How do you interpret the output of noise measuring instruments like sound level meters and noise dosimeters to determine the time-weighted average noise exposure?
Interpreting the output of noise measuring instruments such as sound level meters (SLMs) and noise dosimeters is crucial for accurately assessing workers' noise exposure and implementing effective hearing conservation programs. These instruments provide different types of measurements, and understanding how to interpret these measurements to calculate time-weighted average (TWA) noise exposure is essential.
Sound Level Meters (SLMs):
SLMs are used to measure the instantaneous sound pressure levels at a specific location and time. They provide a snapshot of the noise levels at a particular point in time. The data output from an SLM typically includes:
1. Sound Pressure Level (SPL): Measured in decibels (dB), it indicates the intensity of the sound at a given moment. SLMs can measure sound pressure levels with different frequency weightings (A-weighting, C-weighting, etc.). A-weighting (dBA) is the most commonly used for occupational noise assessments, as it mimics the human ear's sensitivity to different frequencies. For example, an SLM reading may show a sound pressure level of 95 dBA in an area of a factory where a metal press is operating.
2. Maximum Sound Level (Lmax): The highest sound level recorded during a measurement period. It can be important for assessing peak noise levels, particularly in environments with impulsive noises. For instance, if an SLM is used near a nail gun, it may record a high maximum level when the nail gun is fired.
3. Equivalent Continuous Sound Level (Leq): The average sound level over a specific time period. This metric represents a single, constant sound level that would have the same sound energy as the fluctuating noise levels measured. Leq is an important measurement for assessing continuous or variable noise levels over time. For example, if the noise in an office varies throughout the day due to different activities, the Leq would give a representative average of the overall sound level in the office for that given period.
Using SLM Data for TWA Calculation:
To calculate TWA from SLM measurements, you need to sample the noise levels at multiple points across a workday and determine how long employees are exposed to each of these noise levels. A TWA calculation takes into account the changing noise levels and the duration of exposure to each level, combining them to give an average noise exposure over a work period.
A basic TWA calculation is done by combining the measured sound levels at different locations and their respective durations using the following formula:
TWA (dBA) = 10 log [ ( (T1 10^(L1/10)) + (T2 10^(L2/10)) + ... + (Tn 10^(Ln/10)) ) / Ttotal ]
Where:
T1, T2 ... Tn = the time in hours for each measurement
L1, L2 ... Ln = the A-weighted sound level in dB for the time measured
Ttotal = the total time of exposure, usually an 8 hour work day.
A simpler approximation, suitable when levels are not varying widely, is to calculate a simple average of several measurements.
TWA approx = (L1 + L2 + L3... + Ln) / n
Where L1, L2 ... Ln are sound levels in dBA and n is the number of samples taken,
However, this method is an approximation, and not as accurate as the formula given earlier.
For example, if a worker spends 2 hours in an area with 85 dBA, 3 hours in an area with 90 dBA, and 3 hours in an area with 80 dBA, the TWA can be calculated by:
TWA = 10 log [ ( (2 10^(85/10) ) + (3 10^(90/10)) + (3 10^(80/10)) ) / 8 ]
This would result in a TWA of around 86 dBA which means the worker's average noise level over the working day is equivalent to 86dBA
Noise Dosimeters:
Noise dosimeters are personal noise measuring devices that are worn by workers to measure their individual noise exposure over a work shift. The main output from a dosimeter is:
1. Dose Percentage (% Dose): This indicates the percentage of a worker's noise exposure relative to the allowable limit (usually based on an 8-hour workday). For instance, a dose of 100% means that the worker's exposure is exactly at the allowable limit. OSHA uses a criterion level of 90 dBA over an 8-hour period. A dose of 50% indicates an exposure of half the allowable limit, while a dose of 200% indicates twice the allowable limit.
2. Time-Weighted Average (TWA): Dosimeters typically calculate the TWA noise exposure automatically. This measure is also in dBA and represents the average noise level over the measurement period, adjusted for the time duration, and the different noise levels during that time. A dosimeter provides a TWA without having to manually calculate this from separate measurements, and therefore it is a very valuable measuring tool for assessing noise exposure of individual workers. A TWA result of 88 dBA would mean the worker was exposed to 88 dBA during their work shift.
Using Dosimeter Data for TWA Assessment:
Dosimeters directly provide the TWA values, so further calculation isn't necessary.
If a dosimeter reading gives a dose percentage of 150%, and it is based on an allowable noise exposure of 90 dBA, the employee was exposed to 90dBA + (0.5 90dBA) = 135 dBA which is clearly above the PEL and presents a risk of hearing loss.
For example, if a dosimeter displays a TWA of 87 dBA, and the action level is 85 dBA, this indicates that the worker has been exposed above the action level and is required to be included in a hearing conservation program.
In Summary:
SLMs are used to measure noise levels in specific areas at certain times, and the results must be combined to determine a TWA. Dosimeters, however, measure the noise exposure of individuals throughout their entire shift and directly provide the TWA and noise dose for that worker. Dosimeters are more precise than SLMs for assessing individual exposure, but SLM data is required for creating noise maps and understanding the noise levels of specific locations in a work area. To accurately assess noise exposure and implement a successful hearing conservation program, using both types of equipment is generally required, and both must be used with accurate records, and with proper calibration procedures to ensure the accuracy of all noise level data.