The defining characteristic of the barbershop style is the ringing chord. This is a name for one specific and well-defined acoustical effect, also referred to as expanded sound, the angel's voice, the fifth voice, the overtone, or barbershop seventh. (The barbershopper's "overtone" is the same as the acoustic physicist's overtone or harmonic, although the numbers differ. The first overtone, at twice the fundamental frequency, is the second harmonic, etc. The undertone is the difference between the frequencies of two sung notes and is known as heterodyning).
The physics and psychophysics of the effect are fairly well understood; it occurs when the upper harmonics in the individual voice notes, and the sum and difference frequencies resulting from nonlinear combinations within the ear, reinforce each other at a particular frequency, strengthening it so that it stands out separately above the blended sound. The effect is audible only on certain kinds of chords, and only when all voices are equally rich in harmonics and justly tuned and balanced. It is not heard in chords sounded on modern keyboard instruments, due to the slight tuning imperfection of the equal-tempered scale.
Gage Averill writes that "Barbershoppers have become partisans of this acoustic phenomenon" and that "the more experienced singers of the barbershop revival (at least after the 1940s) have self-consciously tuned their dominant seventh and tonic chords in just intonation to maximize the overlap of common overtones." However, "In practice, it seems that most leads rely on an approximation of an equal-tempered scale for the melody, to which the other voices adjust vertically in just intonation."
What is prized is not so much the "overtone" itself, but a unique sound whose achievement is most easily recognized by the presence of the "overtone". The precise synchrony of the waveforms of the four voices simultaneously creates the perception of a "fifth voice" while at the same time melding the four voices into a unified sound. The ringing chord is qualitatively different in sound from an ordinary musical chord e.g. as sounded on a tempered-scale keyboard instrument.
Most elements of the "revivalist" style are related to the desire to produce these ringing chords. Performance is a cappella to prevent the distracting introduction of equal-tempered intonation, and because listening to anything but the other three voices interferes with a performer's ability to tune with the precision required. Barbershop arrangements stress chords and chord progressions that favor "ringing", at the expense of suspended and diminished chords and other harmonic vocabulary of the ragtime and jazz forms.
Harmonic seventh chord
The harmonic seventh chord is a major triad plus the harmonic seventh interval (ratio of 7:4, about 969 cents). This interval is somewhat narrower than and is "sweeter in quality" than an "ordinary" minor seventh, which has a just-intonation ratio of 9:5 (1017.596 cents), or an equal-temperament ratio of 1000 cents (25/6:1). Frequent use of this chord is one of the defining characteristics of blues and barbershop harmony; barbershoppers refer to it as "the barbershop seventh." Since barbershop music tends to be sung in just intonation, the barbershop seventh chord may be accurately termed a harmonic seventh chord. The harmonic seventh chord is also widely used in "blues flavored" music. As guitars, pianos, and other equal-temperament instruments cannot play this chord, it is frequently approximated by a dominant seventh. As a result it is often called a dominant seventh chord and written with the same symbols (such as the blues progression I7 - V7 - IV7).
An often heard example of the harmonic seventh chord is the last word of the modern addition to the song "Happy Birthday to You", with the lyrics, "and many more!" The harmony on the word "more" is typically sung as a harmonic seventh chord.
barbershop seventh chord. A chord consisting of the root, third, fifth, and flatted seventh degrees of the scale. It is characteristic of barbershop arrangements. When used to lead to a chord whose root is a fifth below the root of the barbershop seventh chord, it is called a dominant seventh chord. Barbershoppers sometimes refer to this as the 'meat 'n' taters chord.' In the nineteenth and early twentieth centuries, these chords were sometimes called 'minors.' [as in minor seventh chord]"
The barbershop seventh is the name commonly given by practitioners of barbershop music to the seventh of and the major-minor seventh or dominant seventh chord, when it is used in a barbershop arrangement or performance. "Society arrangers believe that a song should contain anywhere from 35 to 60 percent dominant seventh chords to sound 'barbershop'--and when they do, barbershoppers speak of being in 'seventh heaven.'"
Beginning in the 1940s, barbershop revival singers "have self-consciously tuned their dominant seventh and tonic chord in just intonation to maximize the overlap of common tones, resulting in a ringing sound rich in harmonics" called 'extended sound', 'expanded sound', 'fortified sound', "the voice of the angels". The first positive mention of such practice appears to be Reagan, 'Molly' (1944). "Mechanics of Barbershop Harmony", Harmonizer. The example of a dominant chord tuned to 100, 125, 150, and 175 Hz, or the 4th, 5th, 6th, and 7th harmonics of a 25 Hz fundamental is given, making the seventh of the chord a "harmonic seventh".
There's a chord in a barbershop that makes the nerve ends tingle....We might all our chord a Super-Seventh!...The notes of our chord have the exact frequency ratios 4-5-6-7. With these ratios, overtones reinforce overtones. There's a minimum of dissonance and a distinctive ringing sound. How can you detect this chord? It's easy. You can't mistake it, for the signs are clear; the overtones will ring in your ears; you'll experience a spinal shiver; bumps will stand out on your arms; you'll rise a trifle in your seat."
It is normally voiced with the lowest note (the bass) on a root or a fifth, and its close harmony sound is one of the hallmarks of barbershop music.
When tuned in just intonation (as in barbershop singing), this chord is called a harmonic seventh chord.