Occupational Noise: A-Weighted Vs. C-Weighted Scales
1/25/2021
Often, I have been asked about the noise designations of A-Weighted and C-Weighted measurements. What do these two classifications mean and how do they each relate to our exposures to occupational noise?
A-weighting began with the work of two scientists, Harvey Fletcher and Wilden Munson in the 1930’s when they measured the relationship of loudness to frequencies. In their study, headphones were placed on test subjects while listening to pure tones at various frequencies while measurements were reported based on 10 dB increments of stimulus intensity. For each frequency and intensity, the listener also listened to a reference tone at 1000 hertz (Hz). Fletcher and Munson adjusted the reference tone until the listener perceived that the new sound was the same loudness as the 1000 Hz test tone. Fletcher and Munson then averaged their results over many test subjects to derive reasonable averages. Based on test subjects’ perception of loudness for each frequency (in relation to the 1000 Hz frequency), a line was drawn which resulted in a curve for each frequency. These lines are referred to as “contours.” The lowest equal-loudness contour represents the quietest audible tone; that is, the absolute threshold of hearing. The highest contour is the threshold of pain. It was noted that humans consistently had to adjust the noise levels for the lowest and highest frequencies to a large degree as they perceived them when compared to the reference frequency of 1000Hz.
Fletcher and Munson grouped a number of contours together based on the sensitivity of the human ear. As such, the lowest and highest frequencies, were not provided a significant consideration when evaluating human perception of noise. These frequencies are typically less than 500 cycles per second (aka hertz or Hz) and greater than 6000 Hz, even though humans can detect noise levels as low as 20 Hz and as high as 20,000 Hz; although only those with superb hearing ability can perceive these frequencies (typically that is women in their 20’s and younger).
The various contours that Fletcher and Munson established were then divided into groups or weighted scales. The A-weighted scale represents the contours for which human hearing is most responsive; that is, 500 Hz to 6000 Hz. So, when noise levels are measured in the workplace, instruments such as sound level meters and noise dosimeters reduce the contribution of the low and high noise frequencies may be present, as the human ear does not perceive these “outlying frequencies” with the same weighting as the middle frequencies. Does this represent biased measurements in the field? Well, not if one wants to assessment noise exposures as noise is perceived by workers. Thus, measurements recorded on the A-weighted scale is designed to mimic the human ear.
On the other hand, the C-weighted scale measures a wider range of noise frequencies; typically, from 30 Hz to 10,000 Hz. But unlike the A-weighted scale, these frequencies are measured more evenly, without any consideration to the human perception of how we sense noise from the lower and higher ends. Therefore, a more realistic indication of noise that is present in the workplace is measured with the C-weighted scale, but is not designed to simulate human perception of noise exposure.
The A-weighted scale is the predominant criterion used to measure and assess occupational noise exposures. Instruments such as the sound level meter and dosimeter have the ability to switch between A-weighted and C-weighted measurements. When we need to perform an assessment of a noise level produced by a piece of equipment as well as an area or work location, the A-weighted scale is the option that needs to be considered. The same is true when evaluating employee exposures based on an 8-hour time weighted average; the criterion that OSHA uses to determine full shift worker exposures. This requires a noise dosimeter that is attached to a workers’ collar for a full shift and set on the A-weighted scale.
The C-weighted scale may be used to measure peak measurements for sources that produce very loud noises. This includes impulse noise; that is, noise produced from pile drivers, nail guns, forging, stamping and roof bolting. Thus, instantaneous, short-duration (less than 100 to 200 milliseconds), high-amplitude bursts of noise energy that is much greater than normal peaks which the A-weighted scale typically disregards.
Another setting where the C-weighted scale is used is at rock n’ roll concerts, and certain construction activities such as sandblasting where noise levels can reach as high as 130 decibels. The loudness produced from such activities is similar to a Military jet aircraft take-off from aircraft carrier.
So, while the C-weighted scale measures noise in a truer sense than its A-weighted counterpart, it does not provide noise level information that can be directly translated to the human sensitivities which are necessary to understand how occupational noise can affect worker exposures to noise.
A man, as a general rule, owes very little to what he is born with – a man is what he makes of himself