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Recommended reverberation times for workshops
Nicolas Trompette, Laurent Hardy
To cite this version:
Recommended reverberation times for workshops
Nicolas Trompette
1Laurent Hardy
21INRS, Rue du Morvan, CS 60027, 54519 VANDOEUVRE, France 2CARSAT Midi-Pyrénées, 2, rue Georges Vivent, 31065 TOULOUSE France
nicolas.trompette@inrs.fr
1. ABSTRACT
In architectural acoustics, the reverberation time (RT) is widely used to characterize the acoustical behavior of a room. However, in the particular case of production rooms, workshops or any other industrial halls, RT recommendations or clouds of values cannot be easily found in the literature. The overall objective of the work presented here is to provide recommendations in terms of reverberation time for workshops. In France, the regulation for noisy workshops requires that the rate of decay of sound pressure per distance doubling (DL2) must exceed a certain limit value. In practice, the DL2 values are traditionally measured in workshops together with the RT values; hence, a database was created compiling both values. This database has been used to draw up reference limits for RT values with regards to compliance to the regulation. These reference limits have been further validated with respect to the literature and to a new set of data purposely collected for this comparison. Eight limits are proposed, which are divided in three categories: compliant/non-compliant, for furnished/empty workshops, with a 95/99% confidence interval. These limits are proposed to serve as target values for workshops, either at the design stage or for control purposes, to assess the quality of a workshop acoustic treatment with a simple and affordable measurement protocol.
2. INTRODUCTION
Reverberation Time (RT) is the time it takes for the sound level in a room to decrease by 60 dB after all sound sources have been turned off. It qualifies the reverberation of a room. It is often used for recommendations or by regulations. Limits or recommended values for the RT depend upon the volume and the intended use of the room. However, these values are difficult to settle because of the large variety in terms of size and shape of real rooms. That is why in France, for the particular case of production rooms, workshops or any other industrial halls hereinafter referred as "workshops", this is the rate of spatial decay of sound level per distance doubling (DL2) that was regulated rather than the RT. The regulation [1] specifies target limit values for the DL2indicator as a function of the surface of the industrial hall – see table I. As shown in this table, DL2 is less dependent on the surface than the RT. However, in practice, DL2 is much more difficult to measure than the RT and requires higher-performing equipment; it is also not even measurable in small rooms. In addition, the RT is the usual metric used by stakeholders of the building
industry. It has therefore appeared necessary to issue the RT recommendations for workshops.
Empty workshop Fully-equipped workshop Floor area S Minimum
DL2 Floor area S Minimum DL2 S ≤ 210 DL2≥ 2 S ≤ 210 DL2≥ 3 210< S ≤ 4600 DL2≥ 1,5logS-1,5 210< S ≤ 1000 DL2≥ 1,5logS-0,5 S > 4600 DL2≥ 4 S > 1000 DL2≥ 4
Table I: DL2compulsory limits in France as a function of
the workshop clutter and of its floor area
recommendations in terms of reverberation time for workshops. These recommendations are intended for the various actors in the field, particularly architects and contractors.
3. METHOD AND RESULTS
The method applied, presented below, is fairly simple. This work was initiated by the opportunity to exploit a database containing both TRs and DL2from industrial halls. A Brittany lab whose role is to control the compliance of industries to the regulation constituted this database. In particular, this laboratory measures the DL2for comparison with the limits specified in Table I. While collecting this data, this lab had also measured the RT and recorded various information including the surface area and volume of the room. This is a rather large database (i.e. containing 1001 different cases). On the other hand, the experimental protocol was not robust and the entire data collection lasted several years. As a result, the database can be considered exhaustive, but a wide dispersion could be expected.
The compulsory limits for the rate of decay per distance doubling (DL2) depends on the workshop state (i.e. empty or fully-equipped). The database allows to establish whether the workshop is compliant or not (relative to its surface area and the DL2). Therefore, data were divided into four groups: non-compliant furnished workshops (366 cases), compliant fully-equipped workshops (462), non-compliant empty workshops (51) and non-compliant empty workshops (122).
The first approach used in order to issue RT recommendation consisted in using regression formula to calculate the RT from the DL2, although these two metrics show a disappointing correlation (only 0.5924). Several attempts were performed to try to establish a mathematical relationship between the RT and the DL2together with the dimensions of the workshop. Unfortunately, although the correlation could be improved up to 0.84 with a regression linking the log of the RT to the DL2, the workplace three main dimensions (height, width, length) and the floor area, the mathematical model still gives a bad classification (“compliant” versus “not compliant”) for 16% of the workshops, a number that was not considered satisfactory.
Therefore, a more straightforward approach was tried. To issue these conformity limits for the reverberation time, second order (for empty workshops) or third order (for fully-equipped workshops) regression curves RT=f(V) – where RT is the reverberation time and V the volume of the workshop – were calculated for each of the four groups. Then a constant shift was imposed to the regression curve to shift it to the limit of the other group. This very simple process is illustrated figures 1 and 2: a regression curve is calculated for non-compliant fully-equipped workshops (figure 1). It is then shifted to delimit compliant fully-equipped workshops (figure 2). This allows defining four limits: two for fully-equipped workshops and two for empty workshops. In other words, a limit below which workshops are 99% compliant and a limit above which workshops are 99% non-compliant.
Figure 1. RT versus volume regression curve for
non-compliant fully-equipped workshops.
Figure 2. Shift of the regression to issue the 99%
non-compliance limit.
One drawback of this approach is that there are two limits, one for compliance and one for non-compliance. Thus, RT values between these limits are not classified. As it can be seen on figure 1, 1% of the data (six sets) are over the limits – i.e. the limit is therefore refer to as the 99% limit. This allowed shortening the gap between the compliance and the non-compliance limits. For the same reason, additional limits covering only 95% of the data are also defined. These 95% limits are a weighted linear combination of the 99% conformity and non-conformity limits. Weights were adjusted to achieve a coverage of 95% of the data.
4. VALIDATION 4.1 Comparison with literature
The four limits have been compared to RT clouds given in references [3], [4], [7], [8] and [9].
The limits are consistent with Fry [3] RT clouds of values (figure 3): the lower compliance limit is even identical to the regression for broadcasting studios. The upper compliance limit fits with that for conference rooms. The lower non-compliance limit also fits with that for concert halls. Finally, the upper non-compliance limit corresponds to that for churches.
Compared to the curves proposed by Beranek [4] (figure 4), there is a greater inflection of the slopes of the proposed limits. In Beranek’s, all curves are parallel and linear on a log-log scale, whereas the proposed limits are linear on a log-log scale only above 500 m3. Last, Beranek
Figure 3: Comparison of the RT limits with Fry’s clouds
Figure 4: Comparison of the RT limits with Beranek’s
classification
The comparison with various standards and with Everest recommendations is also conclusive for fully-equipped workshop limits, see figure 5. Curves are linear on a lin-log scale, either for Everest or in the standards, but apart from that, they are consistent with the limits’ slopes. Lower conformity limits for small volumes coincide with that of the standard for offices. The recommendations for low-noise workshops are in line with the lower non-conformity limit. Lastly, the regressions proposed by Everest remain consistent with the proposed limits. Indeed, the lower non-compliance limit fits with that of Everest for music presentation and the compliance upper limit is consistent with Everest limit for speech
Figure 5: Comparison of the RT limits for equipped
workshops with standards’ recommendations
4.2 Comparison with validation data
Limits were then validated throughout comparisons with new data. These validation data were collected according to a specific protocol: sound source and microphones were supposed to be omnidirectional. For the RT measure, a minimum of six measurements, at least three points and for at least three source positions, were requested. Microphones shall be positioned at least 1.5 m away and at least 2 m away from the sound source. The sound source shall be located at least 1 m from any obstacle and 2 m away from walls. Measurements shall also be distributed throughout the workspace. The DL2 was measured at least along the two horizontal dimensions of the workshop. The sound source had to be located at least 4 m from the nearest wall.
Through the use of this methodology, the data can be considered to be more accurate than the data used to establish the limits as seen before, although there is much less of it. The validation database includes 62 fully equipped workshops and 22 empty workshops. The results are shown in Figures 6 and figure 7.
Figure 6: Comparison of the RT limits for empty
workshops with new data collection
non-considered (fully equipped or empty). Finally, the compliant and non-compliant reverberation times are sometimes very close, which confirms, if need be, that the reverberation time does not always allow a workshop to be classified correctly - at least as far as French regulation is concerned. In summary, the control data confirm the relevance of the proposed limits, both in terms of their levels and the gap between compliance and non-compliance.
Figure 5: Comparison of the RT limits for equipped
workshops with new data collection
5. CONCLUSION
There are no recommendations in the literature regarding workshop RT. Moreover, there are obligations in French regulations regarding decay of noise which are difficult to understand by the different stakeholders, building consultants or architects. A work was therefore undertaken based on a large database of RTs and DL2s in order to provide recommendations in terms of reverberation times for workshops that would be consistent with the regulations and the literature. This work resulted in proposals for compliance and non-compliance limits for fully equipped/empty workshops, and for two confidence intervals, 95 % and 99 %, that are available in [10]. Whether at the design stage (empty workshop) or for control purposes (full workshop), they will allow a simple qualification of a workshop. However, they are less accurate than a measure of the noise decay. Thus, some workshops will not be classifiable because they will fall between the limits. In this case, it will be necessary to come back to a more complex measurement, the rate of decay of sound pressure per distance doubling, to qualify the workshop.
6. REFERENCES
[1] Arrêté du 30 août 1990 pris pour l'application de l'article R. 235-11 du code du travail et relatif à la correction acoustique des locaux de travail
[2] W.C. Sabine, Collected Papers on Acoustics, ed. Harvard U.P., 1922
[3] A. Fry, Noise control in building services, Pergamon, 1988
[4] L.L. Beranek, Acoustics, American Institute of Physics, 1993
[5] T.D. Rossing, Handbook of Acoustics, Springer Science, 2007
[6] N. Heerema, M.R. Hodgson, Empirical models for predicting noise levels, reverberation times and fitting densities in industrial workrooms, Applied Acoustics, 57(1),51-60, 1999
[7] ISO 11690-1, Acoustics — Recommended practice for the design of low-noise workplaces containing machinery — Part 1: Noise control strategies, 1996
[8] ISO 9241-6, Ergonomic requirements for office work with visual display terminals (VDTs) — Part 6: Guidance on the work environment, 1999
[9] F.A. Everest, K. Pohlmann, Master Handbook of Acoustics, 5th edition, Mc Graw Hill, 2009
