Transformers are neve silent
Transformers are neve silent (on photo: WEG – A three-phase oil transformer 225 MVA, 275 kV – for distribution of energy at Tealing Grind substation, of Scottish Hydro-Electric Transmission Ltd., in Scotland, one of the largest Utilities in Europe)

Where all this noise is coming from?

Yes, we all know that transformers are never silent. This is actually quite impossible, but in an environmentally aware, highly regulated world, the issue is not the level of noise, but its nature – and it’s very important.

Transformers emit a low-frequency, tonal noise that people living in their vicinity experience as an irritating “hum” and can hear even against a noisy background.

The power industry have a range of solutions to abate humming, which originates in the transformer’s core and, when it is loaded, in the coil windings. Core noise is generated by the magnetostriction (changes in shape) of the core’s laminations, when a magnetic field passes through them. It is also known as no-load noise, as it is dependent of the load passing through the transformer.

An effective and important noise source is the core of the transformer.

The noise of the core depends on the magnetic property of the core material (sheet steel) and flux density. The sound frequency is low (twice the rated frequency). The magnetic forces formed in the core cause vibration and noise. The load noise occurs only on the loaded transforrmers and is added to the no-load (core noise). This noise is caused by the electromagnetic forces due to leakage fields.

The source of the noise are tank walls, magnetic screenings and vibrations of the windings.

The noises caused by the core and windings are mainly in the 100-600 Hz frequency band. The frequency range of the noise (aerodynamic/air and motor/bearing noise) caused by cooling fans is generally wide. The factors effecting the total fan noise are; speed, blade structure, number of fans and arrangements of the radiators.

The pump noise is not effective when the fans are working and it’s frequency is low.

Magnetostriction takes place at twice the frequency of the supply load: for a 50 Hz supply frequency, a lamination vibrates at 100 c/s. What’s more, the higher the density of the magnetic flux, the higher the frequency of the even-number harmonics.

Cant see this video? Click here to watch it on Youtube.

When core or tank resonance frequencies coincide with the exciting frequency, the noise level further increases.

Hum also arises through the vibration caused when the load current passes through the windings, interacting with the leakage flux it generates. This load noise level is determined by the magnitude of the load current. It has always existed, but is becoming proportionally more significant since there are efficient means of reducing the core noise source.

In some situations, the load noise is the dominant noise and is raising increasing concern among new transformer applications.

Note that the broadband noise generated by cooling fans contributes to overall noise levels. But as cooling fans are widely used in the industry, solutions are not specific to transmission and distribution and so are not discussed here.

Vibroacoustic energy sources in the power transformers

Power transformer noise is mainly a low frequency narrow band noise, and the noise spectrum includes the tonal components of the frequency being the multiple of the power line frequency. The power transformers have many sources of vibroacoustic energy.

The most important sources include:

  1. The transformer core vibration as an effect of the magnetostriction phenomena
  2. The transformer winding vibration as an effect of  the electrodynamic forces
  3. The devices of the transformer cooling system, as fans, oil pumps.

Matters of design

Improvements in standard transformer design and materials are cutting the decibel count.

High-permeability (Hi-B) steel, for example, restricts magnetostriction through a surface coating with higher degrees of grain orientation.

Another increasingly popular method is high-precision stacking of the core’s laminations in step-lap patterns, reducing the formation of air gaps in the core joints. Focus on the linkages between the laminations to stop them striking each other includes gluing their edges together, standardizing clamp pressure and removing through-bolts.

In addition, robust, flexible mounts at all points of contact between core and tank inhibit the structure- or oil-borne transmission of resonance from one to the other.

Sound ways of seeing

Areva T&D’s R&D department employs acoustic imaging, acoustic holography and laser vibrometry to locate noise and vibrations. Acoustic maps noise rapidly and comprehensively by differentiating sound levels to determine where it radiates from.

Areva T&D and AB Engineering used 110-microphone arrays 2 m from the tank to measure noise in the 100 Hz to 500 Hz frequency bands.

For each band, an identically scaled map showed red hot spots on noise-free blue backgrounds, making it easy to pinpoint noise sources. Acoustic holography which analyzes near-field noise, was recently used to map transformer noise, arranging a 23-microphone antenna to scan a grid of 20 x 20 cm squares. Algorithm-based software computed the pressure field and sourced the acoustic radiation, displayed as spatially distributed 2-D maps for different frequencies up to 850 Hz.

Laser vibrometry is a no-contact technique for inaccessible or dangerous targets. It uses the Doppler effect, measuring the frequency modulation in the laser beam that rebounds from the vibrating target. Laser vibrometers can automatically scan large numbers of consecutive points, delivering vibration measurements with high spatial resolution.

When a transformer is loaded, vibration energy from the coil and any flux control devices is transmitted to the tank and then to the air and local environment. It is therefore important to design the tank so that it does not resonate at frequencies close to the exciting frequency. Measures like resonance absorbers can gain 3 dB.

On-site solutions

A common on-site method of containing noise radiation is tank-supported wall panels. They generally cover only the sides of the tank, bringing gains of between 4 dB and 10 dB depending on the wall area they cover. They may affect cooling, so acoustic barriers are often used, mounted close to the transformer on one or more sides, or enclosing it.

The simplest solution is a high acoustic screen, which must extend past each end of the transformer by at least as much as it exceeds the height of the transformer. But even single barriers can lower noise levels by 10 – 15 dB, depending on the position of the observer.

Acoustic holography is used to map transformer noise
Acoustic holography is used to map transformer noise

Complete top-bottom-and-side enclosure, of course, produces the most radical results, up to 25 dB of abatement, or even 40 dB if the enclosure is a massive structure made of concrete or steel and fully vibration-insulated. Care should always be taken that the space between tank wall and the barrier is not an even multiple of half of the wave length of the power frequency, e.g., 1,7 m, 3,4 m, etc. for 50 Hz transformers.

The result is standing waves that will cause echoes and amplify sound levels. Attenuation depends on how and how many of these methods are used. Combining Hi-B step-lapped core lamination with core vibration isolators can gain 6 dB. Add tank-mounted wall panels and that is 10 dB.

For greater improvement, a total contact-free enclosure is the answer.

Of course, designers can build low noise into transformers by lowering the core’s induction level, or flux density. But the trade-off is a larger core, larger windings and higher costs.

Need for Research and Development

Reasearch and development is addressing the need for reduced sound levels.

Some abatement techniques are well known, but others can be very innovative, such as resonance absorbers or resilient internal lining absorbers. Most of the selected solutions require a good knowledge of noise field and vibration mapping. New techniques are available to identify this information and to better characterize noise sources.

Benefits can be a reduction of measurement time, facilitated interpretation of measurements, access to other information (as in source localization), and more.

Resources: Transformers make less noise – Information from Areva T&D; Transformer Tests – BALIKESİR ELEKTROMEKANİK SANAYİ TESİSLERİ A.Ş.

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. Bridget Kipp
    Mar 16, 2016

    I live in a rural town, there is a proposal for a converter station. 57 feet high, 55000 square feet. We have nothing like this any where around. It will change dc into ac , or dc into ac. I’m very fearful of the noise along with others in my neighborhood. I will be a little under 3000 feet away. I would take any comments, advice, or any discussions that might be going on.

  2. carol
    May 31, 2015

    Can I ask for some advise please?
    There are 2 x 132/33 kV Double Wound Transformers and Associated Earthing Transformers approximately 120metres from my house. I am hearing a constant low him in every room in my house whether windowsare open or closed. I can also hear it with TV on. I do not hear a hum if I step outside my front door. There is a busy main road in front of substation, between it and my house so quite a lot of ambient noise.
    I have stood beside substation and can hear a ticking noise not really a hum. Could there still be a hum from it that is masked by general environment sounds?
    I have an audio spectrum analyser app on my phone. It shows large peaks at 100hz in my home. Thinking this may be harmonic of electricity….
    I am in a terrace and can hear the noise in each of my neighbours homes either side.
    The tone is driving me crazy! Family members can’t hear it but a few other visitors have. Could it be the substations? Could it be ground borne noise if I can’t hear it in my garden?
    I need some help and advise. (Environmental health are involved in investigating as are Scottish Power.)

    • Keith
      Aug 19, 2015

      What a relief to read your comments! I too have been driven crazy in the same way. Same thing as you say about most other people not hearing it; hearing it through everything, inc. TV; and not hearing it outside the house; I live in the countryside, and for the ten years I’ve lived here it’s just been grinding away at this low level. It’s grumbling away now. It’s incredible that it hasn’t actually driven me insane! I can hear it very faintly outside when there’s no traffic etc. but it sounds a lot quieter outside. Funny thing is I didn’t know until tonight where it was coming from. I’d previously walked a few hundred yards to local farms etc. looking for it, but it would elude me.
      But late tonight about 1.30am I went outside, and by slowly turning my head, I got a fix on the direction, and went out in the car “looking for the noise”. After driving around for miles, stopping several times and using the same direction finding technique, I eventually arrived at a substation 1.9km from my home. It took a while to be sure, because there is a weir right next to it, and the rushing water completely drowns out the rumble from the substation, when you’re within about 50 metres of it.
      I am a sound engineer, and have studied sound and soundproofing to some degree, which all helps. To ‘compare notes’ on your questions: – I too thought it may be travelling through the ground, but now I’m pretty sure it is airborne. I’m also pretty sure there is something resonant about a building (or some buildings) which amplifies the sound when the sound hits it. I live in a 1980’s built annexe, which I believe is stone-clad cement-block construction. I know cement block walls can have a resonance to them, and thought this, possibly combined with the dimensions, may cause or exacerbate the amplification. 100hz is definitely a harmonic of the 50hz mains frequency, and 100hz is the dominant frequency of the transformer noise, according to the article above.
      I will be on to my local distributor, Western Power in the morning. Not too early though.
      I would love to have an update on how you have got on..

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  5. Julia
    Mar 20, 2014

    I live close to an electrical substation and am being driven insane by the noise!! It never stops, 24 hours a day, 365 days a year. It is impossible to sleep at night, Even a white noise machine does not help very much. Cold weather seems to increase the noise and vibration to unbearable levels. I hate electricity and wish that it had never been discovered, even though I use it every day. I hate myself for using it!! Just my thoughts on the subject, having read your article.

    • Gary
      Aug 01, 2016

      I too have the hum, very annoying and I wonder about health effects, but a reply from the electricity company says there is nothing they can do! I am sure there is, but they don’t want to spend the money. I will be looking into class action law suit, though I doubt anything will come of it.

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  8. Md Ebrahim Shah
    Mar 13, 2013

    you are right. i also found that transformers always have some noises.

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