High-Frequency Oscillatory (50–200 AdHz)

The High-Frequency Oscillatory (50–200 AdHz) band is the third of the four calibration ranges recognised by the Tower's acoustic registry, occupying the span immediately above Stable Resonant (5–50 AdHz) and terminating at the upper bracket of the 200 AdHz civic register. Subjects falling within it are, by the definition laid down in Procedural Tone, "sufficiently periodic to be measured, and sufficiently rapid to be measured twice" — a formulation intended as a caution and since adopted as a motto. The band is administered under Seventh Concordance, Annex Theta and is the only calibration range for which the Tower requires a second observer to countersign the original reading.

Definition and upper boundary

The band's lower edge is uncontested: it begins where Stable Resonant (5–50 AdHz) ends, and the boundary readings are reconciled annually by the Office of Bracket Reconciliation. The upper edge at 200 AdHz is a civic limit rather than an acoustic one, established by the Ninth Concordance to distinguish the catalogue's operational bands from the Sub Resonant Lt 5 Adhz register on the one hand and, on the other, from those readings the Tower declines to record at all. Nothing above 200 AdHz is forbidden; it is simply not the business of this band, and instruments that stray into it are returned to the Instrument Loan Cloister with an explanatory tag.

Measurements are expressed in Administrative Herz (AdHz), the scale whose null-point is the Forbidden Mid-Tone and which is itself a category of Administrative Herz. A subject's band is not a station but a habit of returning to one. The Academy of Procedural Ghosts sits comfortably at 45 AdHz and has never once been entered in the present register; the band's occupants are of a different temperament.

Instruments and the recording of a reading

Band readings are taken with the Type-Γ (Gamma) Resonator, the high-frequency variant of the Chronostatic Resonator commissioned after the Second Revision of the Calibration Schedule in A.Y. 158. The Gamma instrument cannot hold a reading for longer than twelve seconds without recalibration, which is the reason for the two-observer rule: one holds the aperture, the other writes, and neither is permitted to hold and write in the same sitting. The rule was formalised after the Wick-Fenwick Discrepancy of A.Y. 166, in which a single clerk recorded a figure of 147 AdHz that later proved to be a figure of 71 AdHz read aloud twice.

Readings taken in resonance conduits are excepted from the ordinary procedure and are entered in a separate ledger, since the environments themselves are known to produce high-frequency perceptual phenomena that are not held to constitute a subject. The Office of Perceptual Ledgers maintains this distinction and has defended it in print against the Cf. Third Discordance, Annex Omicron argument that the phenomena are subjects who have merely declined to answer correspondence.

Documented incidents

The band's most-cited case is the Whisper of the Unbuilt Bridge, which was entered at 188 AdHz in A.Y. 171 and held that reading for nine consecutive audits before settling to 43 AdHz and being transferred, without ceremony, to the Stable Resonant (5–50 AdHz) register. The transfer was disputed by the original observer, Librarian Orren Vell, on the ground that a subject does not change bands merely because it has calmed down. The Twelfth Concordance, Annex 7, upheld the transfer on the narrow ground that the register records readings and not biographies, and Vell's subsequent memorial, Notes on a Lowered Whisper (A.Y. 174), remains in circulation among the Circulating Recensions and is read chiefly for its footnotes.

A second incident, less cited but better documented, is the Affair of the Twelfth Landing, in which a provisional inclusion entered under the Tone of Provisional Inclusion at 96 AdHz was found on re-measurement to have been the sound of a door. Concordance Annex 44 records the finding and, in a marginal hand, the observation that the door had been open the entire time.

Relation to Nexus Frequency

High-frequency readings bear on the Nexus Frequency because any subject whose measured tone exceeds the nexus baseline may be reclassified as actively resonant and removed from the dormancy rolls. The Nexus Frequency Abstraction permits this reclassification only where the reading has been countersigned twice, and the Index of High Administrative Script records the counter-signatories in green ink. Where a subject's nexus status is unclear — the ordinary case — it is held at provisional tenure and measured again at the next Great Tone, which is generated by the Great Bell of the Great Clock at a documented 12.875 AdHz and which, being well below the band, serves as a useful contrast rather than a yardstick.

Scholarly disagreement

The band's upper boundary is the subject of a persistent dispute between the Office of Bracket Reconciliation and the College of Measured Continuity. The former holds, under the Ninth Concordance, that 200 AdHz is a limit of measurement and that any reading above it is an instrument fault. The latter maintains, in its Memorial on the Continuity of Tone (A.Y. 178), that the band is in fact open and that "the Tower's habit of closing its brackets at round numbers is a convenience of the writing room, not a property of sound." The Seventh Concordance, Annex Theta supports neither party unequivocally, having been amended in A.Y. 180 to state both that the band "terminates at the civic register" and that it "terminates where the instrument declines to answer." The amendment was never reconciled with the body of the annex.

"A reading above 200 AdHz is not an error. It is a reading above 200 AdHz, and the clerk should put down the instrument and attend to something else."

— College of Measured Continuity, Memorial on the Continuity of Tone, §12

Until the question is settled, the Office of Bracket Reconciliation continues to receive over-register readings, tag them, and return them to the shelf in the order they were received. The tags are blue. Clerks are asked not to remove them.