Hugues Nélisse

On the modeling of the diffuse field sound transmission loss of finite thickness apertures

The modeling of the diffuse field sound transmission loss (TL) of apertures has been rarely considered in the literature. The aims of this paper are (i) to give a comprehensive review of the existing models, (ii) to propose a general efficient and rigorous numerical method to predict the diffuse field TL of apertures of rectangular and circular cross section, (iii) to provide the reader with numerical results regarding this indicator together with its relation with the normal incidence case for various geometrical configurations, and (iv) to conclude on the relevance of using such a sophisticated model compared to more classical normal incidence ones. The proposed approach is based on the description of the sound field inside the aperture in terms of propagating and evanescent acoustic modes. The radiation of the aperture is accounted for using a modal radiation impedance matrix. The coupled problem is solved in terms of modal contribution factors. The convergence of the approach is then investigated and the model is validated by comparisons with existing models for various configurations and excitations. Several numerical examples are provided regarding the normal incidence and diffuse field TL for various apertures and the relationship between these two indicators is discussed.

Measurement of hearing protection devices performance in the workplace during full-shift working operations.

OBJECTIVES The effectiveness of hearing protection devices (HPDs), when used in workplace conditions, has been shown over the years to be usually lower than the labeled values obtained under well-controlled laboratory conditions. Causes for such discrepancies have been listed and discussed by many authors. This study is an attempt to understand the issues in greater details and quantify some of these factors by looking at the performance of hearing protectors as a function of time during full work shift conditions. METHODS A non-invasive field microphone in the real ear (F-MIRE)-based method has been developed for measuring the effectiveness of different HPDs as a function of time in the workplace. Details of the test procedures, the equipment used, and the post-processing operations are presented and discussed. The methodology was developed in such a way that a complete time and frequency representation are possible. The system was used on a total of 24 workers in eight different companies. Work shifts of up to 9-h long were recorded. Various types of earmuffs and one type of molded earplugs were tested. RESULTS Attenuation data reported as a function of time showed, for most workers tested, considerable fluctuations over entire work shift periods. Parts of these fluctuations are attributed to variations in the low-frequency content in the noise (in particular for earmuffs) as well as poor insertion and/or fitting of earplugs. Lower performances than laboratory-based ones were once again observed for most cases tested but also, important left and right ear differences were obtained for many individuals. When reported as a function of frequency, the attenuation results suggested that the few approximations used to relate the measurements to subjective real-ear-attenuation-at-threshold (REAT) data were realistic. CONCLUSIONS The use of individualized attenuation data and performance ratings for HPDs as well as a good knowledge of the ambient noise in the workplace are key ingredients when evaluating the performance of hearing protectors in field conditions.

Sound transmission loss of rectangular and slit-shaped apertures: Experimental results and correlation with a modal model

Among noise control techniques, enclosures are widely used. It is known that enclosure acoustic efficiency is strongly influenced by the presence of openings or leaks. Modeling of diffuse field sound transmission loss (TL) of apertures and slits is therefore critical when the enclosure acoustic performance characteristics need to be predicted with confidence either for design or for modifying existing enclosures. Recently, a general model for diffuse field sound TL of rectangular and circular apertures has been developed and validated with respect to existing analytical or numerical models. This paper presents an experimental validation of this new model. The aim was to develop a simple, reliable tool for predicting enclosure insertion loss using statistical energy analysis. Twelve out of the 15 test configurations were found to be reliable and were compared with theoretical models, which in fact correlate closely (without adjustment) with the experimental work.

Comparison of sound propagation and perception of three types of backup alarms with regards to worker safety

A technology of backup alarms based on the use of a broadband signal has recently gained popularity in many countries. In this study, the performance of this broadband technology is compared to that of a conventional tonal alarm and a multi-tone alarm from a worker-safety standpoint. Field measurements of sound pressure level patterns behind heavy vehicles were performed in real work environments and psychoacoustic measurements (sound detection thresholds, equal loudness, perceived urgency and sound localization) were carried out in the laboratory with human subjects. Compared with the conventional tonal alarm, the broadband alarm generates a much more uniform sound field behind vehicles, is easier to localize in space and is judged slighter louder at representative alarm levels. Slight advantages were found with the tonal alarm for sound detection and for perceived urgency at low levels, but these benefits observed in laboratory conditions would not overcome the detrimental effects associated with the large and abrupt variations in sound pressure levels (up to 15-20 dB within short distances) observed in the field behind vehicles for this alarm, which are significantly higher than those obtained with the broadband alarm. Performance with the multi-tone alarm generally fell between that of the tonal and broadband alarms on most measures.

Prediction of the acoustical performance of enclosures using a hybrid statistical energy analysis: image source model.

Enclosures are commonly used to reduce the sound exposure of workers to the noise radiated by machinery. Some acoustic predictive tools ranging from simple analytical tools to sophisticated numerical deterministic models are available to estimate the enclosure acoustical performance. However, simple analytical models are usually valid in limited frequency ranges because of underlying assumptions whereas numerical models are commonly limited to low frequencies. This paper presents a general and simple model for predicting the acoustic performance of large free-standing enclosures which is capable of taking into account the complexity of the enclosure configuration and covering a large frequency range. It is based on the statistical energy analysis (SEA) framework. The sound field inside the enclosure is calculated using the method of image sources. Sound transmission across the various elements of the enclosure is considered in the SEA formalism. The model is evaluated by comparison with existing methods and experimental results. The effect of several parameters such as enclosure geometry, panel materials, presence of noise control treatments, location of the source inside the enclosure, and presence of an opening has been investigated. The comparisons between the model and the experimental results show a good agreement for most of the tested configurations.

Prediction of the niche effect for single flat panels with or without attached sound absorbing materialsa)

The sound transmission loss (STL) of a test sample measured in sound transmission facilities is affected by the opening in which it is located. This is called the niche effect. This paper uses a modal approach to study the STL of a rectangular plate with or without an attached porous material located inside a box-shaped niche. The porous material is modeled as a limp equivalent fluid. The proposed model is validated by comparison with finite element/boundary element computations. Using a condensation of the pressure fields in the niche, the niche effect is interpreted in terms of a modification of the modal blocked pressure fields acting on the panel induced by the front cavity and by a modification of the radiation efficiency of the panel modes due to the presence of the back cavity. The modal approach is then used to investigate the impact of (1) the presence of a porous material attached to the panel on the niche effect and (2) the niche effect on the assessment of the porous material insertion loss. A simplified model for the porous material based on a transfer matrix approach is also proposed to predict the STL of the system and its validity is discussed.

Systematic evaluation of the relationship between physical and psychoacoustical measurements of hearing protectors' attenuation

The most commonly used methods to measure hearing protectors attenuation can be divided into two categories: psychoacoustical (subjective) and physical (objective) methods. In order to better understand the relationship between these methods, this article presents various factors relating attenuation values obtained with these methods through a series of tests. Experiments on human subjects were carried out where the subjects were instrumented on both ears with miniature microphones outside and underneath the protector. The subjects were then asked to go through a series of hearing threshold measurements (psychoacoustical method) followed by microphone sound recordings using high-level diffuse field broadband noises (physical method). The proposed test protocol allowed obtaining various factors relating the test methods as well as attenuation values and ratings for different protection conditions (open ear, earmuffs, earplugs, and dual protection). Results are presented for three models of passive earmuffs, three models of earplugs and all their combinations as dual hearing protectors. The validity and the relative importance of various terms used to correct the physical attenuation values when comparing with psychoacoustical attenuation values are examined.

Noise as an explanatory factor in work-related fatality reports

Noise exposure in the workplace is a common reality in Québec, Canada as it is elsewhere. However, the extent to which noise acts as a causal or contributive factor in industrial work-related accidents has not been studied thoroughly despite its plausibility. This article aims to describe the importance or potential importance, during investigations looking into the specific causes of each work-related fatal accident, of noise as an explanatory factor. The written information contained in the accident reports pertaining to contextual and technical elements were used. The study used multiple case qualitative content analysis. This descriptive study was based on the content analysis of the 788 reports from the Commission de la santé et de la sécurité du travail du Québec [Workers′ Compensation Board (WCB)] investigating the fatal work-related accidents between 1990 and 2005. The study was descriptive (number and percentages). Noise was explicitly stated as one of the explanatory factors for the fatal outcome in 2.2% (17/788) of the fatal accidents, particularly when the work involved vehicular movement or the need to communicate between workers. Noise was not typically considered a unique cause in the accident, notably because the investigators considered that the accident would have probably occurred due to other risk factors (for example, disregard of safety rules, shortcomings in work methods, and inadequate training). Noise is an important risk factor when communication is involved in work. Since noise is ubiquitous and may also interfere with vigilance and other risk factors for accidents, it may be a much more important contributing factor to accidents than is currently recognized.

Tunneling effect on the sound transmission loss of a flat structure coupled with a porous material

It is well known that when measuring Sound Transmission Loss (STL) in a laboratory, among all test conditions, the location of a specimen in an aperture affects the results, due to the tunneling effect. Previous studies have considered this effect for flat single panels and double walls but the case of a panel with attached sound package seems to have received very little attention. This paper deals with the application of a modal approach to study the STL of a rectangular plate coupled with a porous material located inside a tunnel. The sound absorbing material is supposed to be either described by a modal approach or a transfer matrix calculated using a Transfer Matrix Method which relates interstitial pressure and total normal stress on both sides of the material. The model is validated by comparison with Finite Element/Boundary Element computations. Numerical results are shown to illustrate the validity of the proposed full modal and hybrid modal-TMM methodologies and their use to investigate the nich...

Effects of ear canal occlusion on hearing sensitivity: A loudness experiment

Over the last century, hearing research has repeatedly reported differences in loudness perception when different types of transducers are being used. One of the effects of using different transducers is that listening may be performed via an open ear (loudspeaker), a cushioned ear (headphones), or an occluded ear (hearing aid receivers, insert earphones). The question of whether varying the acoustic load applied to the ear canal might impact hearing sensitivity has therefore become essential given the need to establish realistic noise damage risk criteria in an attempt to prevent noise-induced hearing loss for any given listening condition. Although such loudness discrepancies in the cushioned ear have been recently proven to be caused by loudness measurement artifacts, currently available data do not exclude a possible impact of ear canal occlusion on loudness perception. This paper presents the results of a loudness balance test carried out on 18 normal-hearing listeners. Using an earplug to occlude the canal, in-ear sound pressure levels were compared between the occluded ear and the cushioned ear at equal loudness. The results show agreement within 1 dB between the two listening conditions, and support the conclusion that loudness does not depend on the type of acoustic load applied to the ear canal.