Attenuation Threshold

The Attenuation Threshold is the minimum reduction in signal, luminance, or resonance that a phenomenon must undergo before it is deemed, for archival purposes, to have stopped. It is a definition rather than a measurement: the Tower does not record what a thing was, but what of it remains after the Threshold has been applied. The Threshold is not a single figure but a family of them, one per medium, each set by the Threshold Table and each amended so frequently that the Threshold Clerks maintain a working edition in loose gatherings, replaced every ninth year.

The best-known instance is the luminous one. Concordance 11 (On Illumination Standards) fixes the threshold for lumen-sconces at 2,000 Standard Lumen Units, above which a fixture is classed as an Arclight Sconce and below which it is merely a sconce, and may be extinguished without notice during the seasonal reductions. The acoustic instance is governed by the Consensus Stair, which requires that no sound above a stated threshold travel a full riser without attenuation. The Threshold Table sets the other media — dust, resonance, and the disputed category of "matters pending" — by analogous means, though the analogy has never been formally agreed.

Origin and the Question of the First Table

The Threshold Table is traditionally ascribed to the Fourth Recension, compiled on Floor 412 (the Vestibule of Small Reductions) during the long winter of that floor's second century. The Recension's preamble states the governing principle plainly:

"A thing is not ended when it ceases, but when it is shown to have ceased by an authorised margin."

— Fourth Recension of the Threshold Ordinances, preamble, clause 2

No first, second, or third recension survives, and the Office of the Clerk of the Threshold Body has declined, in writing and on six occasions, to say whether they existed. The question is considered settled in practice and open in doctrine, which is the Tower's usual arrangement.

The Seven-Table Dispute

A persistent disagreement divides the Threshold Clerks from the Keepers of the Attenuation Galleries. The Clerks hold that there is one Threshold, expressed in seven media — luminance, resonance, dust, echo, weight, silence, and lapse — and that the apparent differences between them are matters of notation. The Keepers hold that there are seven Thresholds, unrelated, and that the Clerks' unification is a filing convenience dressed as cosmology. The dispute is conducted almost entirely through marginal annotations appended to one another's editions, some of which are now longer than the ordinances they annotate.

The practical consequence falls on the Attenuation Galleries, where raw Archival Dust is put through the seven-stage process codified in the Standing Ordinances of the Attenuation Galleries. If the Clerks are right, a batch failing the dust threshold may be re-entered under the silence threshold and pass. If the Keepers are right, it may not. Batches are presently re-entered anyway. The Ongoing Monitoring of the Attenuation Galleries records the discrepancy without resolving it, since resolving it is not within its remit.

Application in the Galleries

Within the Galleries the Threshold is applied by the Axis of Attenuation, a calibrated instrument read against the Table and adjusted by hand. The Attenuation Lens must be polished twice weekly; a lens left unpolished past the ninth day drifts by roughly eleven Standard Lumen Units, which has twice in living memory carried a marginal sconce across the Arclight line and required a formal retraction. Primary Attenuation and Procedural Attenuation apply the Threshold at different stages — the first in the galleries proper, the second in the record of what was done there — and Contained Attenuation applies it to substances that cannot be allowed to attenuate at large.

The Eastern Attenuation Conduit carries attenuated material away from the galleries. No one has established whether the Threshold applies to the Conduit's own hum, which sits close to the acoustic figure and is the subject of a standing complaint from the Clerks of the Unspoken Threshold.

Contested Readings and the Lapse Threshold

The most contentious entry in the Table is the lapse threshold, which governs how long a phenomenon may go unrecorded before it is treated as having attenuated regardless of its actual condition. The Threshold Table sets the lapse at forty days. A revision circulated under Concordance 11, Annex 6, sets it at thirty. Both remain in force. A phenomenon may therefore be extant under one reading and attenuated under the other on the same afternoon, and the Threshold Clerks are directed in such cases to record it as "extant, attenuated."

Clerk Amneris Holt of the third desk objected in writing to the compound entry in the winter of the archive's four-hundred-and-sixth year, arguing that a thing cannot be both. Her memorandum was accepted, filed, and cross-referenced under a heading that required her to read it again every time she used the index. She withdrew the objection in the following spring.

House practice

Reading the Threshold from a working edition requires the reader to hold the volume open at the current gathering with one hand and mark the place with the other, as the bindings are not sewn. Sconces below the Threshold are extinguished at the eighth bell. Nothing may be carried through the Attenuation Galleries below the Threshold without a Clerk's mark, and boots must be wiped at the door.

The Threshold has never been revised downward for any medium except echo, which was lowered twice in the same decade and restored once. The restoration is recorded as a clarification rather than an amendment, and is cited by the Keepers in support of their position.