dBA Generators – What Is It?

dBA Generators – what is it? dBA in Generators is the term used to calculate noise levels emanating from a working diesel generator.

How to Define Noise Levels? Diesel Generator noise levels are calculated in dB(A) and in free field conditions.

‘Free field conditions’ means what it says; the generator noise levels are tested in the open without surrounding buildings.

The common standard for canopied or acoustically enclosed generators is approximately 80dB(A) @1 metre in free field conditions.

Every time a lower sound/ noise level than the average is required the cost increases rapidly. Should a lower dB(A) be required, the next most common is 75dB(A)@ 1 metre in free field conditions.

Any lower dB(A) and the costs are greatly increased and must be weighed against the benefits compared to the ambient background noise of the site where the generator is to be positioned.

It is important to understand that even without the presence of a generator ambient background in the UK is mostly 65 dB(A)

Noise can be defined as unwanted sound, there is an increasing need to define and control noise.

Exposure to excessive noise levels may cause permanent hearing damage and adversely affect working efficiency and comfort.  Recognising this, the US Government created the Occupational Safety and Health Act which established noise limits for industrial environments.

When an employee’s daily noise dose – designated D(8) is composed of two or more periods of noise at different levels, the combined effect is calculated using this formula:

D(8) = (first dose) C1/T1 + (second dose) C2/T2 + etc.

where Cn is the duration of exposure at a specified sound level and Tn is total time of exposure permitted at a specified sound level. The noise dose is considered to be acceptable when the daily dose is equal to, or less than 1.

Note that sound pressure is measured in decibels (dB) and pounds per square inch (psi).  However, to measure the loudness of sound, the frequency of the noise must be considered.  The sound measurements weighted to the human ear are referred to as dB(A), the real impact on the human ear. The dB(A) scale has been accepted world-wide.

For noises of practical interest, the difference in sound power levels are not large.  Knowing that doubling the sound pressure will result in only a 6 dB increase is also helpful.

The following charts will aid in predicting overall engine room noise.  These levels are anechoic (free) field measurements which do not account for the effect of surrounding machines, floor, walls, etc.  The effect of distance on sound is straightforward.  Doubling the distance will decrease the apparent sound by 6dB; halving the distance from the source will increase the apparent sound by 6dB.

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