Audsley lists Harmonic Tuba in his Organ Stops and their Artistic Registration with the following description: The most powerful lingual stop, of 8 ft. pitch, inserted in the Organ. Its pipes are of large scale, having resonators of inverted conical shape and double the normal standard length. The reeds are formed similar to, but slightly larger in scale than, those of the Harmonic Trumpet. The stop is voiced on wind of fifteen inches upward, according to the volume and power of the tone required. Reeds with double tongues have been suggested for this impressive stop; but they are too troublesome to make, too uncertain in speech, and too difficult to tune, ever to favor their adoption: and it is questionable if they are desirable; for sufficient musical noise can be produced by the single-tongued reeds for all legitimate effects in dignified music. The proper tone of the Harmonic Tuba is full, sonorous, and commanding; dominating all the voices of the 8 ft. stops, labial and lingual, in the Organ. Its use is, accordingly, limited to rare and very special effects, chiefly of an orchestral character, and to grand climaxes in which a �full organ� burst of sound is called for. In The Art of Organ-Building, written a decade and a half earlier, Audsley states that this stop is voiced on at least 20" of wind, and that the reeds should have double tongues. He evidently changed his mind on these points. Irwin lists both names with the following description: A very loud Chorus Reed of 8' manual pitch, like the Harmonic Trumpet, furnished with double (or triple) resonators whose added length is designed to remove from the tone many of the very high partials that characterize these two Chorus Reeds when of normal unison length. The harmonic length, usually not in the bass octave, enables these pipes to maintain an even and steady pitch of both fundamental and most overtones, at least the lower-pitched ones. This is generally a louder Reed than the Harmonic Trumpet, and also one of heavier and firmer tone. Some of the freedom of vibration the listener can hear in the Harmonic Trumpet is not in this tone. In many specimens it is completely free from dissonance, at least of the noticeable sort, because of the damping action of the longer tubes. These tubes ... are thick-walled to prevent their own absorption of the reeds' tones. In both of these harmonic Reeds they are of spotted metal for their full lengths. Ordinary pipe metal of high lead content would never resonate the highest overtones. Tin in the proportion of from 35% to 85% is needed in these Reeds, and in most other Reeds, because it permits the voicer to obtain a brighter tone with a resonator of average dimensions. These resonators are tuned by rolling the metal in the slot. This Tuba is not so bright as the Tuba Mirabilis or the Tuba Magna, but is much brighter than the Tuba Sonora. Like the Harmonic Trumpet, it is a solo and ensemble stop of great power and excellent blend with flues as well as other Reeds. The name Harmonic Tuba has also been used as a synonym for Tuba Horn. Other candidates for the loudest organ stop include Tuba Mirabilis and Ophicleide.
Harmonic Tuba 8', Great; Bowdoin College Chapel, Brunswick, Maine, USA; Austin 1927.