Francisco Simón

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

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.]

Assessing Auditory Processing Deficits in Tinnitus and Hearing Impaired Patients with the Auditory Behavior Questionnaire

Background and Purpose: Auditory processing disorders (APD), tinnitus and hearing loss (HL) are typical issues reported by patients in audiologic clinics. These auditory impairments can be concomitant or mutually excluding. APD are not necessarily accompanied by significant HL, whereas many adults exhibit peripheral HL and typical cognitive deficits often associated with APD. Since HL, tinnitus and APD affects to several parts of the ascending auditory pathway from the periphery to the auditory cortex, there could be some interrelationship between them. For instance, tinnitus has been reported to degrade the auditory localization capacity. Tinnitus is believed to be triggered by deafferentation of normal peripheral input to the central auditory system. This peripheral deficit can be accompanied by HL or not, since a type of permanent cochlear damage (thus deafferentation) without an elevation of hearing thresholds might persist. Therefore, a combined study of APD, tinnitus and HL on the same cohort of patients can be audiologically relevant and worthy. Methods: Statistical analysis is applied to a cohort of 305 patients attending an audiology clinic in Madrid (Spain). This group of patients is first categorized in four subgroups, namely, HLTG (with tinnitus and HL), NHLTG (with tinnitus and without HL), HLNTG (with HL but no tinnitus), and NHLNTG (neither tinnitus nor HL). The statistical variables include Age, Average Auditory Threshold (ATT), for assessing HL, Tinnitus Handicap Inventory (THI), for measuring tinnitus, and a new 25-item Auditory Behavior Questionnaire (ABQ), for scoring APD. Factor analysis is applied to arrange these items into 4 subscales. The internal consistency reliability of this ABQ is confirmed by calculating Cronbachs coefficients α. The test-retest reliability is assessed by the intraclass correlation coefficients, ICC. Statistical techniques applied to the data set include descriptive analysis of variables and Spearman rank correlations (ρ) between them. Results: Overall reliability of ABQ is confirmed by an α value of 0.89 and by an ICC of 0.91. Regarding the internal consistency reliability, the four subscales prove a fairly good consistency with α coefficients above 0.7. Average values of statistical variables show significantly lower age of patients with tinnitus and no HL, which can provide a cue of noise overexposure of this segment of population. These younger patients show also decreased ABQ and similar THI in comparison with patients in the other subgroups. A strong correlation (ρ = 0.63) was found between AAT and Age for the HLNTG subgroup. For the HLTG subgroup, a moderate correlation (ρ = 0.44) was found between ABQ and THI. Conclusion: The utilized questionnaire (ABQ), together with AAT and THI, can help to study comorbid hearing impairments in patients regularly attending an audiological clinic.

Noise attenuation by two‐dimensional sonic crystals made of rubber crumb.

It is shown that sonic crystals consisting of cylindrical rods made of rubber crumb can be used as efficient structures to attenuate noise in a wide range of frequencies. A structure made of three rows of cylinders (3‐m height) has been fabricated and its isolation properties have been characterized in a transmission chamber. Two different numerical algorithms have used to simulate the experimental characterization. A good agreement between theory and experiment is obtained and gives support to design barriers based on this technology. [Work supported by MICIIN of Spain.]

Using simulating annealing for the inverse estimation of the non-acoustical parameters of sound absorbers

Acoustic characterization of sound absorbing materials requires the measurement of one to five non-acoustical parameters, depending on the assumed impedance model. The measurement of five non-acoustical parameters, in the case of more refined impedance models, is sophisticated and time-consuming. As an alternative, a much simpler measurement of the normal incidence sound absorption in an impedance tube can be carried out, and the non-acoustical parameters can be estimated by minimizing the difference between the measured and modeled absorption curves. This inverse procedure requires the choice of the impedance model and the inversion method. The aim of the article is to review the goodness of simulating annealing for estimating the non-acoustical parameters of Miki and Champoux–Stinson models of two granular absorbers. Three of these non-acoustical parameters, common to the two models, are also measured, so that a comparison between measured and estimated parameters can be performed.

Characterising elastic layers as non‐lightly damped SDOF systems for the reduction of impact transmission noise

There is a wealth of literature dealing with modal determination of lightly damped viscous systems, however there is relatively little reported for medium and heavily damped systems or with non‐viscous damping. The most commonly reported and applied methods have significant errors in the determination of both modal frequency and damping ratio for systems with high damping. For the lightly damped case simplification can be often conveniently made to allow the performance of each method to be studied theoretically, however for the non‐lightly damped case this is not possible and the analysis must proceed on a heuristic basis. For the application modal parameter determination of elastic floor layers designed to reduce building transmission noise by impact testing, traditional and new methods of modal parameter determination for non‐lightly are presented and heuristically evaluated for both viscous and non‐viscous SDOF systems. It is also shown the performance often depends on the type of damping. This either...

Physical Properties of Noise Barriers and the Development of an Acoustical Protection Rating Index

A study into the physical properties of noise barriers has been carried out and a single rating index to characterise the acoustical protection of a barrier has been developed, in terms of the barrier diffraction coefficient. An expression for a single figure rating index may be formulated by assessing the insertion loss at a receiver point far enough from the barrier to make the difference between direct and diffracted sound paths from barrier to receiver negligible. This makes the geometry between the barrier and the receiver point redundant in the calculation of insertion loss and thus allows the barrier to be characterised by a single rating index. In this work the insertion loss is expressed as a function of the barrier diffraction coefficient, applying the same geometry conditions. Subsequently an expression for a single rating index is obtained as a function of the barrier diffraction coefficient.