Recognize this mobile phone commercial cliché? Well, you might be hearing it in your lab as this phrase is repeated more and more—not relating to poor cell phone service, but the insides of buildings and laboratories. The reason usually results from excessive room noise. All too frequently, even after new construction, we are coming across laboratories (or other rooms such as offices and conference rooms) that present problems with normal conversations. Ventilation systems are typically the culprit. This results from attempts to ensure adequate fresh air and acceptable indoor environmental quality while reducing construction costs, which causes the heating, ventilating and air conditioning (HVAC) system to struggle with the air flows. The ducts often end up undersized for the volumes handled, which leads to vibration and noise that interfere with speech and communication. This situation can be problematic, especially in busy research laboratories where we are scrubbing or filtering the air supply with high efficiency particulate air (HEPA) filters or trying to balance flows in heavily ventilated fume hood systems. The more advanced the air supply and filtering system becomes, the more of a potential there is for these types of problems. Add to this the noise induced by other analytical equipment, vacuum pumps and our normal work activities, and we may have conditions that can make everyday communication difficult.
Excess noise is not a good thing. Background noise from HVAC systems or other laboratory activities can become insidious. It can make conversation difficult, affect concentration, distract workers and increase fatigue, not to mention the potential adverse effects on other support personnel who enter the area. How do we know if the noise is excessive? One rule of thumb is if normal conversation or talking on the telephone is either difficult or impossible. A better way is to have the area assessed by someone knowledgeable about sound, sound measurement techniques and data interpretation, such as an industrial hygienist or acoustical engineer. The National Institute of Health Office of Research Facilities recommends a maximum noise level of NC-45 in research laboratories, operating rooms and similar areas for reasonable speech communication.1 NC-45 refers to a balanced noise criterion curve, which is a set of octave band sound pressure levels used to characterize the noise in a space. (More on NC curves in a minute.) It is important to keep in mind that the NIH recommendation is for “adequate speech intelligibility with normal voice effort,” i.e., normal worker conversation. The NC-45 equates to 53dB (A) and is chosen to account for the frequency distribution based on normal speech. It is considered the maximum design goal for occupied research areas.
NIH recommendations are for background noise, meaning that rooms are unoccupied and all user equipment is turned off when taking measurements and making evaluations. HVAC systems should be running normally and special ventilation equipment designed to run full-time should be on. We suggest that you consider all possible sources of noise in the spaces when evaluating room noise levels. Noise evaluation and control should appear high on the project manager’s list and monitored throughout the facility design as well as during operation.
Evaluating the noise levels
How do we evaluate room noise and what criteria should we be aiming below when designing facilities or making corrections? The American National Standards Institute (ANSI) has published a standard titled “Criteria for Evaluating Room Noise,” S12.2-2008 (2008).2 This consensus standard provides excellent guidance for assessing noise in many different types of spaces. Basically, there are two sets of criteria. Room criterion (RC) curves are defined for the octave bands 16Hz through 8000Hz by a set of curves with a constant slope of –5 dB/octave. The second set, which has gained wider acceptance, are the balanced noise criterion (NC) curves defined by a set of sound pressure levels measured mid-band for octave frequencies 16Hz through 8000Hz. When evaluating noise using this standard, you will need a sound level meter with an octave band filter in order to measure sound levels in decibels at each octave band frequency. The sound levels measured at each frequency are then either plotted or graphed and compared to the NC curve you are trying to meet. One useful online tool is the Engineering Toolbox3 that shows the NC curves, provides recommended NC levels for different types of areas and even allows you to enter your data and plot octave band sound levels on the NC curves.
To determine compliance with a specific NCB curve, the ANSI standard also defines the speech interference level (SIL), rumble and hiss. The SIL is the average of the sound levels measured at 500Hz, 1000Hz, 2000Hz and 4000Hz (the primary human speech frequencies). Rumble is excessive noise in the low frequencies (500Hz and below). Hiss is excessive noise in the high ranges (1000Hz and above). Details on how to evaluate for compliance with these ranges based on the different criteria used are provided in the S12.2 standard. Until more research studies are available, aiming for the NC- 45 criterion curve should be your goal.
In summary, noise control is an important consideration for laboratory facility design and operation. Both the NIH and the ANSI references offer excellent suggestions that can be used for the design phase as well as for evaluating existing facilities. A couple of things to keep in mind:
- Masonry walls, being denser, are more effective than metal or plaster walls in trapping noise and preventing transmission between areas.
- If using sound-attenuating materials, they should be bonded to walls or ceilings and be easy to clean.
On the operations side, noise-producing support functions, such as vacuum pumps and compressors, should be separated from laboratory functions whenever possible. One solution is to locate these in a separate utility “alley” or hallway between laboratories. These isolation techniques may seem like common sense, but they often get overlooked. If you are overwhelmed or lost, call in a professional.
- Design Requirements Manual, National Institute of Health, Office of Research Facilities. September 2009. http://orf.od.nih.gov/NR/exeres/DB66BAA6- 6740-4012-9668-ACBB9031923A.htm
- Criteria for Evaluating Room Noise. American National Standards Institute. S12.2 -2008.
- NC-Noise Criterion Curves, Engineering ToolBox. 2005 http://www.engineeringtoolbox.com/nc-noisecriterion- d_725.html
Vince McLeod is an American Board of Industrial Hygiene– certified industrial hygienist and the senior industrial hygienist in the University of Florida’s Environmental Health and Safety Division. He has 22 years of occupational health and safety experience at the University of Florida, and he specializes in conducting exposure assessments and health-hazard evaluations for the university’s 2,200-plus research laboratories.
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