Room Acoustics affect the sound emitted from a Transformer greatly.
Square shaped rooms are the worst (equal length walls on all four sides). Rectangular shaped rooms are a bit better; and rooms with one "iregular" shaped wall are best.
Hollow wall cavity, framed with steel studs, are easilly vibrated by Transformers. Dampening the walls surrounding a Transformer is an effective way to reduce the transmitted vibrations. Dampen with typical Building Insulation or Sound Battings.
Other things which add to the noise are:
* Reflective Flooring Materials - such as hard VCT, or sealed - bare Concrete floors,
* Setting a Transformer in a corner, where one side of the Transformer is less than 6 feet from one wall, and the front is also less than 6 feet from another wall,
* Non-Flexible Conduit connections to the Transformer,
* Transformers set back - to - back (one facing forward on one side of a hollow wall, another facing backwards on the opposite side of the same wall),
* Setting Transformers against, or near Concrete walls (Tilt-Up Panels),
* Not isolating the frame from floors via dampening materials,
* Loose core laminations, mounting hardware, covers and/or enclosure sections.
The loading of a Transformer does not affect the output sound, nor do Harmonics.
A Transformer will "hum" the same level at idle, as it would at full load / 25% load / 50% load / 75% load.
If everything is tight, the Transformer is connected with flexible whips, and the legs are isolated, then you will need to correct the room's acoustics.
Scott
