Meaningful measures
October 1st 2006 It is important when monitoring noise that the instrument selected is appropriate for the task in hand. Seggy T Segaran of First Stop Safety looks at some of the features offered by sound level meters and explains how they can help the user measure noise levels To reduce the number of people losing their hearing due to high levels of noise, one has to get a measurement of noise levels at work. This has to be reasonably accurate: Just a 2 or 3dB error in measurement could result in a noise dosage figure that is quite misleading. There are a variety of ear defenders available and a reasonably accurate measurement of the noise is necessary to help in the choice of the correct one. If one is carrying out work to reduce the level of noise, a measurement of the level and the pitch of the noise will help in choosing the right method. So, how does one go about this? The choice of sound level meter is important. Below are some notes regarding various features of meters and how they help with measurements of noise levels. A & C Weighting Most Sound level meters have a choice of weighting that can be used. The A-weighting filter has a frequency response that tries to match the response of the ear. The human ear perceives sounds at say 1 kHz as louder than the same level at say 500 Hz or 4 kHz. Sound levels measured by a meter normally have a flat frequency response. The A-weighting filter modifies this to represent that level as perceived by the human ear. This weighting is used for all noise exposure measurements. As the ear behaves quite differently at high levels of sound, the C-Weighting filter is used here, such as in the measurement of impulsive sounds and in the choice of the right hearing protector. Integrating Noise Meter Noise meters usually have Fast and Slow response times and these determine how quickly the unit responds to fluctuating noise. The Fast measurement has a time constant of 125 milliseconds while the Slow is around 1 second. The Slow response is used for Noise at Work exposure measurements. If the noise changes many times over a longer period, say 10 minutes, it becomes quite difficult to take a reading. The Integrating noise meter overcomes this problem by keeping and displaying a running average of the Sound Pressure Level. Peak Measurement As well as slowly varying sounds, one needs also to be able to measure impulsive noise that is very fast, say from a gun shot or a riveting machine. The Fast setting on noise meters cannot react fast enough to these sounds and will read low to give you a false sense of safety. The Peak measurement is made with a time constant of 50 microseconds. Octave Band A sound level meter with this feature allows the user to see which frequencies are contributing to the overall noise level. This is useful for more detailed work such as choosing specialised hearing protection, or reducing noise levels by other means. This tool is very much for the noise professional and not normally required for the day-to-day problems regarding Noise at Work. Precautions Having chosen the right type of meter, what precautions need to be taken when a measurement is taken? First of all, it is important to carry out a field calibration of the sound level meter before every measurement session. When carrying out exposure measurements on an individual, the noise meter is held a few inches away from the ear while they are in their normal working position. When assessing noise levels in a building, one needs to take a number of measurements and work with the maximum one. Certain precautions have to be taken to avoid 'standing waves' due to reflections by keeping the noise meter away from the body and not taking measurements near walls, or large objects. Care also has to be taken to set the range on the sound level meter so that it does not overload. In summary, the choice of the right type of sound level meter and suitable precautions during measurement will lead to an accurate measure of noise levels. More articles from First Stop Safety: | 
