We talk about dB’s (decibels) but what do they really mean? In simple terms, we are trying to take what we hear and relate it to scientifically measurable terms. The decibel as used in acoustics is a measurement comparing the pressure generated by a noise against some standard level. These decibels will vary according to the frequency of the noise, but this is taken care of in the noise level meter.
We refer to dB. The “A” part refers to a position on a sound level meter which more closely follows the human ear. It is important when taking measurements to specify if the noise level was taken on the “A” weighted scale.
Since the transformer is not necessarily symmetrical, we cannot take one reading of noise level from a sound level meter and call the noise level of the unit. It is necessary to take many readings around the transformer and to average them. The resultant will become the transformer noise level.
Standards are laid down on how this should be done. The main ones are ANSI Standard C57-12-90 or NEMA Standard TRI-2-068-1954.
What happens is that you imagine a string following the contours of the transformer. You step back 1 foot from that contour line with the unit excited at the normal voltage, and record a measurement. You take these measurements at 3 foot intervals along the imaginary string. The measurements are totaled and then averaged. The result is the transformer noise level.
To measure amounts of noise in each frequency range you need a frequency analyzer. This is a worthwhile acquisition.

It is always necessary to measure the background (ambient) noise level before you start and when you finish the tests. There has to be a
difference between the ambient reading and the average noise level of 7dB or better, for it to be valid, otherwise you could be increasing the actual reading of the transformer. This sometimes makes night owls of the testers!
SO NOW WE KNOW WHERE THE NOISE COMES FROM AND HOW TO MEASURE IT. WHAT CAN WE DO ABOUT IT?
First of all, accept that there is a noise and you are stuck with it. We have to consider how to
avoid making it a nuisance to people. The most obvious strategy is to place the transformer in a field miles away from habitation. The noise level drops away as the square of the distance from the noise, but even so, it would take a very large field to hide it. However, we invariably have to place transformers near people and we must face up to that fact.
We have both noise and vibration to worry about and as we have said NOISE is usually air born, VIBRATION is structure born.
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