Jaime Pfretzschner

Hybrid passive–active absorption using microperforated panels

A theoretical and experimental study of a proposed hybrid passive–active plane-wave system to provide broadband acoustic absorption is presented. The passive absorber consists of a microperforated panel (MPP), used in place of conventional fibrous materials, in front of an air layer. The active system uses an active transducer (a loudspeaker), an error sensor (a microphone), and an adaptive controller. MPPs are thinner than fibrous materials and provide a better solution in hygienic environments. For two such systems, the dependence of performance on MPP parameters is studied for two control strategies: impedance matching and pressure release. The first condition is found to be better for cases where the acoustic impedance of the system approaches that of air. Otherwise, the pressure-release condition is better, and a wideband solution is the implementation of the active control system up to a frequency where the natural band of the passive system provides higher absorption. Therefore, the use of a low-pa...

Guidelines for the Acoustic Design of Absorptive Devices

12 NOISE & V IBRAT ION WORLDWIDE 1 . Int roduc t ion Acoustic treatments both indoors and outdoors cover many different situations. The designer of such treatments has to take into account not only the characteristics of the noise source, but also its location in order to select the appropriate acoustic material to adapt the solution to the considered problem. If the acoustic improvements are restricted to interior spaces (building halls, theatres, dwellings, factories, vehicle cabins, etc.), usually mineral wools or open pore foams can be used for they are well adapted to solve the problem. On the other hand, for outdoor problems, for instance acoustic noise barriers against traffic noise, the absorption provided by granular materials such as porous concrete or similar materials should be employed, as they behave better with bad weather and other atmospheric phenomena. An additional advantage is that they can be cleaned (with pressurised water) without losing their acoustic properties. In porous materials (fibrous and granular), the absorption process of the acoustic wave takes place through viscosity and thermal losses of the acoustic energy inside the micro tubes forming the material. This kind of material is widely used in room acoustics, in order to control reverberation time, to avoid undesired reflections, to fill double wall cavities, floors and ceilings, etc. In all these cases, the problem starts with the spectra of the emitting source, and then, the selection of the material (bulk density or air flow resistance), as a function of the energy bands to be controlled, and the volume or thickness of the layer required to obtain the necessary absorption to improve the acoustic environment. In front of a wall, usually a layer with enough thickness can be installed in order to absorb an important amount of the incident acoustic energy. Having an absorption coefficient of l means that no reflection energy is re-emitted into the room. The higher the acoustic resistivity, s, of the material, the higher is its dissipation, for a given layer thickness, but at the same time the surface impedance of the layer also increases with s, giving as a result a greater amount of reflections on the surface layer, resulting in a lower absorptive capability. The whole process is frequency dependent, so that for lower frequency bands (higher wavelengths) the necessary layer thickness increases, as s decreases. It is convenient to point out that an anechoic treatment requires that the designed device should provide an outer surface impedance as close to the wave impedance of the air, rc, as possible, and increasing progressively its air flow resistance towards the wall. This can be achieved for a given s, by increasing progressively the volume of the device, for instance through prismatic wedges, or by increasing the air flow resistance of superposed layers. These kind of arrangements provide a surface impedance increasing towards the backing rigid wall, on which they are installed. The purpose of this work is to give some basic ideas that can help the practising noise control engineer in selecting or designing the most adequate solution to the acoustic problem. Guidelines for the acoustic design of absorptive devices

Field evaluation of ear-muffs: Which is more suitable—insertion loss or transmission loss?

Abstract Among the complementary procedures for the subjective measurement of the attenuation characteristics of ear-muffs through the real ear attenuation at threshold, the most widely used are several objective paradigms. Measurements can be performed through Insertion or Transmission Loss procedures; nevertheless, they give different attenuation values in the mid- and high-frequency range, not explained in the literature. This paper describes the origin of these differences, that are attributable to diffraction effects, demonstrating the equivalence of both magnitudes through a measurable parameter.

On the absorption coefficient of porous corrugated surfaces

A classical way of improving acoustical absorption performances of porous materials is the use of corrugated surfaces; this use can obtain lower cut-off frequencies and also improve the overall absorption over a wide frequency range. An analytical approximation is presented for the calculus of the absorption on this kind of surfaces, where the thickness gradient is represented as a series of steps. Reflection coefficient of every step is obtained and will contribute to the net reflection coefficient. Theoretical results will be presented and shown to agree with experimental data.

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.]

Measuring the absorption coefficient of panels at oblique incidence by using inverse filtered MLS signals

This paper describes a method for the measurement of the absorption coefficient of a panel at oblique incidence based upon a new procedure which combines inverse filtered MLS signals with the subtraction technique. This allows for the use of shorter pulses which better discriminate between the different events usually present in measurements, e.g. direct, reflected, and diffracted signals. Therefore, the windows that select the direct and reflected events from the measured trace, which are then used to calculate the absorption coefficient, can be better set up. This in turn optimizes the frequency bandwidth of interest. The method has been validated by applying it to a Microperforated Insertion Unit panel (MIU) in an anechoic room. The MIU consists of a 2.44 m X 2.44 m X 1 mm steel panel, properly perforated to provide a suitable absorption bandwidth, in front of a 2.44 m X 2.44 m X 2 cm wooden panel with an 5-cm air gap between the steel and wooden panels. The inverse filter of the electroacoustic response has been designed so that the radiated pulse has a minimum-phase cosine-magnitude spectrum. Experimental absorption curves agree with the predicted ones.

Ground influence on the definition of single rating index for noise barrier protection

A single number rating index to qualify the acoustical protection of a noise barrier that accounts for the influence of the ground is developed. This is an extension of a previous index for semi-infinite barrier developed by Pfretzschner [Pfretzschner et al., Acust. Acta Acust. 82, 504–508 (1996)]. This index is based on the asymptotic value found for the insertion loss or the acoustic attenuation as physical magnitude, and it is uniquely dependent on the barrier, noise source, and acoustic characteristics of the ground in the source semi-space. The new index agrees well with measured data on a scale model under laboratory conditions, and a direct experimental measuring procedure could be envisaged. In this way the proposed index, which accounts for extrinsic properties (acoustical protection) of barriers, would complete the two previous indices dealing with intrinsic properties (transmission loss and acoustical absorption).

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.

Proposals for the directive characterisation of electroacoustic emitters

Abstract In this paper, the concept of directivity is generalised to acoustic sources radiating transients or signals evolutive with time. (These are the most common cases in the power or reinforcement electroacoustic systems.) The generalisation proposed is based upon the anamorphism relating the signal levels emitted into the free space in different directions. The relationship between the signals observed in two arbitrary directions is essentially independent of time. Therefore, the anamorphical relationship offers the possibility of obtaining the directivity patterns simply by using as a test signal that signal commonly emitted by the system (i.e. speech in a reinforcement system for a conference room). This principle and method can be applied without major restrictions to any other system, or piece or part of machinery emitting acoustic energy in discontinuous form. Concerning electroacoustic sources, it appears advantageous to replace the usual test signal consisting of pure tones by the signal proper to the system (music, speech, etc.) filtered into the standardised frequency bands. The complete signal (not filtered) can also give significant results. As a simplifying and reasonable compromise regarding the directivity for speech and music, bands of white noise are proposed as test signals.

Rubber crumb as an absorbent acoustical material

In order to contribute to the solution of the environmental problem created by the tire wasteways, the possibilities of using rubber crumbs of waste tires as acoustic absorbent materials have been studied in a systematic way. Experimental absorption coefficients of rubber crumb granulates with mesh diameters between 1.4 and 7 mm have been tested in Kundt’s tube and in a reverberation room. Absorption coefficient values so obtained are compared with computational values afforded through mathematical prediction models that use the intrinsical parameters of porous material such as porosity, tortuosity, and flow resistivity, which were measured on layers of the respective materials. The good broadband acoustic absorption of the material encourages its use as an alternative to the current absorbent acoustic screens, used for the protection against traffic noise.