4th Spiral Hum

The 4th Spiral Hum is an 18.5 Hz oscillation maintained in Chamber Theta-14 on Floor 333, and the four-hundred-and-eleventh item to be entered on the standing register of Continuous Hum deviations. It is distinct from the four previous Spiral Hums not by frequency, since all five lie between 18 and 19 Hz, but by the manner of its containment: whereas the others are permitted to propagate along their spirals, the 4th is held in a sealed chamber and released only for assessment. It is the only Hum on the register to have been witnessed by a named archival party from within the source chamber without specialised breathing apparatus.

Description and containment

Chamber Theta-14 occupies the north-east quadrant of Floor 333 and measures eleven by fourteen paces. The Hum is not audible at the door. It becomes perceptible, according to the Manual of Acoustic Assessment (Spiral 71, clause 2.4), at approximately the third pace inward, at which point it is felt rather than heard, principally in the wrists. At the chamber's centre the oscillation registers at 18.5 Hz on a standard brass fork, with a drift of no more than 0.03 Hz across a full working day.

Visitors are required to leave their own tuning forks at the door. The chamber keeps its own.

The Survey of the Canted Halls

The 4th Spiral Hum was first documented by Archivist-Lishener during the Survey of the Canted Halls. Lishener's account, preserved on original wax-cylinder recordings in the Sound Archive of Floor 333 and partly transcribed in the Auditory Annex of Floor 347, describes a tone "persistent as a pulse and about as interested in being noticed" (Lishener, Field Notes, Canted Halls, Cylinder 6). The Survey established the chamber's dimensions but not its age, a gap in the record that the Eleventh Concordance was later amended to excuse on grounds of "prior enclosure" (Annex 2).

Lishener returned to the chamber three times over four years and on each occasion recorded the same single tone, unvarying. The cylinders themselves have since developed a second tone, which the Guild of Cylinder Archivists maintains is not evidence of anything.

Dispute as to origin

Two readings of the Hum's source circulate without resolution. The Seventeenth Concordance (clause 11) classifies the 4th Spiral Hum as a local resonance of Floor 333, "derived from the settling of its own masonry," and cites the chamber's slight tilt as corroboration. The Nineteenth Concordance, drafted eleven years later, rejects this outright and holds that the Hum is "a fragment of a greater oscillation, broken off and retained" (Annex 5), pointing to the frequency's near-identity with the earlier Spiral Hums and with the sub-audible component recorded at Aperture 77-Gamma.

The dispute has real consequences for the chamber's tending. If the Hum is local, its drift belongs to the floor surveyors and may be corrected by shimming. If it is a retained fragment, any correction constitutes an alteration of the greater Hum and falls under the acoustic registry, which has refused to rule. No shims have been fitted. The chamber has been inspected by both offices in alternating years since the Stairwell Jurisdictions settlement of the winter of the year recorded as 3,114.

"The 4th Spiral Hum is to be assessed as though its origin were known, and maintained as though it were not."

— Manual of Acoustic Assessment, Spiral 71, clause 2.9

Relation to other Hums

The 4th Spiral Hum is frequently compared with the Persistent Hum of Aisle Gamma, the audible extent of which reaches the eastern terminus of Spiral 719-Gamma. The two share a spectral signature in the third band but are not linked by any duct or aperture; the comparison is made, in the words of one unsympathetic reviewer, "because there are only so many hums and we have all met them." More materially, the Hum has been entered as a possible distant contributor to the Dual-Frequency Hum from the Gallery of Unfixed Perspectives on Floor 812, though the Office of Sealed Chambers has declined to authorise a carrier test on the grounds that the Hum would have to leave Chamber Theta-14 to participate.

Connections of a looser kind are drawn to the Ventilation Chorus, which integrated the hum of Spiral 9 into its operations by the Settlement of Spiral 9, and to the Pepper-Ghost Hum on Spiral 89, though the latter's active stasis renders any comparison provisional. The Hum of the Lower Floors is generally treated separately: it is loud, and the 4th Spiral Hum is not.

Access and recent incident

Chamber Theta-14 is opened to assessors twice yearly, in the first weeks of the second and eighth months, under the supervision of the Office of Sealed Chambers. Instruments are permitted; written notes are not, as the paper begins to flutter. In the eighth month of the year recorded as 3,408, a visiting assessor from Spiral 77 remained in the chamber beyond the allotted half-hour and reported afterwards that the Hum had "moved to the far end of the room and become polite" (Testimony of Assessor Renn, filed with the Office of Sealed Chambers). The Office found no change in the chamber, no change in the frequency, and no far end.

The report was nevertheless retained in the file, as is customary. The Hum has not been observed to move since, and the Office has recorded its position as "throughout."

See also

• Hum
• The Substrate Hum
• The Hum of Absent Machinery
• The Hum of Anticipated Rain