just intonation
A tuning system based on pure intervals derived from the harmonic series, using simple whole-number frequency ratios
In Depth
In just intonation, intervals are tuned to their acoustically pure ratios: 3:2 for a perfect fifth, 5:4 for a major third, 6:5 for a minor third. These intervals are more consonant than their equal-tempered equivalents but create problems when modulating because the ratios do not add up consistently across all keys. Barbershop quartets naturally gravitate toward just intonation, producing the characteristic ringing overtones that define the genre. Composers like La Monte Young and Ben Johnston have written extended works in just intonation.
When a barbershop quartet locks into perfectly just intervals, the overtones become so strong that audiences perceive a fifth phantom voice above the singers.