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A-Weighting

What Does A-Weighting Mean?

A-weighting is a type of frequency weighting that is used when taking noise measurements. The purpose of A-weighting a noise measurement is to ensure that the noise being measured is the same noise that humans actually hear—in other words, an A-weighted measurement does not include noises that humans don’t hear, and the less sensitive the ear is to a given frequency, the less weight it is given by an A-weighted measurement.

Put simply:

  1. Non-weighted measurements of noise are objective measurements of noise.
  2. Weighted measurements of noise are designed to mimic various subjective perceptions of noise.
  3. A-weighted measurements are designed to mimic how humans subjectively perceive noise that is below 100 decibels in volume.

Safeopedia Explains A-Weighting

Because A-weighting is used to ensure that a noise measurement is measuring noise as it is perceived by humans, it is the most common type of weighting system used in noise measurements. Non-weighted noise measurement is described in decibels (dB); however, an A-weighted noise measurement is described as a dBA or dB(A).

The human ear becomes increasingly less sensitive to noises at very high and very low frequencies, and at sufficient extremes, it cannot hear these noises at all. The A-weighting system is designed to weight noise measurements according to this sensitivity gradient, which means that a very low-frequency noise that measures 50 dB in volume will measure significantly less in dBA. For example, at 315 hertz (Hz), a 0 dB noise (silence) is measured as -40 dBA, because 315 Hz is a low frequency that humans have trouble hearing.

While A-weighting is the most common type of weighting used in sound measurement, it is not always the correct choice in occupational settings. Although human sensitivity to sound varies depending on the frequency, this variance becomes increasingly smaller as the volume of noise increases. This means that A-weighting does not provide an accurate measurement of human-sensitive noise levels in very noisy environments.

For example, the measured difference between a 315 Hz sound that is A-weighted and a 315 Hz sound that is non-weighted is 40 dB; however, above 100 dB, the human ear is only five decibels less sensitive to 315 Hz than it is to mid-frequency noises (e.g., 1000 Hz). Thus, it would be inappropriate to use A-weighting to measure noise in a workplace where the unweighted noise is above 100 dB, such as in many construction and industrial settings.

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