The Bottom Line: A Transformer Hums at 100/120 Hz Because the Core Steel Stretches — and You Cut Noise by Specifying Sound Power, Not by Asking for a “Quiet” Unit
Here is the answer up front. A transformer hums at twice the line frequency — 100 Hz on a 50 Hz supply, 120 Hz on 60 Hz — and the sound is almost entirely magnetostriction: the core steel stretches and contracts microscopically under magnetic flux, not the windings, the load, or the fans. Typical sound power levels run from roughly 45–50 dB(A) for a 100 kVA oil-immersed unit up to 65–70 dB(A) at 5,000 kVA, and dry-type units run about 5–10 dB louder at the same kVA because there is no oil to damp the core. You get a quiet unit by writing a sound-power-level limit (LWA) into the specification — measured per IEC 60076-10 — and by cutting magnetostriction with low-noise core steel (a 3–5 dB saving) or a sound enclosure (15–25 dB).
Key Takeaways
- The hum is magnetostriction — core steel expands and contracts ~2× line frequency, producing the characteristic 100 Hz (50 Hz) or 120 Hz (60 Hz) tone, not winding or load noise.
- Sound power (LWA) rises with kVA — allow roughly +3 dB per doubling of rating; a 100 kVA oil unit is ~45–50 dB(A), a 1,000 kVA unit ~55–60 dB(A) (industry experience, as of 2026).
- Dry-type runs louder — expect 5–10 dB more than oil-immersed at equal kVA, and forced-air (ONAF/fan) cooling adds a few dB on top.
- Specify sound power, not “quiet” — IEC 60076-10 / GB/T 1094.10 define the measurement; NEMA ST-20 covers dry-type sound levels; hold the supplier to the test report.
- The cheap fixes come first — low-noise core steel (−3–5 dB) and placement/sound barriers; a full enclosure removes 15–25 dB but adds cost and reduces cooling.
What Actually Makes a Transformer Hum? Magnetostriction, Not the Windings
When alternating flux passes through the core, the iron crystals in the steel physically change length — a property called magnetostriction. Because the flux reverses direction every half-cycle, the core contracts and expands twice per cycle, which is why the hum sits at double the line frequency: 100 Hz on 50 Hz systems and 120 Hz on 60 Hz systems. That vibration is transmitted from the core through the tank (or the core clamps in a dry-type unit) into the surrounding air as audible noise. Load current also produces a 2× line-frequency force on the windings, but at normal load it is a minor contributor — the dominant source of a transformer’s characteristic hum is the core, which is why a transformer hums even at no load. This is also why no-load loss and noise both trace back to the same root cause: the magnetisation of the core.
Normal Decibel Levels by kVA — and Why Dry-Type Runs 5–10 dB Louder
Sound power level rises with rating because a larger transformer has a larger core surface area radiating noise. As a rough rule, sound power grows by about +3 dB for every doubling of kVA (industry experience). The table below gives typical sound power levels (LWA, dB(A)) — not the pressure level you would measure standing next to the unit, which falls off with distance:
| Rating | Oil-immersed LWA (dB(A)) | Dry-type LWA (dB(A)) |
|---|---|---|
| 100 kVA | 45–50 | 50–56 |
| 500 kVA | 50–55 | 55–60 |
| 1,000 kVA | 55–60 | 60–65 |
| 2,000 kVA | 58–63 | 63–68 |
| 5,000 kVA | 65–70 | 70–75 |
Dry-type transformers run consistently louder than oil-immersed at the same kVA for two reasons: there is no oil to damp the core vibration, and forced-air cooling fans add their own noise. The oil-vs-dry trade-off is broader than sound — see our Oil-Immersed vs Dry-Type Selection Guide for the full comparison. If the unit uses ONAN/ONAF or forced-air cooling, fan noise adds on top of the core hum; how the cooling system is configured is covered in Transformer Cooling Methods & Temperature Rise.
Sound Power (LWA) vs Sound Pressure (LpA): How the Number Is Actually Measured
Two quantities are easily confused. Sound pressure level (LpA) is what a microphone reads at a specific point — it depends on distance and the room, so a “dB” number without a distance and measurement surface is meaningless. Sound power level (LWA) is the total acoustic energy the unit radiates in all directions — it is a property of the transformer itself and does not change with distance. Standards therefore specify the transformer in terms of sound power: IEC 60076-10 (Power transformers — Part 10: Determination of sound levels) and its Chinese equivalent GB/T 1094.10 define how LWA is measured (a microphone array over a measurement surface, then converted to sound power), and IEEE C57.12.90 is the corresponding North American test code for liquid-immersed units. NEMA ST-20 sets sound-level limits for dry-type transformers. When you compare two quotations, make sure you are comparing the same quantity at the same weighting (A) and the same method — a supplier quoting LpA at 1 m will look quieter than one quoting LWA even when the units are identical.
How to Specify a Quiet Transformer: 5 Levers, Ranked by Cost
If the unit will sit indoors, near residential areas, hospitals, schools, or offices, write the noise limit into the specification up front — retrofitting is expensive. The five levers, cheapest first:
- Low-noise core steel — laser-scribed or domain-refined (Hi-B) electrical steel cuts magnetostriction and typically saves 3–5 dB at a modest premium. This is the best value for most applications.
- Lower flux density — designing the core larger so it operates further below saturation reduces both noise and no-load loss, at the cost of a bigger, heavier, pricier unit.
- Vibration isolation — rubber or spring pads between the core-and-coil assembly and the tank base stop structure-borne hum from travelling into the floor and building.
- Sound enclosure / canopy — for dry-type units an acoustic enclosure removes 15–25 dB but adds cost and restricts ventilation, so cooling must be re-checked.
- Placement and barriers — a solid wall or the right room location can cut what reaches the listener by 10 dB or more for essentially no equipment cost; this is an acoustic decision, not an electrical one.
If you are still choosing the unit itself rather than just its noise, start from our Transformer Selection Guide; for sizing the whole system (transformer, switchgear and feeders together) use the Power System Calculator.
Sources
- Source: IEC 60076-10, Power transformers — Part 10: Determination of sound levels.
- Source: GB/T 1094.10, Power transformers — Part 10: Determination of sound levels.
- Source: NEMA ST-20, Dry-Type Transformers for General Applications.
- Source: IEEE C57.12.90, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
- Source: industry experience (typical sound power levels by kVA, dry-type vs oil-immersed delta; QDTB engineering, as of 2026).