Transformer noise: what the dB(A) figure means and how to keep the neighbours happy
Distribution transformers hum at 100 Hz. Typical sound power levels for 100–2500 kVA units, how they translate into what a person hears at 10 m, and the measures that actually reduce noise.
The hum of a transformer comes from magnetostriction – the core laminations change length slightly with every magnetic cycle, twice per mains period, which is why the fundamental tone is 100 Hz (at 50 Hz) with harmonics at 200, 300 and 400 Hz. The level depends on the core induction, the mass of the core and the quality of its clamping, not on the load.
Sound power vs sound pressure
Data sheets state the sound power level LWA according to IEC 60076-10 – the total acoustic power radiated, independent of the room. What a person hears is the sound pressure level LpA at a distance. For a small source in a free field the rule is roughly:
LpA(r) ≈ LWA − 20 · log10(r) − 8 dB
A 48 dB(A) transformer therefore produces about 20 dB(A) at 10 m outdoors – below the background noise of a quiet rural night. Inside a reverberant substation room, however, reflections add 5–10 dB.
Typical PNACHO values
| Rating | PNACHO-O (oil) LWA | PNACHO-D (resin) LWA, AN / AF |
|---|---|---|
| 100 kVA | 39 dB(A) | 39 / 50 dB(A) |
| 250 kVA | 42 dB(A) | 43 / 56 dB(A) |
| 630 kVA | 48 dB(A) | 47 / 61 dB(A) |
| 1 000 kVA | 51 dB(A) | 49 / 63 dB(A) |
| 1 600 kVA | ≈ 55 dB(A) | 52 / 67 dB(A) |
| 2 500 kVA | ≈ 60 dB(A) | 54 / 70 dB(A) |
Cast-resin units are marginally quieter than oil units in natural-air mode; with fans running the fan noise dominates.
What increases the noise on site
- Overvoltage: running a transformer 5 % above rated voltage raises the core induction and can add 3–5 dB. Choosing the right tap position is the cheapest noise measure.
- DC bias and harmonics from PV inverters or rectifiers.
- Rigid mounting on a resonant floor or steel frame.
- Small, hard-walled rooms.
Measures that work
- Anti-vibration pads under the rollers – a standard option for PNACHO units; they decouple the 100 Hz vibration from the floor and are the single most effective measure for indoor installations.
- Flexible connections on the LV busbars and cables, so the vibration does not travel along the busbars.
- Room absorption: mineral-wool panels on two walls; the room should not be a bare concrete cube.
- Distance and orientation: place the long side of the tank away from the nearest window.
- Low-noise design: for hospitals and residential blocks a transformer with reduced core induction can be built – about 5–8 dB lower at a moderate surcharge.
Sound power is measured in the routine test only on request; the figures on the data sheet are guaranteed maximum values per IEC 60076-10 for the standard design. Ask for a measured value if a noise limit is part of your building permit.
The complete noise data for each rating is in the technical data section of every product page in the PNACHO-D and PNACHO-O ranges.
Further reading: MV Transformer noise: Why does a transformer hum? (energeks.com, EN) · Hałas transformatora SN: czemu transformator buczy i co z tym zrobić? (energeks.pl, PL)
Request a quote for your PNACHO transformer
Send us rated power, voltages and the site of installation – our engineers will select the right configuration and send a quote within 24 hours.