Acoustic design of industrial spaces The reduction of noise levels and increase of production
Contents. Introduction. Acoustic measurements Effect of acoustical treatment with suspended ceiling and wall panels > Acoustic design with wall panels > Positioning of wall panels > Case study 4. Case Study: Oatly Health Products Facility > Room > Activity > Acoustic design > Room acoustic measurements > Results > Additional actions 6. Appendix Results of measurements before and after
Introduction The main purpose of acoustic design in industrial spaces is usually to reduce the overall noise levels. In addition acoustic treatment with sound absorbers with good high frequency absorption gives increased speech intelligibility and better directional hearing, making it easier to hear the location of different sound sources. This document explores the key measurement criteria, the effects of adding absorption to a space and demonstrates through a case study what can be achieved in real world applications. Before acoustic design it is recommended to interview the staff and identify the main acoustic problems and how noise affects the people working there. Besides the acoustics, it is also important to establish if other demands have to be fulfilled, for example cleaning or hygiene criteria. Once the key objectives have been defined, appropriate acoustic treatments can be developed to meet the specific challenges of the project. By lowering the noise levels, decreasing the reverberation time and increasing the speech intelligibility the acoustic comfort in the space will be improved, which studies have shown will have a positive effect on staff wellbeing. Before acoustic design it is recommended to interview the staff and identify the main acoustic problems and how noise affects the people working there. The main purpose of acoustic design in industrial spaces is usually to reduce the overall noise levels.
Acoustic Measurements To objectively measure the acoustic conditions in an industrial space several room acoustic parameters have to be measured. It s common to measure parameters related to reverberance, sound strength and speech clarity. The parameters are described in table. TABLE Parameter Designation Unit Explanation Standard Reverberation time T s Speed at which sound disappears ISO 8-/ Speech Clarity C5 or D db or % Measure the perception of speech in a room. If these values increase the speech intelligibility increases. ISO 8-/ To objectively measure the acoustic conditions in an industrial space several room acoustic parameters have to be measured. Sound level decrease L db Measure the decrease in sound pressure level
Effect of acoustical treatment with suspended ceiling and wall panels It s very important to keep in mind that acoustical treatment with acoustic suspended ceilings and wall absorbers in industrial spaces mainly influences the general noise level. The sound field normally consists of direct sound and a reverberant sound. The direct part reaches the operators ears without hitting the room surfaces. Thus, this part is not affected by the acoustic ceiling and wall panels. To decrease the direct sound you often have to use screens, hoods or changing the mechanical process that generates the sound. The sound level that originates from the reverberant sound will be affected by the absorbing ceiling and wall panels. A realistic goal concerning the reduction of the db(a) level in the reverberant field by adding porous absorbers is in the region 5 to db(a). The perception of change in sound pressure level is given in table. TABLE Changes in sound level Subjective perception Only the noise that originates from the reverberant sound will be affected by the absorbing ceiling and wall panels. db Reduction or increase is hardly perceivable 5 db Reduction or increase is clearly perceivable db Reduction or increase results in halving or doubling of perceived sound level
The effect of added absorption in an industrial space is illustrated in Figure. The effect of the acoustic treatment depends on the amount of absorption before treatment. In Figure the effect of additional absorption is measured by the reverberation time. If the space is very reverberant and consequently has a long reverberation time, the effect of added absorption will be higher compared to a space with shorter reverberation time. In Figure below the effect of added absorption is calculated for an industrial space with floor area 5 m and for two different ceiling heights. The room without added absorbers is supposed to have a reverberation time of seconds. The alpha w for the added absorbers is.95. When the amount of added absorbers exceeds the floor area it is assumed that the absorbers are attached to the walls. Figure refers to an industrial space with a volume of m. 8 Sound level reduction db 7 6 5 4 5 5 5 5 4 Added amount of absorbers (m) alphaw=.95 (Class A) 4s s s Sound level reduction db 9 8 7 6 5 4 ceiling height 4.5 m, floor area 5 m ceiling height 8. m, floor area 5 m 4 6 8 Added amount of absorbers (m). Figure. The effect of added absorbers on the sound level in a room with a volume of m. The figures in the diagram refer to the reverberation time before acoustical treatment. Figure. The sound reduction in an industrial space with a floor area of 5 m and for two different ceiling heights. 4
stdev 9 8 Sound level reduction db 7 6 5 4 ceiling height 4.5 m, floor area 5 m ceiling height 8m, floor area 5 m 4 6 8 6 4 8 Added amount of absorbers (m).,5,5 5 5 5 K K 4K [Hz] Figure. The sound reduction in an industrial space with a floor area of 5 m and for two different ceiling heights. Figure 4. Reverberation time for the octave-band frequencies 5 to 4 Hz measured in a food factory before acoustic treatment It s common in industrial spaces that the reverberation time at low frequencies is lower than the reverberation time at high frequencies. This is illustrated in figure 4. The factory in this case is a food producer and the volume of the space is 576 m. This also means that acoustic treatment with porous absorbers will be very efficient since there is a lot of high frequency energy that will be absorbed by the absorbing ceiling and wall panels. It s not uncommon in industrial spaces that the reverberation time at low frequencies are lower than the reverberation time at high frequencies. 5
Acoustic design with wall panels From an acoustic point of view it s often advantageous to locate the absorbing material as close to the noise sources as possible. In this case an acoustic wall panel may be useful. If there is a risk for mechanical impact the panel could be protected by a cage. Normally, the wall panels works as a complement to a suspended ceiling. Positioning of wall panels The reduction of sound pressure level is related to the amount of absorbing material that is added to the room. When reducing sound pressure, the location of absorbers is of minor importance. However, when we are concerned with reducing reverberation time and improving speech clarity the distribution of the absorbers is important. Even a small quantity of wall panels in a room with absorbent ceiling could have a great influence on reverberation time and speech clarity. It s preferable to cover two adjacent walls with panels rather than two parallel walls to have the greatest effect on reducing reverberation within a room. The reduction of sound pressure level is related to the amount of absorbing material that is added to the room. Field study In the appendix the effect of acoustic treatment with a suspended ceiling and wall panels is shown for a factory building. The effect of acoustic treatment with a suspended ceiling and wall panels is shown for a factory building. 6
Case Study: Oatly Health Products Facility Oatly manufacture lactose-free oat based drinks that are consumed as a healthy alternative to milk. The company prides itself on its entrepreneurial spirit, curiosity and friendliness, often welcoming visitors to their factory located in Landskrona, Sweden. However, as with most industrial processes, the production facilities were noisy which was creating welfare issues for both staff and visitors. Employee issues: Table Property Designation Parameter Explanation Reverberance EDT, T Early decay time (s), Reverberation time (s) Speed at which sound disappears. A shorter reverberation time means that the room is perceived as more damped. > Complaints from employees about the noise level > Use of hearing protection limited conversation and observations within teams > Increased stress levels made it hard to relax when at home > Some employees reported disturbed sleep Speech clarity Sound strength C5, D, RASTI L Speech Clarity (db), Definition (%), RASTI Sound level decrease (db) Measure the perception of speech in a room. If these values increase the speech intelligibility increases Measure the decrease in sound pressure level Room Oatly s factory in Landskrona, Sweden. Volume: BxLxH= 9m x 7m x 6m=458 m Activity Machinery (conveyor belt) food packaging. Acoustic design To lower the general noise in the room and to create more favourable Acoustics, Hygiene Performance mm tiles were attached to the ceiling (complete coverage) and Hygiene Performance 4 mm to areas on three walls (see photo). Room acoustic measurements Before acoustic treatment the environment was judged as noisy with difficulties perceiving speech and having a conversation. The main purpose of the room acoustic measurements was to establish the effect of the acoustic treatment on noise level and speech intelligibility. Figure. Placement of ceiling and wall absorbers in Oatly factory premises. Table shows the room acoustic parameters measured. A brief explanation of the parameters is also given in the table. 7
Table shows the average values of the room acoustic parameters for the octave band 5 Hz and Hz. According to ISO 8- this average is suitable as a representative value for the room acoustic parameters. The values are given before and after acoustic treatment. In appendix the values are given for the octave band 6 Hz to 4 Hz. Table Property Parameter Before treatment After treatment EDT (s),4 s,6 s Reverberance T (s),4 s,6 s Extra action As an additional measure to improve the acoustics in certain areas of the factory the wall absorber Ecophon Advance Protection C was used. The wall absorbers were mounted in the corridor between the long walls and machines, see Figure. The tiles were placed at head height and mounted in a grid for protection against mechanical damage. The purpose of the absorbers is to reduce the high noise levels caused by multiple reflections between the wall and machine. The noise level was measured at three positions along the corridor before and after installation of the wall absorbers and show a decrease in db (A) level of between and db. Decrease in Sound Strength L (db) - 5 db C5 (db) -,9 db,7 db Speech clarity D (%) 4 % 7 % RASTI,47 (poor/fair),7 (good) Results Table shows that after the acoustic treatment, the noise level (general noise) in the room dropped by about 5 db. Before treatment the general noise level in the room was in the region 8-84 db(a) (Industrial Health measurements 98). After treatment the general noise level dropped to about 77 db(a) which means that the levels are below the action values according to Arbetsmiljöverket set (AFS 5:6). The values of speech perception also show a clear improvement. One reason for this is that the influence of so-called late reflections in the room has been reduced. Also the reverberation times decreased significantly. In the vicinity of some machines, the noise level is still above 8 db(a). This is because the direct sound from the machines dominate. Direct sound is not affected by the ceiling and wall absorbers, but must be addressed with direct measures of the sound source such as encapsulation or enclosure. Figure. Wall absorber Advance Protection C placed at head height along the walkways. Examples of spontaneous comments from staff after acoustic treatment were: > easier to hear what is said > easier to detect where the sound comes from > nice not having to wear hearing protection > not so tired when I get home, sleep better 8
Appendix Results of measurements before and after Measurement results in octave bands before and after treatment EDT (s) T (S),5 Early decay time (s),5,5 Reverberation (S),5,5,5 6 5 5 5 4 6 5 5 5 4 Frequency (Hz) Frequency (Hz) EDT (S) Before EDT (S) After T (S) Before T (S) After EDT [s] T [s] Before After Before After 6,, 5,,9 5,8,7 5,,6,4,6,9,6 4,6,7 6,6,5 5,7, 5,,8 5,5,6,4,6,9,6 4,7,6 9
L (db) C5 (db) Reduction in Sound Pressure (db) 7 6 5 4 6 5 5 5 4 Speech Clarity db 5 4 - - 6 5 5 5 4 Frequency (Hz) - -4 AL [db] Frequency (Hz) C5 (db) Before C5 (db) After ΔL [db] 6,5 5, 5, 5 5,8 4,7 4, 4,5 4,6 C5 [db] Before After 6,,6 5,,6 5 -,,6 5 -,4,6 -,5,8 -,9, 4 -,,4