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Influence of source directivity on noise levels in industrial halls: Simulation and experiments
Applied Acoustics 68 (2007) 682–698 www.elsevier.com/locate/apacoust
Influence of source directivity on noise levels in industrial halls: Simulation and experiments Jacques Chatillon
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Institut National de Recherche et de Se´curite´, Avenue de Bourgogne, BP 27, 54501 Vandoeuvre cedex, France Received 3 October 2005; received in revised form 5 July 2006; accepted 7 July 2006 Available online 27 September 2006
Abstract Noise exposure of workers in industrial halls is mainly induced by noisy machines whose acoustical features are often globally known by Sound Pressure Level. The evaluation of the directivity of these noise sources can help to anticipate specific solutions for noise reduction. This study shows how the directivities of three wood-working machines have been characterized. Some characterisations have been achieved with a simple and fast acoustical intensity mapping which meets the constraints of industrial areas. When source directivity is evaluated, its influence on the noise field in industrial halls can be assessed. Some simulations and some experiments allowed the estimation of the noise field induced in workshops by both directional and omnidirectional sources. Comparison of the fields prove that the noise distribution is influenced by the source directivity if the halls are empty. As soon as the halls contain scattering objects, the directivity effect is reduced a lot and the noise field remains nearly the same far from the source whatever source used. Nevertheless, workers close to a machine are exposed to noise according to their position with respect to the machine. Exposure at the workplace can vary from 4 to 8 dB(A) according to the directivity of machines such as those measured in the trials. 2006 Elsevier Ltd. All rights reserved. Keywords: Occupational noise; Source directivity; Noisy machines; Industrial hall
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Tel.: +33 3 83 50 98 70; fax: + 33 3 83 50 21 96. E-mail address: [email protected].
0003-682X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2006.07.010
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1. Introduction Noisy machines are the main cause of worker noise exposure in industrial halls. In order to reduce this noise pollution, actions to lower noise emission of the source and actions to improve acoustical treatments of the halls are both usable. The acoustical characteristics of industrial sources are often known globally and are often reduced to a single representative: the Sound Pressure Level (SPL). Knowing the spatial features of noise emission by measuring the directivity of the machines could help to find new solutions to reduce the noise level in industrial halls. For instance, these solutions could be a partial screening of the noisy sources, a change of their position or orientation, or a partial acoustical treatment of a close hall wall. This study aims to characterize the directivity of several noisy machines and to evaluate the influence of this feature on noise exposure all over an industrial hall. Three noisy wood-working machines and a test-source are studied. A laboratory method is firstly used by means of numerous measurements of pressure points around the source in an anechoic room. In the industry world, constraints of measurements are linked to work organization and to the building features. The hall is generally reverberant and it often involves a lot of noise sources. Work organization implies that the measurement area is reduced and that the time required for measurements must be shortened. In order to meet these constraints, a faster and simplified method for directivity characterization has been implemented by using acoustical intensity mapping. Source directivity being evaluated, the noise level induced by directive sources in industrial halls is then assessed thanks to simulations and experiments. 2. Characterizations of source directivity 2.1. Methods 2.1.1. Simple vs. complete characterization Characterization of source directivity can be evaluated with the required accuracy by laboratory or standard methods using pressure measurements [1] or intensity measurements [2–4]. For instance, in the laboratory, a complete characterization can be achieved by measuring sound pressure on numerous microphones on a hemisphere around the source. This technique is accurate if there is a large collection of measured points, but requires time and space to fan the microphone system out. There is also a need for source stability over time. The directivities of both a test-source and a table-saw have been measured by this method in a hemi-anechoic chamber. However, in a real industrial hall, some constraints can prevent from producing laboratory standard measurements. These constraints include the presence of secondary sources and the reflection on the hall walls which can cause a high level of background noise capable of screening the source of interest. Furthermore, constraints of production can sometimes restrict the measurement area around the source or time for measurements. For these reasons, a simpler characterization of the source directivity has to be devised. By mapping acoustic intensity with a sweeping scan on a parallelepiped around the source, time of measurement can be shortened and secondary source contributions can be eliminated as far as they are time stable.
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This mapping was used in the study after validation by comparison with point-to-point intensity measurements in a hemi-anechoic chamber. This simple characterization, using intensity sweeping scan and reconstructed virtual points, was validated on the same test-source, and used to predict the effect of two wood-working machines put in a real production hall. 2.1.2. Test-source and wood-working machines A test-source was selected for the study because its directivity can be theoretically calculated. Moreover it is quite directive in the hearing frequency band for showing noise field differences if they exist. This source has a simple directivity pattern (cardinal sine) with a main lobe narrow enough (50 at 1 kHz). This typical test-source is easy to compare – for its directivity and its influence – to the noisy machines. On another hand, simulations of sound field in an industrial hall use simulated sources whose directivity is similar to testsource. The test-source is composed of a rectangular box with four loud-speakers (Fig. 1) emitting a pink noise from 125 Hz to 8 kHz. The theoretical directivity pattern of this source is those of one loud-speaker convoluted with those of a line. It is approximately composed of a main lobe with a 3 dB width equal to about 50 at 1000 Hz in the Px plane (Fig. 1) with secondary lobes decreasing in level as a cardinal sine function. By construction, this source is four times less directive in the Py plane (Fig. 1). Furthermore, the noise level transmitted to the back-side is reduced a lot by the box acting as a baffle. Three configurations of this source were used (Fig. 2): (A) source at 1 m from the ground, emission to a wall, (B) source at 1 m from the ground, emission to the ceiling, (C) source on the ground, emission to a wall.
Fig. 1. Test-source.
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Fig. 2. Configurations of test-source.
The two first configurations A and B induce an image source at 2 m from the real one in case of hard ground. The set-up number C allows the real source and the image source to be identified for a frequency band up to about 1 kHz. Three wood-working machines have been used, without any load in order to obtain a very stable emission during the times of measurements. The table-saw used in the study is shown inside the hemi-anechoic chamber in Fig. 3. This machine is approximately a cube 1.2 m · 1.2 m · 1.2 m, with a blade of 30 cm diameter. A loading system with a small motor on the top is not used. The blade and the motor are the main sources of noise. The two other wood-working machines are a planing machine and a surface-planing machine. The planing machine dimensions are about 1 m · 1 m · 1.2 m, it includes an aspiration system on the top which was turned off during the experiment. The loading is achieved through a way crossing the machine, that means these are two holes at two opposite sides of the machine, and the tool inside at the middle (Fig. 4 left). The main noise contribution is emitted by these holes. The surface-planing machine looks like the table-saw (Fig. 4 right) except from its length (2.5 m) and the tool which is a planer, the noise mainly comes from the tool (on the table top) and the motor inside. 2.1.3. Experimental set-up of the two characterization methods The complete characterizations of the test-source in three configurations and of the table-saw were achieved in a hemi-anechoic chamber with concrete floor. Sound pressure levels were measured at points on a hemisphere of 2 m radius around the sources. Sixteen microphones were used for a vertical scan from 0 to 90 and the system was rotated around the vertical axis in steps of 5 (Fig. 5). This configuration defines 16 · 72 = 1152 positions of pressure measurements to obtain the directivity of the sources with enough accuracy up to the maximum frequency band (8 kHz). A simple characterization was also achieved by acoustic intensity mapping on a parallelepiped around the source (Fig. 5). The intensity probe scan of a plane is split in time into
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Fig. 3. Table-saw in anechoic chamber.
several parts each corresponding to a virtual point of measurement similar to those obtained by point-to-point intensimetry. In order to validate the scanning and virtual point method, a classical point-to-point intensimetry measurement was firstly done with the test-source. 2.2. Source directivity evaluation by means of complete characterization The complete characterization of the test-source shows the expected directivity patterns: a main lobe, narrowing with frequency increase, with side lobes appearing at highest frequencies and a screening of the noise at the rear. Fig. 6 shows an example of directivity patterns obtained for configuration ‘‘A’’ for the octaves centred at 1 kHz and 8 kHz. These patterns are displayed as a top view of the hemisphere around the source emitting at the right side, the azimuth angle varying from 0 to 360 all around the circle, and the elevation varying from 0 to 90 as a radius of the circle. The table-saw displayed in Fig. 3 was characterized with the same method while the blade was rotating without any load, in order to record a very stable noise during the time required to operate the microphone system (less than 1 h).
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Fig. 4. Planing machine (top) and surface-planing machine (bottom).
Fig. 7 shows an example of two directivity patterns obtained with the table-saw for the octaves centred at 1 kHz and 8 kHz. These patterns are displayed with the same colour levels as those in Fig. 6 in order to compare the directivity of the table-saw with those of the test-source. Top view of the table and the blade is superimposed on the 1 kHz pattern. One can note that the table-saw looks less directive than the test-source at 1 kHz, where the levels are nearly only gathered in red colour. A typical pattern is visible at 8 kHz looking as butterfly wings which show more energetic emissions at +45 or 45 from the blade plane. These complete characterizations allow the estimation of the directivity indices of the sources in accordance with the ISO 14257 standard [5]. This standard proposes to calculate, by third octaves, the directivity indices of a source by comparing the mean quadratic pressure level of the source found on a circle around the source with the maximum and the minimum quadratic pressure levels found on smaller angular sectors on the same circle. The circles can be scanned in the two angular directions (azimuth and elevation). The standard defines a template according to the frequency (2 dB up to 630 Hz, 2–8 dB up to 1 kHz, 8 dB above 1 kHz) defining the maximum indices of a so-called ‘‘omnidirectional’’ source.
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Fig. 5. Complete and simple characterization.
Fig. 6. Test-source configuration (A). Examples of directivity patterns (left 1 kHz; right 8 kHz) measured on the hemisphere around the source.
Plotting the test-source and the table-saw indices together with the template indicates how the sources are rather directive or omnidirectional. Fig. 8 displays the directivity indices of the test-source in configuration A compared to the directivity indices of the tablesaw. Each plot contains the template of ISO 14257. For the test-source in configuration ‘‘A’’ and ‘‘C’’, the directivity indices are reaching 15–25 dB in azimuth for frequencies above 1 kHz. The test-source is rather directive in azimuth for this configuration, while it should not be as directive on elevation if there was not the screening effect of the box back-side. For the configuration ‘‘B’’, the directivity is higher in site than in azimuth, as the geometry of the source can explain. For the table-saw, the plots related to ISO 14257 show that this machine is quasi-omnidirectional even if indices are outside the template for thirds of octave between 200 and 800 Hz.
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Fig. 7. Table-saw. Examples of directivity patterns (left 1 kHz; right 8 kHz) measured on the hemisphere around the source.
Fig. 8. Directional indices of the test-source in configuration ‘‘A’’ (black circles TS) and of the table-saw (white diamonds Saw) together with the ‘‘omnidirectional’’ template given by ISO 14257 (solid line).
Finally there is a large difference in directivity between the two sources, while the testsource is rather directive, the table-saw is not. This observation may have large consequence on the noise level radiated in an industrial hall with these sources. 2.3. Source directivity evaluation by means of simplified characterization The simple characterizations have been achieved on the test-source in the hemi-anechoic chamber as well as on two wood-working machines already described and put in a real production hall. The machines were unloaded in order to record a very stable noise during the intensity probe scan (10–20 min for all the planes). The acoustic intensity mapping of the test-source in configuration ‘‘A’’ has been achieved by a sweeping scan of the five planes of a virtual box closing the source. Each plane is scanned in 2 min and the total time record of the intensity probe is then split into 81 parts each corresponding to a virtual mean fixed point. Fig. 9 shows how the mapping of the five planes around the source are displayed.
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Fig. 9. Display of intensity mapping.
Fig. 10 displays two examples of intensity map for the test-source in configuration ‘‘A’’ for the octaves centred at 1 kHz and 8 kHz. One can notice the frequency effect of the azimuth directivity of the source already seen with the complete characterization. The acoustic intensity mapping of the two wood-working machines has been achieved in the same way. The planes around the sources are different from those of the test-source because the machine dimensions are not the same. The planing machine is scanned by 80 virtual points by vertical plane (1.5 m by 2 m) and 100 for the top side (2 m by 2 m). The surface-planing machine is scanned by about 70 or 140 points on vertical planes (2.5 m or 5 m by 1.2 m) and by about 250 points on the top side (2.5 m by 5 m). Fig. 11 shows two examples of intensity maps for the two machines for the octaves centred at 1 kHz and 8 kHz. One can see that the planing machine looks to have little
Fig. 10. Test-source configuration ‘‘A’’. Examples of intensity mapping (1 kHz; 8 kHz). Rectangular scales in metres.
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Fig. 11. Planning machine (top) and surface-planing machine (bottom). Examples of intensity mapping (left 1 kHz; right 8 kHz). Rectangular scales in metres.
directivity as the levels are nearly equally shared on the map. However, the loading-holes (left and right planes on the map) show more energetic emissions at low frequency than the rest of the machine. This observation is less clear at higher frequency because the spectrum of this machine is mostly below 2 kHz. For the surface-planing machine, the tool at the top of the table is clearly the noise source, and the table acts as a reflecting plane to send the most energetic emissions to the ceiling. The surface-planing machine looks to be more directive than the planing machine. Processed in dB(A), the same intensity maps show differences between the mapping of the planes between 3 and 8 dB(A). These simple characterizations do not allow the estimation of the directivity indices of the sources in accordance with the ISO 14257 standard. In order to compare the directivity of the three sources, specific directivity indices have been set up by calculating mean quadratic pressure levels on lines scanning the map, and maximum and minimum levels compared to this average value. The scan of the map is achieved in two directions: horizontal then vertical (Fig. 12). For a given scan line, the values are smoothed with three adjacent points of the line, and the difference between the maximum and the minimum with the average value, for each third of octave, define the directivity indices. This processing of the indices looks like those
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Fig. 12. Scans of the intensity map to define new directivity indices.
Fig. 13. Horizontal and vertical directivity indices calculated on the intensity maps for the test-source (black circles TS), the planing machine (white diamonds, PM), and the surface-planing machine (white squares, SPM).
of the ISO 14257 standard, but final values are not comparable, since a circular or square scanning motion is used according to the calculation methodology. Fig. 13 shows the indices calculated with this last method. The indices of the test-source are increasing with frequency, similar to those indices calculated following the ISO standard. However the horizontal scan over squares induces higher indices than expected at low frequency. The analysis of the indices values shows that the planning machine is always much less directive than the test-source. However the surface-planing machine is more directive than the test-source for lower frequencies using the vertical scan. These remarks are in agreement with the first observations of the intensity maps displayed in Fig. 11. 3. Influence of the directivity on noise radiated in halls 3.1. Comparison criterion When the source directivity is estimated, its influence on the noise distributed in the halls can be evaluated by means of simulations or experiments. In both cases this influence
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is processed by comparing either the noise field caused by an omnidirectional source with those radiated by a directional source, or the noise field caused by the same directional source whose orientation has changed. In an industrial hall, the acoustic field can be split in two different areas: the direct field and the reverberant field. The direct field, close to the source, involves mainly the machine operator exposure. In this case, the levels of exposure are directly connected to the noisy machine sound pressure level and the acoustic features of the hall are of secondary importance. Then the machine directivity, estimated by methods such as those described above, gives immediately an assessment of the worker exposure according to his position. The measurements achieved by the methods of characterizations described in Section 2 allow to estimate the differences in exposure due to the machine directivity. The noise exposure can vary up to 4 dB(A) for a worker close to the table-saw, up to 5 dB(A) for a worker close to the planing machine and up to 8 dB(A) in the case of the surface-planing machine. The reverberant field, far from the source, involves mainly the other workers in the rest of the hall. The levels are still linked to the noisy machine spectrum and directivity but also to the propagation in the hall whose acoustic characteristics becomes very important. As we are rather interested in the reverberant field aspects, only the propagation in the hall has to be estimated in this section. For this reason the comparisons of the radiated fields will exclude the noise levels close to the machine. This can be achieved, considering the position of the operator of the machine and the assumed positions of the other workers in the hall, by leaving out the sound levels contained in a circle of radius 4 m around the source. Then the comparisons between the sound field induced by the directive source and the field induced by an omnidirectional source (or the directive source with another orientation) have to satisfy several constraints to take into account mainly the reverberant field, to give results independent from the noise source spectra, to reach the standard noise exposure assessment. These constraints can be satisfied by the following actions: to remove the near sound levels from the calculation, to remove noise source spectrum from the sound levels, to measure sound field at operator level (1.6 m) and to give results in dB(A). The comparison criterion can then be defined. Noise mapping is naturally the best mean to compare noise fields in an industrial hall. However, maps contain too much information to be used directly. We propose to simplify the analysis by using noise map differences accumulated to provide a histogram. The mean value and the dispersion of each histogram will be simple quantifiers of the difference of the sound fields induced by sources which have to be compared. Fig. 14 explains the principle of the sound field comparison: after source spectrum correction and (A) weighting, two noise maps are subtracted to obtained the difference map. The cells near the source (<4 m) are left out the calculation. Cells of same difference level are accumulated to build a difference histogram. Top value and dispersion of this histogram can quantify the influence of the source directivity over the whole workshop.
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Fig. 14. Noise field comparison criterion: example of difference map histogram (simulated data). Number of cells is finally processed in percentage.
3.2. Room acoustical software A room acoustical software package was used to calculate the noise distributed in the industrial halls based upon the case of a directive source. This software is a ray-tracing model simulating the building as a collection of planes with given Sabine coefficients [6]. The rays coming from the source are reflected between these planes loosing energy with each reflection. When a ray is crossing a reception cell, its energy (no phasing) is accumulated in the cell to define the final level of this element of the noise map. The software was originally working with one or several omnidirectional sources and has been modified to accept a directional noise source under single source conditions. This restriction allows the use of only the configuration ‘‘C’’ (source level 0 m) for the test-source as an input parameter of the simulations because it is assumed that real source and image source are identified for this configuration. The other test-source configurations ‘‘A’’ or ‘‘B’’ (source level 1 m) induce an image source too far from the real source. The wood-working machines showed also either image or multiples sources. With this software version, these last noise sources cannot be simulated but they will be widely used during the experiments. 3.3. Simulated halls A database of 10 halls was used for simulations. The features of these existing typical industrial halls are known thanks to a measurement campaign achieved previously. Their dimensions are varying from 22 m to 75 m (length), from 7 m to 40 m (width) and from 3.7 m to 11.1 m (height). They include both flat or sloping roofs and have been described with a lot of architectural details (windows, doors, inhomogeneous materials, etc.). Each hall is simulated with its actual acoustical treatment and with four other acoustical treatments: two more absorbing and two more reverberating. This choice of the wall and ceiling materials allows the definition of five simulated halls from fully reverberating (mean Sabine coefficient 0.06) to near anechoic (mean Sabine coefficient 0.9). Each hall can be modelled either empty or full. A full hall meaning that the simulation is achieved with volume absorption and scattering from the floor up to a given height
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(usually 1.5 m) as if the hall contained machines, stocks, pieces of furniture, etc. This fitting is modelled for limiting the ray trajectory to a length equal to 2.5 m on average before absorption and scattering to a random direction. The directive or omnidirectional source is laid down at a hall corner (4 m–4 m from the walls). In the case of the simulation of the directive test-source in configuration ‘‘C’’, the beam-pattern can be pointed to four different directions: east, west, north, south, viewing the hall from the top as a geographical map. These configurations lead to define 400 cases of simulation: 10 halls, five cases of acoustical treatment, two cases of fitting (empty or full), four cases of directional source each compared to the omnidirectional case. 3.4. Simulation results For the 400 simulated cases, the results can be summarized as following. 3.4.1. Mean top value Mean difference between noise fields induced by directional source and by omnidirectional source: 4.2 dB(A) in empty halls, 2.5 dB(A) in fitted halls. 3.4.2. Mean dispersion Mean percentage of cells having difference values at ±1 dB(A) from the mean top value: 77% in empty halls, 92% in fitted halls. These summarized results and average values can be completed by more detailed observations: the mean values of the histograms or the dispersions are not linked to the hall dimensions, for a given acoustic treatment, the statistical differences from one building to another are similar to the differences from one acoustic treatment to another in the same building. These observations show that there is no noticeable coherent link between the acoustical treatment or the hall dimensions and the field differences for a given source orientation. The parameters implying field differences are reduced to: the source orientation, the fact that the hall is empty or not. The simulations do not show an average difference greater than 3 dB(A) when the hall is fitted. Previous studies showed that the accuracy of the software is ±3 dB(A) due to approximate modelling and approximate knowledge of the actual hall parameters (dimensions, Sabine coefficients, architecture details), the influence of the source directivity could be weak when the hall contains objects which cause sound waves scattering. On the contrary, in empty halls, the source directivity influences the noise distribution but this influence can not be linked to the building features by simple relationships.
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3.5. Hall experiments Several experiments have been undertaken in two different medium size halls. The first is empty with an average Sabine coefficient of 0.26. Its dimensions are 30 m · 7.7 m · 3.7 m. The second hall is a production workshop full of numerous wood-working machines, stocks, ventilation pipes, pieces of furniture, cupboards, etc. Its dimensions are 26 m · 11 m · 8.3 m. It also has an average Sabine coefficient of 0.2. In both halls, comparisons of measured maps are achieved between the fields created by an omnidirectional source and those created by a directional source, also between fields created by the directional sources rotated on a different orientation (see Fig. 2). The omnidirectional source is a mechanical source with a know spectrum (SPL 116 dB Lin) and directivity index in accordance with the ISO 14257 standard assuming it is omnidirectional. The test-source (used in configuration A, B, or C), the table-saw, the planning machine and the surface-planing machine have been described above. Sources have been moved, re-orientated or replaced by the omnidirectional source depending on the test requirements (see Table 1). Machines are always used without any load in order to get a stable noise during the measurement. In the empty hall, maps are composed of 6 · 28 microphone positions separated each by 1 m and distributed all over the hall. In the second hall, two long corridors allow the installation of the microphones on 6 columns of 18 rows. The rest of the room is full of scattering ‘‘clutter’’. Each microphone position is considered as a measurement cell, as in the case of the simulations. The experimental comparisons are also achieved by means of histograms of differences obtained cell by cell, after turning the spectra noise pink. Also the cells close to the source (<4 m) are excluded from the process. Table 1 summarizes the results. In the empty hall, the effect of source directivity can modify the noise distribution with an average difference reaching 3 dB(A). The histogram pattern is very narrow showing that nearly all the cell differences are contained within the interval ±1 dB(A) around the mean value. Even if the test-source is directive, comparisons between two orientations or with the omnidirectional source show that the noise distribution is not fully changed. The table-saw causes same differences with the omnidirectional source, even if its directivTable 1 Experimental results summary (Vm: mean value of the differences between the sound fields ; Pm: percentage of cells having difference values at ±1 dB(A) from the mean value Vm) Configuration of sources TS: test-source OS: omnidirectional source
Vm (dB(A))
Pm (%)
Empty hall TS (configuration ‘‘A’’) vs. TS (configuration ‘‘B’’) OS vs. TS (configuration ‘‘C’’) OS vs. table-saw oriented to south OS vs. table-saw oriented to west Table-saw oriented to south vs. table-saw oriented to west
2 3 3 3 0
97 96 100 98 99
Full hall OS vs. TS (configuration ‘‘C’’) oriented to south OS vs. TS (configuration ‘‘C’’) oriented to west Planing machine oriented to west vs. planing machine oriented to north OS vs. surface-planing machine
2 1 0 2.5
80 84 75 67
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ity indices are much lower. The measured field is nearly the same when the table-saw orientation is modified. This is due to its symmetrical directivity pattern looking like butterfly wings around the blade. In the fitted hall containing a lot of scattering ‘‘clutter’’, the differences are lower although the distribution is a little bit spread. Compared to the omnidirectional source, the test-source leads to differences of approximately 1 or 2 dB(A) and the surface-planing machine about 2.5 dB(A). As these values can be considered inside the measurement accuracy interval, they are not really significant. The planing machine causes no difference in the field in this hall when its orientation is modified, this is due to its low directivity indices as well as to scattering. Finally, the scattering by all the objects contained in the hall breaks the directivity effect of the sources. 4. Discussion The directivity of a test-source and of three wood-working machines have been characterized by two methods. The method using acoustical intensity scanning has been validated et allows a fast estimation of the directivity in an industrial hall. In the studied frequency band, which corresponds to the domain of hearing losses, directional indices of the machines have been quantified by an ISO standard or an equivalent measurement. The wood-working machines have low directivity indices but this observation is inadequate to claim that the noise field induced in industrial halls is equivalent to those induced by an omnidirectional source. In order to clarify this question, noise fields have been simulated in the case of a directional test-source, inside several industrial halls. The results show that the noise field can be influenced by the directivity of the source in an empty hall. This influence is not strictly linked to the hall features. As soon as the hall is not empty and contains objects (or ‘‘clutter’’) participating to wave absorption and mainly scattering, the noise field is not very different with the two kinds of sources (directional or omnidirectional). Measurements achieved in two different halls confirm these observations. The noise field can be influenced by the source directivity if the hall is empty but the source directivity effect is broken by the scattering due to the fittings of a production hall. In a fitted production hall, the noise exposure of the workers far from the sources is quasi-independent from the directivity of sources such as the wood-working machines studied in this paper. Nevertheless, intensity maps of the same machines showed that the noise exposure at the workplace, close to the machine, can vary from 4 to 8 dB(A) according to the position of the worker. The worker position in the near-field remains a crucial factor of the noise exposure. References [1] ISO 3745:2003 Ed. 2. Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for anechoic and hemi-anechoic rooms. [2] GADE S. Sound intensity. theory, instrumentation and applications. Tech Rev 1982;3:3–39. [3] ISO 9614-1:1993 Ed. 1. Acoustics – Determination of sound power levels of noise sources using sound intensity – Part 1: Measurement at discrete points. [4] ISO 9614-2:1996 Ed. 1. Acoustics – Determination of sound power levels of noise sources using sound intensity – Part 2: Measurement by scanning.
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[5] ISO 14257:2001. Acoustics – Measurement and parametric description of spatial sound distribution curves in workrooms for evaluation of their acoustical performance. [6] Ondet AM, Barbry JL. Modelling of sound propagation in fitted workshops using ray tracing. J Acoust Soc Am 1989;85:787.
Influence of source directivity on noise levels in industrial halls: Simulation and experiments Jacques Chatillon
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Institut National de Recherche et de Se´curite´, Avenue de Bourgogne, BP 27, 54501 Vandoeuvre cedex, France Received 3 October 2005; received in revised form 5 July 2006; accepted 7 July 2006 Available online 27 September 2006
Abstract Noise exposure of workers in industrial halls is mainly induced by noisy machines whose acoustical features are often globally known by Sound Pressure Level. The evaluation of the directivity of these noise sources can help to anticipate specific solutions for noise reduction. This study shows how the directivities of three wood-working machines have been characterized. Some characterisations have been achieved with a simple and fast acoustical intensity mapping which meets the constraints of industrial areas. When source directivity is evaluated, its influence on the noise field in industrial halls can be assessed. Some simulations and some experiments allowed the estimation of the noise field induced in workshops by both directional and omnidirectional sources. Comparison of the fields prove that the noise distribution is influenced by the source directivity if the halls are empty. As soon as the halls contain scattering objects, the directivity effect is reduced a lot and the noise field remains nearly the same far from the source whatever source used. Nevertheless, workers close to a machine are exposed to noise according to their position with respect to the machine. Exposure at the workplace can vary from 4 to 8 dB(A) according to the directivity of machines such as those measured in the trials. 2006 Elsevier Ltd. All rights reserved. Keywords: Occupational noise; Source directivity; Noisy machines; Industrial hall
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Tel.: +33 3 83 50 98 70; fax: + 33 3 83 50 21 96. E-mail address: [email protected].
0003-682X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2006.07.010
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1. Introduction Noisy machines are the main cause of worker noise exposure in industrial halls. In order to reduce this noise pollution, actions to lower noise emission of the source and actions to improve acoustical treatments of the halls are both usable. The acoustical characteristics of industrial sources are often known globally and are often reduced to a single representative: the Sound Pressure Level (SPL). Knowing the spatial features of noise emission by measuring the directivity of the machines could help to find new solutions to reduce the noise level in industrial halls. For instance, these solutions could be a partial screening of the noisy sources, a change of their position or orientation, or a partial acoustical treatment of a close hall wall. This study aims to characterize the directivity of several noisy machines and to evaluate the influence of this feature on noise exposure all over an industrial hall. Three noisy wood-working machines and a test-source are studied. A laboratory method is firstly used by means of numerous measurements of pressure points around the source in an anechoic room. In the industry world, constraints of measurements are linked to work organization and to the building features. The hall is generally reverberant and it often involves a lot of noise sources. Work organization implies that the measurement area is reduced and that the time required for measurements must be shortened. In order to meet these constraints, a faster and simplified method for directivity characterization has been implemented by using acoustical intensity mapping. Source directivity being evaluated, the noise level induced by directive sources in industrial halls is then assessed thanks to simulations and experiments. 2. Characterizations of source directivity 2.1. Methods 2.1.1. Simple vs. complete characterization Characterization of source directivity can be evaluated with the required accuracy by laboratory or standard methods using pressure measurements [1] or intensity measurements [2–4]. For instance, in the laboratory, a complete characterization can be achieved by measuring sound pressure on numerous microphones on a hemisphere around the source. This technique is accurate if there is a large collection of measured points, but requires time and space to fan the microphone system out. There is also a need for source stability over time. The directivities of both a test-source and a table-saw have been measured by this method in a hemi-anechoic chamber. However, in a real industrial hall, some constraints can prevent from producing laboratory standard measurements. These constraints include the presence of secondary sources and the reflection on the hall walls which can cause a high level of background noise capable of screening the source of interest. Furthermore, constraints of production can sometimes restrict the measurement area around the source or time for measurements. For these reasons, a simpler characterization of the source directivity has to be devised. By mapping acoustic intensity with a sweeping scan on a parallelepiped around the source, time of measurement can be shortened and secondary source contributions can be eliminated as far as they are time stable.
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This mapping was used in the study after validation by comparison with point-to-point intensity measurements in a hemi-anechoic chamber. This simple characterization, using intensity sweeping scan and reconstructed virtual points, was validated on the same test-source, and used to predict the effect of two wood-working machines put in a real production hall. 2.1.2. Test-source and wood-working machines A test-source was selected for the study because its directivity can be theoretically calculated. Moreover it is quite directive in the hearing frequency band for showing noise field differences if they exist. This source has a simple directivity pattern (cardinal sine) with a main lobe narrow enough (50 at 1 kHz). This typical test-source is easy to compare – for its directivity and its influence – to the noisy machines. On another hand, simulations of sound field in an industrial hall use simulated sources whose directivity is similar to testsource. The test-source is composed of a rectangular box with four loud-speakers (Fig. 1) emitting a pink noise from 125 Hz to 8 kHz. The theoretical directivity pattern of this source is those of one loud-speaker convoluted with those of a line. It is approximately composed of a main lobe with a 3 dB width equal to about 50 at 1000 Hz in the Px plane (Fig. 1) with secondary lobes decreasing in level as a cardinal sine function. By construction, this source is four times less directive in the Py plane (Fig. 1). Furthermore, the noise level transmitted to the back-side is reduced a lot by the box acting as a baffle. Three configurations of this source were used (Fig. 2): (A) source at 1 m from the ground, emission to a wall, (B) source at 1 m from the ground, emission to the ceiling, (C) source on the ground, emission to a wall.
Fig. 1. Test-source.
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Fig. 2. Configurations of test-source.
The two first configurations A and B induce an image source at 2 m from the real one in case of hard ground. The set-up number C allows the real source and the image source to be identified for a frequency band up to about 1 kHz. Three wood-working machines have been used, without any load in order to obtain a very stable emission during the times of measurements. The table-saw used in the study is shown inside the hemi-anechoic chamber in Fig. 3. This machine is approximately a cube 1.2 m · 1.2 m · 1.2 m, with a blade of 30 cm diameter. A loading system with a small motor on the top is not used. The blade and the motor are the main sources of noise. The two other wood-working machines are a planing machine and a surface-planing machine. The planing machine dimensions are about 1 m · 1 m · 1.2 m, it includes an aspiration system on the top which was turned off during the experiment. The loading is achieved through a way crossing the machine, that means these are two holes at two opposite sides of the machine, and the tool inside at the middle (Fig. 4 left). The main noise contribution is emitted by these holes. The surface-planing machine looks like the table-saw (Fig. 4 right) except from its length (2.5 m) and the tool which is a planer, the noise mainly comes from the tool (on the table top) and the motor inside. 2.1.3. Experimental set-up of the two characterization methods The complete characterizations of the test-source in three configurations and of the table-saw were achieved in a hemi-anechoic chamber with concrete floor. Sound pressure levels were measured at points on a hemisphere of 2 m radius around the sources. Sixteen microphones were used for a vertical scan from 0 to 90 and the system was rotated around the vertical axis in steps of 5 (Fig. 5). This configuration defines 16 · 72 = 1152 positions of pressure measurements to obtain the directivity of the sources with enough accuracy up to the maximum frequency band (8 kHz). A simple characterization was also achieved by acoustic intensity mapping on a parallelepiped around the source (Fig. 5). The intensity probe scan of a plane is split in time into
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Fig. 3. Table-saw in anechoic chamber.
several parts each corresponding to a virtual point of measurement similar to those obtained by point-to-point intensimetry. In order to validate the scanning and virtual point method, a classical point-to-point intensimetry measurement was firstly done with the test-source. 2.2. Source directivity evaluation by means of complete characterization The complete characterization of the test-source shows the expected directivity patterns: a main lobe, narrowing with frequency increase, with side lobes appearing at highest frequencies and a screening of the noise at the rear. Fig. 6 shows an example of directivity patterns obtained for configuration ‘‘A’’ for the octaves centred at 1 kHz and 8 kHz. These patterns are displayed as a top view of the hemisphere around the source emitting at the right side, the azimuth angle varying from 0 to 360 all around the circle, and the elevation varying from 0 to 90 as a radius of the circle. The table-saw displayed in Fig. 3 was characterized with the same method while the blade was rotating without any load, in order to record a very stable noise during the time required to operate the microphone system (less than 1 h).
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Fig. 4. Planing machine (top) and surface-planing machine (bottom).
Fig. 7 shows an example of two directivity patterns obtained with the table-saw for the octaves centred at 1 kHz and 8 kHz. These patterns are displayed with the same colour levels as those in Fig. 6 in order to compare the directivity of the table-saw with those of the test-source. Top view of the table and the blade is superimposed on the 1 kHz pattern. One can note that the table-saw looks less directive than the test-source at 1 kHz, where the levels are nearly only gathered in red colour. A typical pattern is visible at 8 kHz looking as butterfly wings which show more energetic emissions at +45 or 45 from the blade plane. These complete characterizations allow the estimation of the directivity indices of the sources in accordance with the ISO 14257 standard [5]. This standard proposes to calculate, by third octaves, the directivity indices of a source by comparing the mean quadratic pressure level of the source found on a circle around the source with the maximum and the minimum quadratic pressure levels found on smaller angular sectors on the same circle. The circles can be scanned in the two angular directions (azimuth and elevation). The standard defines a template according to the frequency (2 dB up to 630 Hz, 2–8 dB up to 1 kHz, 8 dB above 1 kHz) defining the maximum indices of a so-called ‘‘omnidirectional’’ source.
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Fig. 5. Complete and simple characterization.
Fig. 6. Test-source configuration (A). Examples of directivity patterns (left 1 kHz; right 8 kHz) measured on the hemisphere around the source.
Plotting the test-source and the table-saw indices together with the template indicates how the sources are rather directive or omnidirectional. Fig. 8 displays the directivity indices of the test-source in configuration A compared to the directivity indices of the tablesaw. Each plot contains the template of ISO 14257. For the test-source in configuration ‘‘A’’ and ‘‘C’’, the directivity indices are reaching 15–25 dB in azimuth for frequencies above 1 kHz. The test-source is rather directive in azimuth for this configuration, while it should not be as directive on elevation if there was not the screening effect of the box back-side. For the configuration ‘‘B’’, the directivity is higher in site than in azimuth, as the geometry of the source can explain. For the table-saw, the plots related to ISO 14257 show that this machine is quasi-omnidirectional even if indices are outside the template for thirds of octave between 200 and 800 Hz.
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Fig. 7. Table-saw. Examples of directivity patterns (left 1 kHz; right 8 kHz) measured on the hemisphere around the source.
Fig. 8. Directional indices of the test-source in configuration ‘‘A’’ (black circles TS) and of the table-saw (white diamonds Saw) together with the ‘‘omnidirectional’’ template given by ISO 14257 (solid line).
Finally there is a large difference in directivity between the two sources, while the testsource is rather directive, the table-saw is not. This observation may have large consequence on the noise level radiated in an industrial hall with these sources. 2.3. Source directivity evaluation by means of simplified characterization The simple characterizations have been achieved on the test-source in the hemi-anechoic chamber as well as on two wood-working machines already described and put in a real production hall. The machines were unloaded in order to record a very stable noise during the intensity probe scan (10–20 min for all the planes). The acoustic intensity mapping of the test-source in configuration ‘‘A’’ has been achieved by a sweeping scan of the five planes of a virtual box closing the source. Each plane is scanned in 2 min and the total time record of the intensity probe is then split into 81 parts each corresponding to a virtual mean fixed point. Fig. 9 shows how the mapping of the five planes around the source are displayed.
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Fig. 9. Display of intensity mapping.
Fig. 10 displays two examples of intensity map for the test-source in configuration ‘‘A’’ for the octaves centred at 1 kHz and 8 kHz. One can notice the frequency effect of the azimuth directivity of the source already seen with the complete characterization. The acoustic intensity mapping of the two wood-working machines has been achieved in the same way. The planes around the sources are different from those of the test-source because the machine dimensions are not the same. The planing machine is scanned by 80 virtual points by vertical plane (1.5 m by 2 m) and 100 for the top side (2 m by 2 m). The surface-planing machine is scanned by about 70 or 140 points on vertical planes (2.5 m or 5 m by 1.2 m) and by about 250 points on the top side (2.5 m by 5 m). Fig. 11 shows two examples of intensity maps for the two machines for the octaves centred at 1 kHz and 8 kHz. One can see that the planing machine looks to have little
Fig. 10. Test-source configuration ‘‘A’’. Examples of intensity mapping (1 kHz; 8 kHz). Rectangular scales in metres.
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Fig. 11. Planning machine (top) and surface-planing machine (bottom). Examples of intensity mapping (left 1 kHz; right 8 kHz). Rectangular scales in metres.
directivity as the levels are nearly equally shared on the map. However, the loading-holes (left and right planes on the map) show more energetic emissions at low frequency than the rest of the machine. This observation is less clear at higher frequency because the spectrum of this machine is mostly below 2 kHz. For the surface-planing machine, the tool at the top of the table is clearly the noise source, and the table acts as a reflecting plane to send the most energetic emissions to the ceiling. The surface-planing machine looks to be more directive than the planing machine. Processed in dB(A), the same intensity maps show differences between the mapping of the planes between 3 and 8 dB(A). These simple characterizations do not allow the estimation of the directivity indices of the sources in accordance with the ISO 14257 standard. In order to compare the directivity of the three sources, specific directivity indices have been set up by calculating mean quadratic pressure levels on lines scanning the map, and maximum and minimum levels compared to this average value. The scan of the map is achieved in two directions: horizontal then vertical (Fig. 12). For a given scan line, the values are smoothed with three adjacent points of the line, and the difference between the maximum and the minimum with the average value, for each third of octave, define the directivity indices. This processing of the indices looks like those
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Fig. 12. Scans of the intensity map to define new directivity indices.
Fig. 13. Horizontal and vertical directivity indices calculated on the intensity maps for the test-source (black circles TS), the planing machine (white diamonds, PM), and the surface-planing machine (white squares, SPM).
of the ISO 14257 standard, but final values are not comparable, since a circular or square scanning motion is used according to the calculation methodology. Fig. 13 shows the indices calculated with this last method. The indices of the test-source are increasing with frequency, similar to those indices calculated following the ISO standard. However the horizontal scan over squares induces higher indices than expected at low frequency. The analysis of the indices values shows that the planning machine is always much less directive than the test-source. However the surface-planing machine is more directive than the test-source for lower frequencies using the vertical scan. These remarks are in agreement with the first observations of the intensity maps displayed in Fig. 11. 3. Influence of the directivity on noise radiated in halls 3.1. Comparison criterion When the source directivity is estimated, its influence on the noise distributed in the halls can be evaluated by means of simulations or experiments. In both cases this influence
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is processed by comparing either the noise field caused by an omnidirectional source with those radiated by a directional source, or the noise field caused by the same directional source whose orientation has changed. In an industrial hall, the acoustic field can be split in two different areas: the direct field and the reverberant field. The direct field, close to the source, involves mainly the machine operator exposure. In this case, the levels of exposure are directly connected to the noisy machine sound pressure level and the acoustic features of the hall are of secondary importance. Then the machine directivity, estimated by methods such as those described above, gives immediately an assessment of the worker exposure according to his position. The measurements achieved by the methods of characterizations described in Section 2 allow to estimate the differences in exposure due to the machine directivity. The noise exposure can vary up to 4 dB(A) for a worker close to the table-saw, up to 5 dB(A) for a worker close to the planing machine and up to 8 dB(A) in the case of the surface-planing machine. The reverberant field, far from the source, involves mainly the other workers in the rest of the hall. The levels are still linked to the noisy machine spectrum and directivity but also to the propagation in the hall whose acoustic characteristics becomes very important. As we are rather interested in the reverberant field aspects, only the propagation in the hall has to be estimated in this section. For this reason the comparisons of the radiated fields will exclude the noise levels close to the machine. This can be achieved, considering the position of the operator of the machine and the assumed positions of the other workers in the hall, by leaving out the sound levels contained in a circle of radius 4 m around the source. Then the comparisons between the sound field induced by the directive source and the field induced by an omnidirectional source (or the directive source with another orientation) have to satisfy several constraints to take into account mainly the reverberant field, to give results independent from the noise source spectra, to reach the standard noise exposure assessment. These constraints can be satisfied by the following actions: to remove the near sound levels from the calculation, to remove noise source spectrum from the sound levels, to measure sound field at operator level (1.6 m) and to give results in dB(A). The comparison criterion can then be defined. Noise mapping is naturally the best mean to compare noise fields in an industrial hall. However, maps contain too much information to be used directly. We propose to simplify the analysis by using noise map differences accumulated to provide a histogram. The mean value and the dispersion of each histogram will be simple quantifiers of the difference of the sound fields induced by sources which have to be compared. Fig. 14 explains the principle of the sound field comparison: after source spectrum correction and (A) weighting, two noise maps are subtracted to obtained the difference map. The cells near the source (<4 m) are left out the calculation. Cells of same difference level are accumulated to build a difference histogram. Top value and dispersion of this histogram can quantify the influence of the source directivity over the whole workshop.
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Fig. 14. Noise field comparison criterion: example of difference map histogram (simulated data). Number of cells is finally processed in percentage.
3.2. Room acoustical software A room acoustical software package was used to calculate the noise distributed in the industrial halls based upon the case of a directive source. This software is a ray-tracing model simulating the building as a collection of planes with given Sabine coefficients [6]. The rays coming from the source are reflected between these planes loosing energy with each reflection. When a ray is crossing a reception cell, its energy (no phasing) is accumulated in the cell to define the final level of this element of the noise map. The software was originally working with one or several omnidirectional sources and has been modified to accept a directional noise source under single source conditions. This restriction allows the use of only the configuration ‘‘C’’ (source level 0 m) for the test-source as an input parameter of the simulations because it is assumed that real source and image source are identified for this configuration. The other test-source configurations ‘‘A’’ or ‘‘B’’ (source level 1 m) induce an image source too far from the real source. The wood-working machines showed also either image or multiples sources. With this software version, these last noise sources cannot be simulated but they will be widely used during the experiments. 3.3. Simulated halls A database of 10 halls was used for simulations. The features of these existing typical industrial halls are known thanks to a measurement campaign achieved previously. Their dimensions are varying from 22 m to 75 m (length), from 7 m to 40 m (width) and from 3.7 m to 11.1 m (height). They include both flat or sloping roofs and have been described with a lot of architectural details (windows, doors, inhomogeneous materials, etc.). Each hall is simulated with its actual acoustical treatment and with four other acoustical treatments: two more absorbing and two more reverberating. This choice of the wall and ceiling materials allows the definition of five simulated halls from fully reverberating (mean Sabine coefficient 0.06) to near anechoic (mean Sabine coefficient 0.9). Each hall can be modelled either empty or full. A full hall meaning that the simulation is achieved with volume absorption and scattering from the floor up to a given height
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(usually 1.5 m) as if the hall contained machines, stocks, pieces of furniture, etc. This fitting is modelled for limiting the ray trajectory to a length equal to 2.5 m on average before absorption and scattering to a random direction. The directive or omnidirectional source is laid down at a hall corner (4 m–4 m from the walls). In the case of the simulation of the directive test-source in configuration ‘‘C’’, the beam-pattern can be pointed to four different directions: east, west, north, south, viewing the hall from the top as a geographical map. These configurations lead to define 400 cases of simulation: 10 halls, five cases of acoustical treatment, two cases of fitting (empty or full), four cases of directional source each compared to the omnidirectional case. 3.4. Simulation results For the 400 simulated cases, the results can be summarized as following. 3.4.1. Mean top value Mean difference between noise fields induced by directional source and by omnidirectional source: 4.2 dB(A) in empty halls, 2.5 dB(A) in fitted halls. 3.4.2. Mean dispersion Mean percentage of cells having difference values at ±1 dB(A) from the mean top value: 77% in empty halls, 92% in fitted halls. These summarized results and average values can be completed by more detailed observations: the mean values of the histograms or the dispersions are not linked to the hall dimensions, for a given acoustic treatment, the statistical differences from one building to another are similar to the differences from one acoustic treatment to another in the same building. These observations show that there is no noticeable coherent link between the acoustical treatment or the hall dimensions and the field differences for a given source orientation. The parameters implying field differences are reduced to: the source orientation, the fact that the hall is empty or not. The simulations do not show an average difference greater than 3 dB(A) when the hall is fitted. Previous studies showed that the accuracy of the software is ±3 dB(A) due to approximate modelling and approximate knowledge of the actual hall parameters (dimensions, Sabine coefficients, architecture details), the influence of the source directivity could be weak when the hall contains objects which cause sound waves scattering. On the contrary, in empty halls, the source directivity influences the noise distribution but this influence can not be linked to the building features by simple relationships.
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3.5. Hall experiments Several experiments have been undertaken in two different medium size halls. The first is empty with an average Sabine coefficient of 0.26. Its dimensions are 30 m · 7.7 m · 3.7 m. The second hall is a production workshop full of numerous wood-working machines, stocks, ventilation pipes, pieces of furniture, cupboards, etc. Its dimensions are 26 m · 11 m · 8.3 m. It also has an average Sabine coefficient of 0.2. In both halls, comparisons of measured maps are achieved between the fields created by an omnidirectional source and those created by a directional source, also between fields created by the directional sources rotated on a different orientation (see Fig. 2). The omnidirectional source is a mechanical source with a know spectrum (SPL 116 dB Lin) and directivity index in accordance with the ISO 14257 standard assuming it is omnidirectional. The test-source (used in configuration A, B, or C), the table-saw, the planning machine and the surface-planing machine have been described above. Sources have been moved, re-orientated or replaced by the omnidirectional source depending on the test requirements (see Table 1). Machines are always used without any load in order to get a stable noise during the measurement. In the empty hall, maps are composed of 6 · 28 microphone positions separated each by 1 m and distributed all over the hall. In the second hall, two long corridors allow the installation of the microphones on 6 columns of 18 rows. The rest of the room is full of scattering ‘‘clutter’’. Each microphone position is considered as a measurement cell, as in the case of the simulations. The experimental comparisons are also achieved by means of histograms of differences obtained cell by cell, after turning the spectra noise pink. Also the cells close to the source (<4 m) are excluded from the process. Table 1 summarizes the results. In the empty hall, the effect of source directivity can modify the noise distribution with an average difference reaching 3 dB(A). The histogram pattern is very narrow showing that nearly all the cell differences are contained within the interval ±1 dB(A) around the mean value. Even if the test-source is directive, comparisons between two orientations or with the omnidirectional source show that the noise distribution is not fully changed. The table-saw causes same differences with the omnidirectional source, even if its directivTable 1 Experimental results summary (Vm: mean value of the differences between the sound fields ; Pm: percentage of cells having difference values at ±1 dB(A) from the mean value Vm) Configuration of sources TS: test-source OS: omnidirectional source
Vm (dB(A))
Pm (%)
Empty hall TS (configuration ‘‘A’’) vs. TS (configuration ‘‘B’’) OS vs. TS (configuration ‘‘C’’) OS vs. table-saw oriented to south OS vs. table-saw oriented to west Table-saw oriented to south vs. table-saw oriented to west
2 3 3 3 0
97 96 100 98 99
Full hall OS vs. TS (configuration ‘‘C’’) oriented to south OS vs. TS (configuration ‘‘C’’) oriented to west Planing machine oriented to west vs. planing machine oriented to north OS vs. surface-planing machine
2 1 0 2.5
80 84 75 67
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ity indices are much lower. The measured field is nearly the same when the table-saw orientation is modified. This is due to its symmetrical directivity pattern looking like butterfly wings around the blade. In the fitted hall containing a lot of scattering ‘‘clutter’’, the differences are lower although the distribution is a little bit spread. Compared to the omnidirectional source, the test-source leads to differences of approximately 1 or 2 dB(A) and the surface-planing machine about 2.5 dB(A). As these values can be considered inside the measurement accuracy interval, they are not really significant. The planing machine causes no difference in the field in this hall when its orientation is modified, this is due to its low directivity indices as well as to scattering. Finally, the scattering by all the objects contained in the hall breaks the directivity effect of the sources. 4. Discussion The directivity of a test-source and of three wood-working machines have been characterized by two methods. The method using acoustical intensity scanning has been validated et allows a fast estimation of the directivity in an industrial hall. In the studied frequency band, which corresponds to the domain of hearing losses, directional indices of the machines have been quantified by an ISO standard or an equivalent measurement. The wood-working machines have low directivity indices but this observation is inadequate to claim that the noise field induced in industrial halls is equivalent to those induced by an omnidirectional source. In order to clarify this question, noise fields have been simulated in the case of a directional test-source, inside several industrial halls. The results show that the noise field can be influenced by the directivity of the source in an empty hall. This influence is not strictly linked to the hall features. As soon as the hall is not empty and contains objects (or ‘‘clutter’’) participating to wave absorption and mainly scattering, the noise field is not very different with the two kinds of sources (directional or omnidirectional). Measurements achieved in two different halls confirm these observations. The noise field can be influenced by the source directivity if the hall is empty but the source directivity effect is broken by the scattering due to the fittings of a production hall. In a fitted production hall, the noise exposure of the workers far from the sources is quasi-independent from the directivity of sources such as the wood-working machines studied in this paper. Nevertheless, intensity maps of the same machines showed that the noise exposure at the workplace, close to the machine, can vary from 4 to 8 dB(A) according to the position of the worker. The worker position in the near-field remains a crucial factor of the noise exposure. References [1] ISO 3745:2003 Ed. 2. Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for anechoic and hemi-anechoic rooms. [2] GADE S. Sound intensity. theory, instrumentation and applications. Tech Rev 1982;3:3–39. [3] ISO 9614-1:1993 Ed. 1. Acoustics – Determination of sound power levels of noise sources using sound intensity – Part 1: Measurement at discrete points. [4] ISO 9614-2:1996 Ed. 1. Acoustics – Determination of sound power levels of noise sources using sound intensity – Part 2: Measurement by scanning.
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[5] ISO 14257:2001. Acoustics – Measurement and parametric description of spatial sound distribution curves in workrooms for evaluation of their acoustical performance. [6] Ondet AM, Barbry JL. Modelling of sound propagation in fitted workshops using ray tracing. J Acoust Soc Am 1989;85:787.