Transformers emit a continuous 120 Hertz hum with harmonics when connected to 60 Hertz circuits. The fundamental frequency is the “hum” which annoys people primarily because of its continuous nature.
For purposes of reference, sound measuring instruments convert the different frequencies to 1000Hertz and a 40 db level. Transformer sound levels based on NEMA publication TR-1 are listed in the Table below:
Since values given in Table are in general higher than typical, the difference must be attenuated by distance and by proper use of materials in the design of the building. An observer may believe that a transformer is noisy because the level in the room where it is located is high.
Two transformers of the same sound output in the same room increase the sound level in the room approximately 3 db, and 3 transformers by about 5 db, etc. Sounds due to structure-transmitted vibrations originating from the transformer are lowered by mounting the transformers on vibration dampeners or isolators. There are a number of different sound vibration isolating materials which may be used with good results.
Dry-type power transformers are often built with an isolator mounted between the transformer
support and case members. The natural period of the core and coil structure when mounted on vibration dampeners is about 10% of the fundamental frequency.
The reduction in the transmitted vibration is approximately 98%. If the floor or beams beneath the transformer are light and flexible, the isolator must be softer or have improved characteristics in order to keep the transmitted vibrations to a minimum. (Enclosure covers and ventilating louvers are often improperly tightened or gasketed and produce unnecessary noise.)
The building structure will assist the dampeners if the transformer is mounted above heavy floor members or if mounted on a heavy floor slab. Positioning of the transformer in relation to walls and other reflecting surfaces has a great effect on reflected noise and resonances.
Often, placing the transformer at an angle to the wall, rather than parallel to it, will reduce noise. Electrical connections to a substation transformer should be made with flexible braid or conductors; connections to an individually-mounted transformer should be in flexible conduit.