INTRODUCTION
We measure sound intensity (also referred to as sound power or sound pressure) in units called decibels. Decibels (dB) are named in honor of Alexander Graham Bell, the inventor of both the telephone and the audiometer. An audiometer is a device that measures how well a person can hear certain sounds. A modern version of it is still used today to diagnose hearing loss.
Decibels are different from other familiar scales of measurement. While many standard measuring devices, such as rulers, are linear, the decibel scale is logarithmic. This kind of scale better represents how changes in sound intensity actually feel to our ears. To understand this, think of a building that is 80 feet tall. If we build up another 10 feet, the building will be 12.5 percent taller, which would seem just slightly taller to us; this is a linear measurement. Using the logarithmic decibel scale, if a sound is 80 decibels, and we add another 10 decibels, the sound will be ten times more intense, and will seem about twice as loud to our ears.
What techniques are we using for measuring noise?.
Before taking any field measurements, it is important to determine the type of information required. Do the workplace noise levels vary throughout the day? Are the workers fairly mobile? Do workers operate different equipment?
In a bottle washing and filling facility in Ontario, for example, the noise levels vary over the work shift. Instantaneous noise measurements, taken with an SLM (Type 2, SLOW RESPONSE, A-filter), at one person's work station, ranged from 63 dBA to 114 dBA over the day, although levels most commonly ranged from 90 to 96 dBA and 104 to 107 dBA. This information strongly suggested that there was a potential for excessive noise exposure.
The worker was asked to wear a noise dosimeter over a full eight-hour work shift. At the end of the shift, the noise dosimeter indicated a 270% dose. This was a substantial exposure. In addition, the dosimeter provided an equivalent noise level of 97 dBA. In other words, a constant eight-hour exposure to a steady, continuous noise of 97 dBA would have resulted in the same exposure.
An ISLM could also have been used in this example, particularly if the worker spent most of the work shift in a defined location, or the first half of the shift in one area and the remainder in another area. The ISLM could have provided equivalent sound level measurements and a fairly accurate exposure assessment.