Today we introduce a very important aspect related to noise level measurements. That is “decibel –A weighting filter”
Sound or Noise level in an occupational environment is a very important safety related measurement dictating the relative safety and the time duration that a person can safely work within a certain noisy environment. Therefore sound pressure level (in dB), often referred as the noise level, have to be measured whenever workers are suspected to be exposed to unsafe noise conditions in their working environment, or when we need to determine the maximum duration that workers are allowed to work within a certain environment.
Audible noise occurs at various frequencies throughout the 20 Hz – 20 kHz frequency range (i.e. approximate audible frequency range for human hearing). But, (similar to many manmade microphones, and speaker sets), the human ear is not equally sensible to all frequencies within this audible frequency range. It has been discovered that human ear is highly sensitive to the sounds within the frequency range of 1 kHz to 4 kHz, while human ear is less sensitive to sounds of frequencies below this range and also above this range. That means, humans can tolerate more loud noises outside this sensitive frequency range.
Due to the above mentioned reason, if we measure the total noise level in an environment using a measuring instrument without any “human like” adjustment, that measurement will not represent the actual noise condition perceived by a human.
So, acoustic engineers have developed various types of “noise weighting curves” (or “noise filters”) to adjust the noise measurements in accordance with human perception of noise under different conditions.
There are several noise filters denoted as, decibel-A filter, decibel-B filter, and decibel- C filter (a rarely used decibel – D also existed). The graph in the image indicates the characteristics of above mentioned filters.
Out of these, decibel –A filter is the most common one and it gives a satisfactory representation of human sound perception at “not too loud” noise conditions. Many modern safety engineering standards also use this decibel – A weighted noise filter for their reference values. Hence many noise measuring instruments have incorporated this db-A filter into their instruments. What it really does is that the instrument automatically subtracts a certain dB level from the actually measured decibel level at each frequency (based on the A weighting curve -see the graph). For example, at 100 Hz, the instrument shall reduce 20 dB from the measured noise level. Eventually the instrument will collect such adjusted dB levels for all measured frequencies and give you the total noise level as a single value with the unit denoted as db-A. (Remember, this is not a simple arithmetic addition process. You cannot just add 2 decibel levels together to take a summation. This is due to the fact that dB range is a logarithmic range, not a normal linear measurement unit. We will discuss this phenomenon further in future posts).
Out of these, decibel –A filter is the most common one and it gives a satisfactory representation of human sound perception at “not too loud” noise conditions. Many modern safety engineering standards also use this decibel – A weighted noise filter for their reference values. Hence many noise measuring instruments have incorporated this db-A filter into their instruments. What it really does is that the instrument automatically subtracts a certain dB level from the actually measured decibel level at each frequency (based on the A weighting curve -see the graph). For example, at 100 Hz, the instrument shall reduce 20 dB from the measured noise level. Eventually the instrument will collect such adjusted dB levels for all measured frequencies and give you the total noise level as a single value with the unit denoted as db-A. (Remember, this is not a simple arithmetic addition process. You cannot just add 2 decibel levels together to take a summation. This is due to the fact that dB range is a logarithmic range, not a normal linear measurement unit. We will discuss this phenomenon further in future posts).
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| The image shows the characteristics of db-A, db-B, and db-C weighting filters. |
The equipments for measuring the noise is used majorly in the commercial sector, it has very few applications in the domestic sector. In the recording studios, radio stations and the places where the broadcast is made, noise measuring instruments are used. In general, noise is defined as an unpleasant and unwanted sound which is not required and comes out as a loss or as an accident.
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