Proceedings of Meetings on Acoustics

Transcription

1 Proceedings of Meetings on Acoustics Volume 19, ICA 2013 Montreal Montreal, Canada 2-7 June 2013 Noise Session 4aNSa: Effects of Noise on Human Performance and Comfort I 4aNSa7. Acoustical characteristics of Technology Educational Shops in British Columbia Ahmed Summan* and Murray Hodgson *Corresponding author's address: University of British Columbia UBC, Vancouver, V6T1Z3, BC, Canada, ahmedsumman@gmail.com Technology Educational Shops (TES) are designed to develop high school students' technological literacy. Their acoustical conditions play a dominant role in the quality of these environments. TES are, at the same time, classrooms for learning and industrial workshops for making things. Each use has its own standards governing its acoustical characteristics: ANSI S for classrooms and the Ondet & Sueur DL2 criteria for workshops. A major conflict could exist by using the same room for two different purposes. This study investigated this conflict by evaluating the acoustical characteristics of 20 unoccupied wood, metal and automotive shops. It conducted measurements of background noise level (BNL), reverberation time (RT), speech intelligibility index (SII) and DL2. Results showed that BNLs and RTs in most TES were higher than the acceptability criteria for unoccupied core learning spaces. SII values indicated bad/poor speech intelligibility for normal and raised voice levels and reasonable/good speech intelligibility for loud and shout voice levels. DL2 values were found acceptable in about 80% of the TES. In general, these results indicate the poor acoustical conditions of TES as classrooms, and the need for special sound control measures. Published by the Acoustical Society of America through the American Institute of Physics 2013 Acoustical Society of America [DOI: / ] Received 21 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 1

2 INTRODUCTION Technology Educational Shops (TES) are designed to develop high-school students technological literacy. Their acoustical conditions play a dominant role in the quality of these environments. TES are, at the same time, classrooms for learning and industrial workshops for making things. Each use has its own standards governing its acoustical characteristics: ANSI S background-noise-level and reverberation-time criteria for classrooms and the Ondet & Sueur, 1995 DL 2 and Warnock, 1980 reverberation-time criteria for workshops. A major conflict could exist by using the same room for two different purposes. This study investigated this conflict by evaluating the acoustical characteristics of 20 unoccupied woodwork, metalwork and automotive shops relative to these, and additional verbal-communication-quality, criteria. The measurements conducted included background-noise level (BNL), reverberation time (RT), speech-intelligibility index (SII), and sound-level reduction with distance doubling (DL 2 ). STUDY-SITE DESCRIPTION This study was conducted in 20 different unoccupied TES including metalwork (M), woodwork (W), and automotive (A) shops, all of them in three British Columbia school districts. The sizes of the TES floorplans ranged from 107 m 2 to 305 m 2 and the volumes ranged from 323 m 3 to 2055 m 3. Only five of these TES were new, the others were old. The 20 TES were considered of traditional construction; none of them was of sustainable ( green ) design. Construction materials of the walls included bricks, wood and concrete with or without acoustical panels. Ceilings were made of steel and aluminum, suspended ceiling tiles, or wood with or without suspended acoustic panels. Floors were mostly made of concrete, and of hardwood in an old woodwork TES. The heights of the ceilings ranged from low ceilings at around 2.8 m to very high ceilings at around 11 m. All of these factors could play a real role in the quality of acoustics in these TES. Figure 1 shows an image of a metalwork TES; Figure 2 shows a floorplan of a woodwork TES. MATERIALS AND METHODS In order to investigate the acoustical conflict of using TES for two different purposes (classrooms and industrial rooms) with respect to the governing criteria for each use, relevant acoustical measurements, in octave bands from Hz, and calculations were carried out as follows: - Background-noise levels (BNL) were measured using a calibrated integrating sound-level meter (ISLM) with octave-band analyzer [Rion NA-29E] in each TES at about five positions for 10 seconds/position and with slow response. Presented BNLs were measured in 20 unoccupied TES where the building ventilation system was on, but all machines and dust collectors were off. BNL was also measured while the dust collectors were on in 8 TES. A- weighted BNL was calculated from the unweighted levels in each octave band. The average BNL in each TES was calculated. FIGURE 1: Image of a metalwork TES. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 2

3 FIGURE 2: A floor plan of a woodwork TES. - Reverberation time (RT) was measured using an omni-directional loudspeaker as a sound source that was placed randomly at the centers and corners of each TES in unoccupied conditions when all machines and ventilation were off. An audio amplifier was connected to the omnidirectional speaker and laptop to generate and control sound through the omnidirectional speaker. The calibrated ISLM with octave-band analyzer [Rion NA-29E] was connected to the laptop and placed at random receiver positions at each TES. The WinMLS software was used to generate the test signal through the loudspeaker and as an acoustical analyzer. Finally, the average of the RT values in each TES was calculated. - Surrogate Speech Levels were obtained by measuring sound-pressure levels from a speech source radiating broadband noise, pre-calibrated for its sound-power output. The calibrated manikin loudspeaker simulating a human talker was placed where the TES teacher is usually supposed to stand to teach. An audio amplifier was connected to the manikin loudspeaker and a laptop to generate and control sound through this loudspeaker. The ISLM [Rion NA- 29E] was placed at student listener positions at three random positions close to and far from the manikin loudspeaker in each TES, to measure and record levels generated by the manikin loudspeaker. The heights of both the noise source and the receiver were around 1.5 m. Measured levels were corrected for background noise. - Speech Intelligibility Index (SII) was calculated from the measured octave-band BNL, RT and surrogate speech levels (corrected to realistic talker values). The calculation was performed according to ANSI S SII values were calculated for a teacher speaking at four assumed vocal outputs (normal, raised, loud, and shout) in the TES. - Reduction of sound level with doubling of distance (DL 2 ): In order to obtain DL 2, a single sound source (omnidirectional loudspeaker connected to the audio amplifier) was located at different positions corresponding to doublings of the distance (2, 4, 8, 16 m, etc.) from a receiver (the sound level meter), taking into account each TES length to assign feasible distances for measuring. The heights of the noise source and the receiver were around 1.5 m. A sound-level meter at those positions measured the generated sound-pressure levels from the sound source. These measured sound-pressure levels in each TES were presented as a spatial decay curve modeled by a best-fit straight line as a function of logarithmic-distance, and the slope determined. RESULTS - Background-noise levels (BNL): In general, average BNLs in all unoccupied TES ranged from 45 to 68 dba when the dust collectors were off. These values are clearly higher than the dba criterion for core learning spaces (ANSI S ), as shown in Figure 3. Moreover, average BNLs when dust collectors were on are much higher than those when they were off in the 8 TES that have dust collectors, as shown in Figure 4. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 3

4 35 FIGURE 3:. Background-noise levels (dba) in the 20 TES in unoccupied conditions. FIGURE 4: Background-noise levels (dba) in 8 TES with dust collectors off and on. - Reverberation Time (RT): The measured reverberation times, as averages of the octave-band values, in each TES are shown in Figure 5. Results indicate that most of the TES have RTs higher than the 0.7 s criterion for core learning spaces (ANSI S ), and higher than the 1 s criterion for industrial rooms (Warnock, 1980). For example, woodwork TES WB1 and automotive TES AB1 have RTs of 3.4 and 2.3 s, respectively. These two TES are brand new and made of concrete walls and floors, and high steel ceilings, without any sound-absorbing materials. However, there are only six TES with RT values that meet the standards or are lower. For example, WS4 has an RT at 0.57 s. This particular old TES has a hardwood floor, and walls and a low ceiling that are covered with acoustical panels and acoustical tiles that can absorb radiated sound. - Speech Intelligibility Index (SII): Results for the calculated SII averages in each TES are shown in Table 1. Results were compared with the following proposed criteria: SII<0.3 (Bad); 0.3<SII<0.5 (Poor); 0.5<SII<0.6 (Good); 0.6<SII<0.8 (Very Good); SII>0.8 (Excellent). The SII quality at normal voice level in most TES were bad or poor. However, at shouting voice level SII quality was good or very good in about 12 TES. In two TES (AB1 and WB2) SII quality was found to be bad at all assumed voice levels. Indeed, poor or bad SII quality was found in the TES with high BNLs and high RTs that are unacceptable acoustically in TES as industrial workrooms and as classrooms. Very good and good SII quality were only found at loud and shouting voice levels. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 4

5 0.7 FIGURE 5: Measured reverberation-time values for the 20 TES. TABLE 1: Results for the calculated Speech Intelligibility Index (SII) and SII quality for the 20 unoccupied TES. TES Averages of SII values Overall Averages of SII values Overall SII TES Normal Raised Loud Shout Normal Raised Loud Shout SII Quality Quality WR Poor AR Poor-Good WR Poor-Good AB Bad WR Poor-Good MS Poor-Good WR Good MS Bad-Poor WR Poor-Good MS Poor-VG WS Bad- Good MR Bad-Poor WS Bad-Poor MR Good-VG WS Poor-Good MR Poor-Good WS Poor-VG WB Poor-Good AR Bad-Poor WB Bad - Reduction of sound level with doubling of distance (DL 2 ): The measured DL 2 values in 20 TES are shown in Table 2. The results were compared with the criteria for industrial rooms according to Ondet & Sueur, 1995 as a function of the floor area of the TES. From Table 2 it is seen that only four TES have unacceptable DL 2 values that are smaller than the criteria. This suggests that these four TES are reverberant industrial rooms. For instance, woodwork shop WB2 and metalwork shop MR1 have DL 2 values lower than the criteria. However, all of the other 16 TES have acceptable DL 2 values that are equal to or greater than the criteria, which makes these 16 TES considered as acoustically treated. CONCLUSION In conclusion, acoustical measurements conducted in the selected TES indicate that BNLs and RTs in most TES were higher than the acceptability criteria for unoccupied core learning spaces. SII values indicated bad/poor speech intelligibility for normal and raised voice levels and good/very good speech intelligibility only for loud and shout voice levels. This leads to the conclusion that teachers have to shout or at least speak loudly for students to understand their words in other words the verbal-communication quality is unacceptable. The DL 2 results indicated that about 80% of the investigated TES (as industrial rooms) are treated acoustically. Indeed, these results indicate the poor acoustical conditions in the TES considered as classrooms, and the need for special sound control measures. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 5

6 TABLE 2: Measured DL 2 values in the 20 TES. TES Area, m 2 DL 2 TES Area, m 2 DL 2 Measured Standard Pass/Fail Measured Standard Pass/Fail WR P AR P WR F AB P WR P MS P WR P MS P WR P MS P WS P MR F WS P MR P WS P MR F WS P WB P AR P WB F ACKNOWLEDGMENTS It was a real pleasure to get support and companionship from the UBC School of Environmental Health supervisory committee, Richmond, Burnaby and Surrey School Districts and the Acoustics & Noise Research Group. The research was funded by WorkSafe BC. REFERENCES American National Standards Inst. (1997). ANSI S3.5 (1997). American National Standard Institute - Methods For Calculation of the Speech Intelligibility Index. New York: Acoustical Society Of America. American National Standards Inst. (2002). ANSI S12.60 (R2009). American National Standard Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools (American National Standards Inst., New York). Ondet A.M., Sueur J. (1995). Development and Validation of a Criterion for Assessing the Acoustic Performance of Industrial Rooms. J. Acoust. Soc. Am. 97, Warnock, A.C.C. (1980). Reverberant Noise Control in Rooms Using Sound Absorbing Materials. Institute for Research in Construction, National Research Council of Canada, BRN Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 6