Magnetic hum, ventilation, neighbours: understanding the sound levels (LwA, LpA, IEC 60076-10) and the real levers, at design stage as at installation.
Two sources dominate. The magnetic hum first: the core vibrates to the rhythm of the flux (a fundamental at 100 Hz on a 50 Hz network, plus its harmonics); it is the continuous acoustic signature, present even at no load. Then ventilation, where it exists: forced cooling (ONAF, AF) adds an aerodynamic noise that only appears at high load. Non-linear loads (converters, chargers) enrich the spectrum and can make the hum more noticeable.
Two quantities coexist and are often confused: sound power (LwA), a property of the machine, and sound pressure (LpA), what you measure at a given distance; the latter varies with the room and the distance. A transformer's guaranteed levels are determined by test per IEC 60076-10 and appear on its test report: the only serious basis to compare two offers, at identical measurement conditions.
At design stage, the main lever is the core's flux density: working the magnetic circuit less "hard" reduces the hum, at the cost of more material, which converges with the reduced-losses logic (low-loss machines are often the quieter ones too). The core's assembly quality also counts, as does the technology: at equal rating, an oil-immersed unit is generally more discreet than a dry-type, the tank and the oil damping the vibrations.
A quiet machine can still make a noisy site if the installation invites it: anti-vibration pads under the transformer (cutting the structure-borne path to the slab), no rigid contact between the unit and the walls, a treated room or enclosure where needed, and distance from sensitive areas; the perceived level drops fast with distance. On a prefabricated substation, the enclosure is specified accordingly at quotation stage.
General overview; guaranteed levels are set at quotation stage and verified on each unit's test report.
At equal rating, generally yes: the oil-immersed unit benefits from the damping of the tank and the oil. A well-designed, well-installed dry-type remains perfectly compatible with an office environment; it is a matter of specification.
Yes: a reduced level is specified at order stage and verified by measurement per IEC 60076-10, recorded on the test report. Specifying it up front costs incomparably less than acoustic treatment after a complaint.
The magnetic hum is present as soon as the unit is energised, even at no load. Fan noise, on ONAF or AF units, only appears at high load when the fans start.
Whether dwellings or occupied rooms are close, the planned location, and any imposed level. With that we specify the machine, the enclosure and the mounting accordingly.