euharmonic

from The Century Dictionary.

• Producing perfectly concordant sounds, as opposed to sounds produced by tempered instruments.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Mus.) Producing mathematically perfect harmony or concord; sweetly or perfectly harmonious.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective rare Pleasingly melodious or harmonious — applied especially to the euharmonic organ invented by Henry Ward Poole (1825-1890), in whose design mathematical principles of harmony were employed.