Overview of Researches in Acoustics at DIN
Since many years, a considerable research effort has been devoted to the theoretical, experimental and numerical study of room acoustics. A portable electronic system, using MLS pseudo-random sequences, for the objective qualification of concert halls was realised. Using this system, the acoustics of several ancient Italian theatres was evaluated, obtaining useful indications on the past design criteria and for the actual restoration criteria. The same evaluation technique has also been applied to modern auditoriums. An experimental research on the relevance of specific cavities made by the builders under the orchestra pit of some ancient theatres is actually going on. The developments of the research include the definition of non conventional methods for the restoration of ancient buildings of the historical heritage; the new proposals are studied by computer simulation. The results of the research were presented at national as well as international congresses, like ICA '92, Euro-noise '95 and ICA/ASA '98, and also on a dedicated international publication (Y.Ando & D.Noson Editors, Music and concert hall acoustics, Academic Press, 1996). The paper on Boxes and sound quality in an Italian opera house has been selected for Internet presentation at the ICA/ASA '98 meeting and then published in an expanded version on the Journal of Sound and Vibration (2000). The study has been enlarged to the comparison of the different existing techniques for computer simulation, both from theoretical and applied point of view, and to auralisation by convolution.
A first research dealt with a comparative analysis of the Italian and ISO standards on acoustics in buildings, coupled with the verification of the applicability of the test codes in critical situations. A thorough experimental research was performed, about quick methods for estimating, in the field, the sound insulation rating of walls; it produced results presented in an international congress and on the international journal Applied Acoustics (1989). New experimental results on the in situ sound insulation of walls and floor are being collected at any time, in the light of the new measurement methods proposed on the international scene. This activity is linked to the technical activity done by researchers of DIENCA for UNI, CEN and ISO in building acoustics. The research has then been extended to the evaluation of the acoustic comfort offered by the whole building. The applicability of pr EN 12354-1 and 12354-2 to Italian buildings is under study. Also, a specific research project for the measurement of the flanking transmission in building structures, both in laboratory and in situ, is going on. A scale model investigation on the influence of boundary conditions on the airborne sound insulation of lightweight double walls has been published on the Journal of Building Acoustics (2000).
The research deals either with the methodological aspects and their relationship with existing standards, either with the mathematical models of outdoors noise propagation. A computer program for the evaluation of the environmental impact of the noise from large roadways was developed; it comprises the design of sound attenuating devices, like screens, low-noise road surfaces, etc. The outlined methods and program were successfully verified during the environmental impact assessment of some major Italian roadways and presented to European experts during an Eurosymposium (1992). Specific criteria were also developed to perform the environmental impact assessment of a clay-pigeon shooting-range, source of long sequences of noise impulses. The procedures for accomplishing a correct city planning according to the most recent Italian laws were studied in details. A fluid dynamic model of sound propagation in real atmosphere is now under refinement; it is based on the W.K.B. expansion of the governing equations and leads to a treatment of the Blokhintzev invariant valid also for non adiabatic lapse rates. The phenomena related to the transit of high speed trains in tunnels have been investigated using a 2D numerical model. It shows the stationary pressure field associated with the train and the superimposed shock wave generated when the train enters the tunnel. This wave travels at the sound speed forwards and backwards along the tunnel, partially reflected back and partially radiated outdoors.
Low-noise road surfaces
The Group of Applied Acoustics of the University of Bologna is carring on a specific research project for the measurements of the sound absorption coefficient of road surfaces in situ. The research project can exploit the general techniques developed for testing the acoustic properties of materials using MLS signals. The method has been selected by ISO/TC43/SC1/WG38 to become the forthcoming international standard ISO 13472-1 with the title "Acoustics - Procedure for measuring sound absorption properties of road surfaces in situ - Part 1: Extended surface method". Qualification tests are now under work, in close cooperation with other research teams, like: LCPC, Nantes (France); ARPAT, Firenze (Italy); ARPA, Modena (Italy).
Since many years, a lot of research has been done on testing the acoustic performances of noise reducing devices in situ. DIENCA has been a main partner in the Adrienne project (1995-97), a research project funded by the European Commission on the in situ measurement of the acoustic characteristics of road traffic noise reducing devices. At DIENCA is under development a software system, ALFA, for the post-processing of data measured on samples of various materials and products (in laboratory as well as in situ) in order to obtain the sound absorption coefficient and the acoustic impedance. DIENCA is consultant of Italferr (the engineering company of Italian Railways) for the qualification of noise barriers to be installed alongside the high speed Italian railway. the In September 2005 DIENCA demonstrated the feasibility of a demanding on site verification, checking the acoustic intrinsic characteristics of the noise screens just installed alongside the new high speed railway Torino-Novara. The airborne sound insulation and reflection characteristics were measured on several sample sections of the noise barriers, selected at random, working mainly overnight during the final step of the construction work: read more clicking here.
Acoustic properties of materials
Interdisciplinary research which aims to apply digital signal processing techniques and mathematical models to obtain an in-depth knowledge of the mechanical and acoustic properties of materials. Starting from an investigation of the usefulness of cepstral analysis, the research evolved toward the development of mathematical techniques relying on pseudo-random binary sequences of maximum length (MLS) for the measurement of the sound absorption coefficient in situ; a prototype device was built. The results, remarkably new, have been presented in an international congress and, with more details, on a special issue of the international journal Applied Acoustics (1993). Advanced frequency analysis methods are also under consideration to improve the way of using the traditional impedance tube. A detailed study on the application of new measurements techniques for the design of sound absorbing materials with regard to the new international rating system was published on an international journal: M.Garai "On the single number rating of sound absorption" J. Building Acoustics, 1(3), 217-233 (1994). DIENCA was a main partner in a research project funded by the European Commission on the in situ measurement of the acoustic characteristics of road traffic noise reducing devices (Adrienne project, 1995-97). At DIENCA is under continuous development a software system, ALFA, for the post-processing of data measured on samples of various materials and products (in laboratory as well as in situ) in order to obtain the sound absorption coefficient and the acoustic impedance. An airflow resistance measurement device was built (1998), complying with UNI EN 29053; the data measured on polyester fibre samples have been analysed to check the reliability of the traditional semi-empirical models of porous/fibrous materials and a new model has been proposed: M. Garai, F. Pompoli "A simple empirical model of polyester fibre materials for acoustical applications" Applied Acoustics, 66,1383-1398 (2005).
Standards and legislation
Research connected with the activity made for UNI, CEN and ISO. Low-noise road surfaces properties are studied in connection with the participation to the ISO working group ISO/TC 43/SC 1/WG 38 to draft ISO 13472-1. DIENCA participated as main partner to a research project funded by the European Commission (Adrienne project, 1995-97) and having the aim to assess a new method for testing the absorption and airborne sound insulation of noise reducing devices. The new method is under consideration by CEN/TC 226/WG 6 for inclusion in the standard package EN 1793 "Road traffic noise reducing devices - Test methods for determining the acoustic performance", which includes five parts. Airflow resistance measurement according to UNI EN 29053 and related mathematical modelling is being studied intensively. In the field of general metrology, an in-depth study on the consequences for practical acoustics of a change in the definition of the logarithmic quantities has been undertaken.
Application of modern analysis techniques to the evaluation and guidance of the restoration of ancient pianos of the romantic period.