Why Is Factory Soundproofing Important?
Factories typically have high ceilings with lots of hard reflective surfaces. When sound impacts a surface it is either reflected, absorbed or transmitted. In factory spaces, much of the machinery noise is reflected into the space. This noise can interfere with production and ultimately cost your company money.
Noise can be hazardous to the health and safety of employees. Employees may not be able to communicate effectively, resulting in unnecessary accidents. Over time, exposure to loud noise can permanently damage hearing.
We are going to look at how to calculate noise exposure, identify problematic noise sources, identify the best acoustical treatment approach and look at noise mitigation options.
Computing Worker Noise Exposure
It is important to understand how noise exposure is computed to know how to approach noise mitigation.
The United States Department of Labor Occupational Safety and Health Administration (OSHA) Noise Exposure Code (1910.95(b)(1)) states that when employees are subjected to sound exceeding those listed in Table 1, feasible administrative or engineering controls shall be utilized. If such controls fail to reduce sound levels within the levels of Table 1, personal protective equipment shall be provided and used to reduce sound levels within the levels of the table.
|Table 1 – TABLE G-16 – PERMISSIBLE NOISE EXPOSURES (1)|
|Duration per day, hours||Sound level dBA slow response|
|1/4 or less||115|
The OSHA Occupational Noise Exposure Code (1910.95(c)(1)) states that the employer shall administer a continuing, effective hearing conservation program whenever employee noise exposures equal or exceed an 8-hour Time-Weighted Average sound level (TWA) of 85 dBA without regard to any attenuation provided by the use of personal protective equipment. An 8-hour TWA of 85 dBA equals a 12-hour TWA of 82 dBA.
Conversion Between “Dose” and “8-Hour Time-Weighted Average” Sound Level
Compliance with paragraphs (c)-(r) of this regulation is determined by the amount of exposure to noise in the workplace. The amount of such exposure is usually measured with a dosimeter which gives a readout in terms of “dose.” In order to better understand the requirements of the amendment, dosimeter readings can be converted to an “8-hour time-weighted average sound level.” (TWA).
In order to convert the reading of a dosimeter into TWA, see Table A-1, below. This table applies to dosimeters that are set by the manufacturer to calculate dose or percent exposure according to the relationships in Table G-16a. So, for example, a dose of 91 percent over an eight hour day results in a TWA of 89.3 dB, and, a dose of 50 percent corresponds to a TWA of 85 dB.
If the dose as read on the dosimeter is less than or greater than the values found in Table A-1, the TWA may be calculated by using the formula: TWA = 16.61 log(10) (D/100) + 90 where TWA=8-hour time-weighted average sound level and D = accumulated dose in percent exposure.
D = 100 (C(1)/T(1) + C(2)/T(2) + … + C(n)/T(n)),
C is the total time spent at each noise level. The reference duration, T, is shown in Tables 1 and 2 and can be computed by:
T = 8 / (2^((L-90)/5))
where L is the measured A-weighted sound level.
|Table 2 – TABLE G-16A|
|Sound Level (dBA)||Reference duration, T (hours)|
Table 3 shows the conversion between dose and 8-hour time-weighted average sound level.
|Table 3 – TABLE A-1 – CONVERSION FROM “PERCENT NOISE EXPOSURE” OR “DOSE” TO “8-HOUR TIME-WEIGHTED AVERAGE SOUND LEVEL” (TWA)|
|Dose or percent noise exposure||TWA|
There are generally four noise mitigation options. Attenuate the:
- Noise source
- Noise path
- Noise receiver
- Limit time in certain areas
The final treatment may be a combination of treating these different areas.
There are several options when evaluating reducing the noise radiating from loud equipment.
- Replacing the equipment with quieter equipment – smaller units, newer units, different process.
- Equipment maintenance – replace belts, lubricant, balancing.
- Lagging the equipment – lag with loaded vinyl (normally with quilted fiberglass on the equipment side to decouple lagging and equipment).
- Enclosing equipment – usually with acoustical absorption on equipment side of the hard enclosure.
- Location of equipment – isolate the equipment from other noise making equipment to reduce the overall noise level.
Noise is propagated in two ways:
- Direct path – where there is line-of-sight between the noise source and the receiver (person being impacted). The direct path is attenuated through the use of barriers (blocking the line-of-sight).
- Reverberant (reflected) paths. Noise that bounces off the facility’s walls, floor and ceiling and then reaches the receiver. The reverberant by adding absorption to the room.
The amount of noise reaching any location will be a combination of the two paths and can be calculated. This is useful in predicting the effectiveness of various acoustical treatments.
The direct path is calculated assuming spherical radiation with no reflections. The noise level is 6 dBA lower for every doubling of distance.
The reverberant noise level is calculated using the room constant or reverberation time (the ratio of absorption to surface area). The reverberation time can be measured by playing loud broad band noise and turning it off. The reverberation time (RT60) is the time it takes for the noise to drop by 60 dBA. It is measured in each octave or 1/3 octave band.
The sound pressure level Lp can be calculated using the following formula:
Lp = Lw + 10log[Q/(4πd^2) + 4/R]
d is the distance from the equipment. Q is the directivity coefficient (1 uniform spherical, 2 uniform half spherical (single reflecting surface), 4 uniform radiation over 1/4 sphere (two reflecting surfaces, corner)). R is the room constant, described below.
Lw is the sound power level of the equipment. Lw can be calculated from a free field noise measurement or if the measurement is close enough to the equipment where it is much greater than the reverberant noise level. R goes to infinity.
Lw = Lp – 10log[Q/(4πd^2)]
If Q is 1, this can be simplified to:
Lw = Lp + 20log(d) + 10.5
R is the room constant, calculated:
R = Sα/(1-α)
Where α is the average absorption and S is the surface area. The absorption (and room constant) can be calculated from the measured reverberation time (RT60):
RT60 = 0.049V/Sα
Where V is the volume.
The direct noise can be separated from the reverberant noise. This will help determine how effective a barrier will be (impacting the direct path but not reverberant). This will show how effective adding absorption will be in different parts of the facility (impacting just the reverberant path).
The third treatment option is to treat the worker (receiver). Using hearing protection and/or local shields for the individuals is an option.
Limit Time in Certain Areas
Reducing the time a person spends in a high noise area is the final treatment options. Shifts can be split or time shortened to lower the overall worker exposure to the high noise levels.
Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.