What causes transformer noise? How to stop transformer noise?
These are two questions eNoise Control frequently hears. We have measured transformer and reactor
noise in the field and have several years of experience in treating this low frequency humming noise.
The following excerpt from a 2012 Federal Pacific article details transformer noise and potential solutions.
Our company can help with sound measurements, property line noise readings, and possible solutions,
including sound walls and sound enclosures.
Transformer noise is caused by a phenomenon called magnetostriction. In very simple terms this means
that if a piece of magnetic sheet steel is magnetized it will extend itself. When the magnetization
is taken away, it goes back to its original condition. A transformer is magnetically excited by an
alternating voltage and current so that it becomes extended and contracted twice during a full cycle
We have established that the transformer hum is caused by the extension and contraction of the core
laminations when magnetized. Under alternating fluxes, we can expect this extension and contraction
to take place twice during a normal voltage or current cycle. This means that the transformer is
vibrating at twice the frequency of the supply, i.e. for 60 cycles per second supply frequency,
the noise or vibration is moving at 120 cycles per second. This is called the fundamental noise
- Transformer noise is produced by the core.
- The amount of noise is generally fixed by the design of the transformer.
- Adjustments to a design to reduce the noise level can be made at cost, but don't expect a
huge reduction in the noise level.
- Loading a transformer has little effect on the noise level.
- Vibrations are produced as well as noise and these are just as important as the noise.
Summary regarding noise control of transformers and reactors:
We have established that the core and coils of a transfomer will, when magnetized, produce
a hum (noise) and mechanical vibrations, but, the transformer category will also have an effect
on what happens once the noise and vibration is produced.
Source: Understanding transformer noise. In (2012). Federal Pacific, Dry-Type Transformers
(pp. 113-118). Retrieved from
- Transformer noise is difficult to change at the source.
- Transformer core constructions help to a degree. Reputable manufactures will use good joints,
flat steel, consistent thickness, good core supports, few bolts, etc.
- Transformer current loading has little or no effect on the noise level.
- Placing transformers in liquid (oil) does not help since oils are incompressible.
- Vibration-isolating core and coils within a tank does assist vibration isolation although
isolation of the whole tank is still needed.
- Noise reduction by distance is the simplest form of attenuation. If it can be achieved
without cost - excellent. Usually it cannot.
- Noise reduction by sound screens or sound walls would be next best option for sound attenuation.
- Full enclosure is usually the only option left to a troublesome transformer.
- Full enclosure can be made of any material with a high mass/weight ratio. Brick, concrete, steel
have been used. Expect 25 to 30 dB(A) reductions.
- Full enclosures using masonry products are not easily demountable.
- Acoustic steel panel techniques make good demountable enclosures. A 15 to 20 dB(A) reduction
is possible with properly designed enclosures.