The decibel, which is a unit of measurement for sound levels and expressed as dB, is used in the fields of communication, electronics and signals -- and by various industries that have equipment that generate excessive noise. The terms dBA and dBC refer to the types of filters used to measure dB -- either an A filter or a C filter. Each filter has a different sensitivity to various frequencies. Understanding the difference is important to businesses that must filter sound for employee safety reasons or when setting safe sound levels in movie theaters and on telecommunications devices.
The A Filter
Measurements made on A filters are expressed in dBAs. The dBA sound level meter applies to the mid-range frequencies as opposed to the dBC sound level meter that measures low and high frequencies. The Occupational Safety and Health Administration provides the country's guidelines for employers regarding workplace noise exposure limitations based on dBA measurements in increments of three. For example, the permitted exposure length starts at 85 dBA, with a maximum exposure in a 24-hour period at 139 dBA for only 0.11 seconds.
The C Filter
Measurements made using C filters are expressed in dBCs. Unlike dBA, its measurements suit low and high frequency sound levels. The C filter literally filters the sounds the microphone picks up in the sound level meter, used more in entertainment venues. The frequency response function, sometimes called a weighting characteristic, controls the tone by giving more weight to some frequencies than other less important frequencies. When transmitted sound has bass issues or problems, the C filter is typically used.
A-and-C Filter Weighting Applications
The A-weighting measures the risk of hearing loss. Specifically, it helps determine OSHA compliance that states the allowable noise exposure by a time-weighted dBA average sound level or by a maximum daily dose of noise. On the other hand, C-weighting is used by comparing its measurements to those of A-weighting. For example, C-weighting helps when doing computations regarding hearing protectors and the noise reduction rating calculations.
If dBA sound levels exceed allowable, safe or comfortable levels, suggestions for reducing the sound include limiting the level or volume of the sound, moving farther away from the source of the sound or using ear plugs or ear muffs to protect the ears. C-weighting occurs for peak measurements and in noise measurement for the entertainment industry, as in a live stage event or in running a movie theater business where bass noise transmissions can become problematic.
Business and professional sound systems sometimes list an A-weighted rating in their printed specifications. If you find this, it indicates an A filter actively hides or filters certain hums or other background sounds. That manufacturer obviously felt the need to filter some objectionable noises in its sound system. You could see this as a positive addition to the system, or you could presume the sound system is not of top quality in the presence of A-weighted filters. Otherwise, the manufacturer would not feel compelled to filter these unwanted sounds from coming through the system.