Massimo Garai

Measurement of the sound-absorption coefficient in situ: The reflection method using periodic pseudo-random sequences of maximum length

Abstract The problem of measuring the sound-absorption coefficient in situ is approached in a systematic way, accounting for parasitic reflections and background noise. The basic reflection method is improved by using pseudo-random binary sequences of maximum length as the test signal. The measurement procedure is orientated towards digital processing techniques and uses the advantages of modern instruments. The inherent cross-correlation process greatly improves the noise immunity. The relationship between the loudspeaker characteristics and the sample size is clarified. The use of a non-rectangular analysis window is suggested. The proposed method was tested in a normally furnished room using portable instruments; the normal-incidence results are in good agreement with values measured using the conventional Kundts tube method.

European methodology for testing the airborne sound insulation characteristics of noise barriers in situ: Experimental verification and comparison with laboratory data

In the frame of the 1994-1997 Standard, Measurement and Testing program, the European Commission funded a research project, named Adrienne, to define new test methods for measuring the intrinsic characteristics of road traffic noise reducing devices in situ. The research team produced innovative methods for testing the sound reflection/absorption and the airborne sound insulation characteristics of noise barriers. These methods are now under consideration at CEN (European Committee for Standardization), to become European standards. The present work reports a detailed verification of the test method for airborne sound insulation over a selection of 17 noise barriers, representative of the Italian and European production. The samples were tested both outdoors, using the new Adrienne method, and in laboratory, following the European standard EN 1793-2. In both cases the single number rating for airborne sound insulation recommended by the European standard was calculated. The new method proved to be easy to use and reliable for all kinds of barriers. It has been found sensitive to quality of mounting, presence of seals, and other details typical of outdoor installations. The comparison between field and laboratory results shows a good correlation, while existing differences can be explained with the different sound fields and mounting conditions between the outdoor and laboratory tests. It is concluded that the Adrienne method is adequate for its intended use.

A new in situ method for the acoustic performance of road traffic noise reducing devices

When they drew the European standards for ‘‘road traffic noise reducing devices’’ (NRD), the members of CEN/TC226 (road equipments)/WG6 (antinoise devices) found problems in using ISO 354 and ISO140 for characterizing sound absorption and airborne sound insulation of NRD. The problems are due to the fact that these standards have been drawn for building products, used inside buildings. The measurement of the sound absorption under a reverberant field does not reflect the real absorption coefficient found with road traffic. In addition, many road administrations wish to get a field test method for testing installed products. A European funded research (‘‘ADRIENNE’’) has been started in January 1995 and finished in December 1997 with the objectives to be able to measure the in‐situ sound absorption and airborne sound insulation of NRD for the widest range of existing products (i.e., flat and nonflat ones). This paper shall present the results of the research, which is based on the combination of MLS signal ...

Repeatability and Reproducibility of In Situ Measurements of Sound Reflection and Airborne Sound Insulation Index of Noise Barriers

In Europe, in situ measurements of sound reflection and airborne sound insulation of noise barriers are usually done according to CEN/TS 1793-5. This method has been improved substantially during the EU funded QUIESST collaborative project. Within the same framework, an inter-laboratory test has been carried out to assess the repeatability and reproducibility of the newly developed method when applied to real-life samples, including the effect of outdoor weather variability and sample ageing. This article presents the statistical analysis of the inter-laboratory test results, and the values of the repeatability and the reproducibility, both in one-third octave bands and for the single-number ratings. The estimated reproducibility values can be used as the extended measure of uncertainty at the 95% credibility level in compliance with the ISO GUM. The repeatability and reproducibility values associated with airborne sound insulation are also compared with the corresponding values for laboratory measurements in building acoustics and an acceptable agreement is found.

Extraction of the envelope from impulse responses using pre-processed energy detection for early decay estimation

The Schroeders backward integration method and its applications have been widely studied in the literature; some papers analyze the performance of the method, some others suggest various enhancement techniques. In spite of these findings, there exist several cases where the energy decay curve extracted using the classical backward integration method and the parameters computed from it seem not always representative of the phenomenon under study. Among them, the cases where the early decay is dependent on strong, distinct reflections occurring just after the direct wave, as in most Italian opera houses. Other cases are measured impulse responses with a very low signal-to-noise ratio or missing the direct wave. In the literature, alternatives to the Schroeders method have been proposed, ranging from Hilbert transform to non-linear processing techniques. In this work a method for the extraction of the envelope based on pre-processed energy detection for early decay estimation is proposed. It is shown that it returns an envelope well matching the first part of the decay even in non-linear cases, returning detailed information on the first part of the decay. The performance of the proposed method is presented and discussed for some exemplary impulse responses measured in historical opera houses. A preliminary study on the perceptive relevance of the method is finally presented.

In situ measurements of the intrinsic characteristics of the acoustic barriers installed along a new high speed railway line

In September 2005 the new high speed railway line Torino-Novara, Italy, was near completion and acoustic barriers had just been installed according to specifications. At this site, the authors conducted in situ verification of the intrinsic characteristics of the noise reducing devices. It is the first European experience of this kind on a large construction workplace. The conditions were extremely demanding and the time scheduled for the task very short. The challenging task was successfully completed applying CEN/TS 1793-5 and taking advantage of the logistic support of the customer. The paper reports the key points of this successful experience and shows some exemplary results. The values measured in situ are compared with the results obtained some years before on products of the same kind. Regarding sound reflection, the in situ method proved to be reliable and to give values more realistic than the laboratory method. Regarding sound insulation, the comparison with previous measurements indicates that, as long as the barriers are well installed, similar results can be expected and that their variance is comparable to that of laboratory tests. On the other hand, large differences (4-5 dB or more) indicate poor quality of construction and installation work, that can be confirmed by a careful inspection. This sensitivity of the in situ method to detect faults paves the way to establish minimum construction and installation criteria. It is concluded that the selected method is fully adequate to in situ verification and could be repeatedly applied to check the acoustic durability of noise reducing devices over time.

Experimental measurements of flanking transmission in CLT structures

This paper discusses the results of an experimental campaign that led to the characterization of flanking transmission in CLT structures with different connection systems. The vibration reduction indices Kij were measured in accordance with the ISO 10848-1 standard in order to provide data suitable to calculate the apparent reduction index according to the EN 12354-1 standard. The research explored several vertical and horizontal junctions on which different number and the type of screws and plates, different CLT panels, and resilient materials at the wall-floor junction were tested. The results show that the Kij along different transmission paths have a specific trends in frequency that depend on the fastening system and the mutual connection of the panels. Moreover, the analysis shows great discrepancies among measurements performed on CLT panels provided by different manufacturers. The results call for a deeper investigation concerning the mechanical/acoustical characterization of the panels.

Strategic Noise Mapping of the Agglomeration of Bologna, Italy

This paper describes how transportation noise makes the largest contribution to environmental noise. The European Directive 2002/49/EC relating to the assessment and management of environmental noise requires European Union (EU) Member States to assess and reduce environmental noise by means of noise mapping and subsequent action plans (to manage noise issues and effects) for roads, railways, airports and urban agglomerations. The agglomeration of Bologna, Italy, consists of the Municipality of Bologna and four other smaller municipalities, overall accounting for more than 450 000 inhabitants. It includes some major highways, railways and a national airport. The regional Authority, Regione Emilia- Romagna, assigned the task to fulfil the END to DIENCA. This paper describes the organization and methods implemented to make the strategic noise mapping of the agglomeration of Bologna, reporting on the man problems encountered, e.g. the harmonization of data coming from different sources, the selection of a digital ground model suited for the subsequent acoustic simulation, etc. , and the ways to solve them. The overall noise maps of the agglomeration of Bologna are presented. Some remarks about the accuracy of predictions and the main sources of uncertainty are also presented in the paper.

Signal analysis in the sound absorption measurement procedure: The importance of time subtraction and reference surface corrections

The sound absorption coefficient measurement (following ISO 13472‐1 normative) of highly absorbing materials, which are tipically employed in room acoustic correction, presents some inherent difficulties. These materials present highly sound absorption coefficient indeed, but usually low sound insulation index and are installed over highly reflective surfaces. This leads to some measurement problems, partially solved by means of the reference surface method. In this work some measurement examples on the same materials but with different contour conditions are analyzed and the improvements on the results, due to reference surface normalization and time domain subtraction of free field response, are discussed.

Recordings of Italian opera orchestra and soloists in a silent room

Anechoic recordings of symphony orchestras have been proposed in the literature and have been used in a multitude of studies concerning both innovative measurements and psychoacoustic experiments. Using the same approach, the present work shows the results of a recording campaign focused on the Italian Opera. Different motifs from Italian Operas have been played by professional musicians and soloists in the silent room of the Bologna University. The excerpts have been chosen because of their musical style characteristics and their acoustic properties (dynamics, timbre, vibrato). The chosen motifs come from scores of Donizetti, Verdi and Puccini, in order to consider various orchestrations and Opera styles.