It’s important to monitor ambient noise levels in forensic laboratories both in the design phase and during operation.
Are you tired of hearing this plea that has turned into a commercial cliché? Yet, this is a phrase we are hearing more and more, not relating to poor cell phone service, but inside buildings and laboratories in particular. The reason is excessive room noise. All too frequently, and even after new construction, we are coming across rooms or laboratories in which it is difficult to hold normal conversations. Ventilation systems are typically the culprit. It is our attempt to ensure adequate fresh air and acceptable indoor environmental quality while reducing construction costs that pushes the heating, ventilating, and air conditioning (HVAC) system to struggle with the air flows. The ducts often end up undersized for the volumes handled and that leads to vibration and noise that interferes with speech and communication. This situation can be problematic even in forensic facilities especially if 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 potential for these types of problems. Add to this the noise induced by other analytical equipment, vacuum pumps, and our normal work activities and we likely have conditions that can make every day 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 that 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 difficult or impossible. A better way is to have the area assessed by someone knowledgeable about sound, its measurement techniques, and data interpretation, such as an industrial hygienist or acoustical engineer. The National Institutes 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 be monitored throughout facility design as well as during operation.
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 at octave band midband frequencies also for bands 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 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.