characteristics of ear muffs. To this end, the most common methods (subjective, objective on people, and objective on a test fixture), under free and pseudo-diffuse field conditions have been used. The relative importance of these factors in the different measuring methods, their variation with frequency, and the influence of field conditions and the type of head (human or artificial) are studied. Empirical equations are derived that specify the contribution of the individual factors in the different paradigms. In the light of our results, the apparently discrepant behaviour of ear protectors under different conditions can be explained and correlated.

Absorption of thin micro-perforated partitions lined with anisotropic fibrous materials

Recent trends in broadband noise reduction have considered the design of bio-inspired multi-layer sound absorbers. The layout usually comprises a suitable combination of thin micro-perforates, fibrous materials and airspaces with a view to mimic the remarkably low acoustic emissions of the owl flight. One of the main features is the anisotropic texture of the highly porous material beneath the micro-perforate. This study examines how the absorption properties of rigidly-backed micro-perforated panels (MPPs) are modified when lined with anisotropic fibrous materials with specified inclination of the parallel fibers within the material thickness. One discusses the effects of the material constitutive parameters, such as the bulk density, the flow resistivities and the structure factors along and normal to the fibers axis, on the MPP air-frame relative velocity. The model of propagation in the anisotropic medium accounts for energy losses and is compared against a multiple scattering approach in case of fibe...

Absorption of anisotropic fibrous materials partitions shielded by thin micro-perforated panels

Recent trends in broadband noise reduction have considered the design of bio-inspired multi-layer sound absorbers. In particular, the silent flight of certain species of night bird predators can offer an inspiration for the design of acoustic partitions for the control of the airframe noise in terrestrial and aeronautic transport systems. The layout usually comprises a suitable combination of thin micro-perforates, fibrous materials and airspaces with a view to mimic the remarkably low acoustic emissions of the owl flight. One of the main features is the anisotropic texture of the highly porous material beneath the micro-perforate. This study examines how the absorption properties of rigidly-backed micro-perforated panels are modified when lined with anisotropic fibrous materials with specified inclination of the parallel fibers within the material thickness. A fully anisotropic model has been proposed that accounts for anisotropy and frequency dependence in complex densities and bulk modulus. This model ...

Micro-Perforated materials for the reduction of flow-induced noise

Turbulent Boundary Layer (TBL) induced noise is one of the dominant noise sources in modern aircraft. The aim of this work is to study the behavior of insulating partitions composed of Micro-Perforated Panels (MPPs) exposed to a TBL excitation. We have carried out a set of simulations to establish a comparison between the performance of the MPP control devices when varying the physical configurations of the partition and the nature of the primary noise excitation. It can be shown that when exciting the partition with less correlated random loads, the corresponding TL increases progressively. Experiments have been performed in the low-speed wind tunnel in order to determine the acoustic and aerodynamic TL performance of a number of MPP multilayer partitions. The effect of inserting a micro-perforated panel within the cavity at unequal distances from the front and back panels dampens more efficiently the Mass-Air-Mass controlled resonances of the Panel-Cavity-Panel system with respect to those of the MPP-Ca...

Nonlinear aspects of outdoor propagation of acoustic pulses produced by weak explosions

This paper deals with some nonlinear characteristics of acoustic pulses generated by weak explosions with regard to establishing a probe signal for the experimental study of ground characteristics and outdoor sound propagation. These explosions cover the intensity range of signals commonly used on experimental studies, e.g., studies of the impulse response of rooms and the transmission loss and absorption characteristics of noise barriers. After a brief introduction of time history and spectra of such signals, compared to linear pulses and nonlinear pulses of explosions of a higher nonlinear range particular attention is paid to air absorption up to 20 kHz. Evolution of the pulse signature as it propagates as well as global pick attenuation and attenuation in 1/3 octave frequency bands are discussed compared to linear processes and well established nonlinear theories, offering as a conclusion a simple and practical model that accounts well for the transformations undergone by the pulse when it propagates ...