The National Electrical Manufacturers Association (NEMA) published NEMA TR 1-2013 Transformers, Regulators and Reactors.
This publication applies to single-phase and polyphase power and distribution transformers (including step-voltage regulators and reactors). Step-voltage regulators have been added to the scope in this edition and there is now more alignment with IEEE transformer standards. NEMA TR 1 is one of the few standards on the market that lists audible sound levels for these particular transformers.
NEMA TR 1-2013 may be downloaded at no cost or purchased in hardcopy for $55 on the NEMA website.